Fast Plants 2002

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Artificial Selection

in

Brass ica rapa

Biology 100 – Advances in Biology in the Age of theHuman Genome Project

I242 – Light, the Universe, and Everything

Spring 2002

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Table of Contents

Artificial Selection in Brassica rapa

Introduction............................................................................................................................... 1

Selective Breeding ................................................................................................................ 1Artificial vs. Natural Selection.............................................................................................. 2Research Origins of Fast Plants ............................................................................................ 4Life Cycle of rapid cycling Brassica rapa............................................................................. 5Variable Traits in Brassica rapa.......................................................................................... 13Fast Plants and their Butterfly, Pieris rapae........................................................................ 13

Our Experiment....................................................................................................................... 16Sowing Fast Plant Seed....................................................................................................... 16Counting Hairs.................................................................................................................... 17Selecting the parents of the next (second) generation......................................................... 18Pollination of selected plants .............................................................................................. 19

Harvesting seeds ................................................................................................................. 20Establishing the second generation..................................................................................... 20Counting the hairs in the second generation....................................................................... 20

Data Analysis .......................................................................................................................... 21Inheritance Patterns............................................................................................................. 21Analyzing the data to describe the variation in the population........................................... 26Thinking about variation..................................................................................................... 29

Writing the report.................................................................................................................... 30General Guidelines.............................................................................................................. 30The Conventional Format ................................................................................................... 31

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Artificial Selection in Brassica rapa

Adapted and edited by Katherine Dorfman1

Introduction

Select ive Br eeding

A trip to a supermarket, farm, pet store or garden center will offer nearly endless examples ofthe products of selective plant and animal breeding by humans. Over hundreds, and in somecases thousands of years, humans have altered various species of plants and animals for ourown use by selecting individuals for breeding that possessed certain desirable characteristics,and continuing this selective breeding process generation after generation. In many cases theend results have been dramatic. Domestic dog varieties, from Chihuahuas to great Danes,trace their separate lineages to a common wild ancestor, the wolf (Canis lupus). Domesticfowl varieties are all derived from the wild jungle fowl (Gallus gallus), while most modern breeds of domestic cattle originate from the now-extinct giant wild ox ( Bos primigenius).One example that particularly impressed Charles Darwin was that of domestic pigeonvarieties (such as tumblers, fantails, carriers, pouters, and many others), which are derived

from wild rock doves (Columba livia) over a period of some 5000 years.

There are many similar examples among plants, including those that humans have bred forfood as well as beauty. One plant group especially important to humans for food is Brassica genus of plants in the mustard family. A wide variety of familiar and highly nutritiousvegetables originate from just a few species of wild Brassicas, in particular B. rapa, B.

oleracea, and B. junce. Some varieties have been bred specifically for root production,others for leaves, flower buds, oil production, etc. The group is of great economicimportance, and because of this, the genetic relationships among the various forms have beenthoroughly studied and are now fairly well understood. It may surprise you to learn that thesefamiliar vegetables so different in appearance have the same species as a common ancestor.

Centuries of artificial selection have produced such widely divergent forms. For example, thefollowing varieties originate from these wild species (see Figure 1):

Brassica oleracea: cauliflower, broccoli, cabbage, Brussels sprouts, kohlrabi, collards,savoy cabbage, kale

Brassica juncea: leaf mustard root mustard, head mustard, and many more varieties ofmustard

Brassica rapa: turnip, Chinese cabbage, pak choi, rapid-cycling

Figure 1. Varieties (cultivar groups) of Brassica r apa .

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2 Artificial Selection in Brassica rapa

The next time you’re in a grocery with a large produce section or a large farmers’ market, seehow many of these different varieties you can find; there are numerous other varieties notlisted above.

That plant and animal breeders have been able to dramatically change the appearance ofvarious lineages of organisms in a relatively short period of time is an obvious yet profoundfact. It certainly did not escape the attention of Charles Darwin, who devoted the firstchapter of The Origin of Species to the topic of artificial selection by humans (“VariationUnder Domestication”). He used many examples of selection by humans to help support thecase for his proposed mechanism resulting in evolution of natural populations—naturalselection.

In order to gain better understanding of selection and inheritance, researchers haveexperimented with artificial selection involving a wide variety of traits in many differentspecies of plants and animals. The results, obtained in a relatively short period of time, areoften impressive.

For example, in one experiment (started in 1896), the average oil content of corn kernels wasincreased from 5% to 19% in 75 generations of selective breeding. In another, average annualegg production in a flock of white leghorn chickens was doubled in only 33 years (from 126to 250 eggs per hen). Similar examples abound, for characters useful to humans (as above)as well as more esoteric ones (such as the number of abdominal bristles on fruit flies, or thefecundity of female flour beetles).

A general finding from these studies is that most variable traits in organisms respond toartificial selection (i.e., it is usually possible—even easy—to increase or decrease thefrequency or average value of a trait in a lineage through careful selective breeding). In thislab exercise, you will attempt to accomplish the same thing.

Art if ic ial vs. Natural Selectio n

Natural selection is a deceptively simple concept, relatively easy to understand at a basiclevel, but with profound implications that are intellectually challenging. The followingexercise in artificial selection will serve as an introduction to natural selection, and we hopewill help you to better understand this important concept. Fundamentally, artificial selectionand natural selection are quite similar, but there are a few important differences.

Natural selection

Very briefly, natural selection occurs when certain variants within a population of organismsexperience consistently greater reproductive success (i.e., leave more offspring) than do other

variants. Generally, within a population of organisms (of the same species), there is variationamong individuals for a great variety of obvious and not so obvious traits. Some of thisvariability is a result of genetic differences among individuals, while some is probably aresult of different environmental influences. Here we are concerned only with variability thathas a genetic basis (i.e., is inheritable—passed on from parents to offspring). As a generalrule, organisms produce more (often far more) offspring than can survive to reproducethemselves (in Darwin’s words, there is a “struggle for existence”). If individuals withcertain inheritable characteristics have an advantage in survival and reproduction overindividuals with other characteristics, then it follows that the next generation will differ frothe previous one—perhaps slightly, perhaps considerably. Why? Because individuals with

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Mount Holyoke College, Spring 2002 3

those certain favored inheritable characteristics are more likely to be parents by virtue of possessing these characteristics; the next generation will contain a disproportionate numberof offspring of these “naturally selected” parents, and these offspring will tend to resembletheir parents. As a result, over many generations the genetic structure of the populationchanges (in other words, the population evolves).

Artificial Selection

Artificial selection is essentially this same process. A population of organisms exhibitsconsiderable variability among individuals for a trait or traits that might be of interest tohumans for one reason or another (such as running speed in thoroughbred horses, petal sizeand shape in tulips, number of kernels in an ear of corn, egg size in chickens, etc.). By virtueof possession of certain traits, some individuals are selected by the plant or animal breeder orscientist to be parents of the next generation, while the remainder of the population not possessing those traits are excluded from breeding. To the degree that the desired trait isinheritable, the offspring will tend to resemble their parents, and hence on average the nextgeneration will differ from the previous one—i.e., will be faster, or have larger petals, or

more kernels, or larger eggs, etc. Over time, substantial differences can be achieved betweenthe original population and the artificially selected subsequent ones.

As noted above, there are some important differences between artificial and natural selection.In contrast to natural selection, artificial selection:

1) favors traits that for one reason or another are preferred by humans;

2) has a goal or direction toward which the selection process is directed;

3) generally is much faster than natural selection, because the next generation can beabsolutely restricted to offspring of parents that meet the desired criteria (rarely isnatural selection such an all-or-none phenomenon).

In artificial selection, humans are doing the selecting—purposely restricting breeding toindividuals with certain characteristics. In natural selection, the “environment” does theselecting—individuals that survive and reproduce better in a given environment, for whateverreason, are “naturally selected”. The environment includes such influences as competitors, parasites, predators, food supply, climatic conditions, soil nutrients, and many, many others.

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Research Orig ins o f Fast Plants 2

Rapid-cycling Brassica rapa, Fast Plants, were developed by Dr. Paul H. Williams, Professorof Plant Pathology at the University of Wisconsin-Madison. Fast Plants are a product of Dr.Williams’ research to improve the disease resistance of plants in the family Cruciferae, alarge diverse group that includes mustards, radishes, cabbages, and other cole crops. In order

to speed up the genetic research in the crucifers, he began breeding Brassica rapa and sixrelated species from the family Cruciferae for shorter life cycles. The end result was a geneticline of small, prolific, rapid-cycling Brassicas. These plants are now known as “Fast Plants”.

Dr. Williams continued to refine the rapid-cycling Brassicas to have characteristics mostsuitable for laboratory and classroom use. He selected seed from stock that met the followingcriteria:

• Shortest time from planting to flowering

• Ability to produce seeds at high planting density

• Petite plant type

• Rapid seed maturation• Absence of seed dormancy

• Ability to grow under continuous fluorescent lighting

• Ability to grow well in a potting mix

After about 20 years of planting, growing, and selecting, his breeding process had reduced asix-month life cycle to five weeks. Further breeding produced relative uniformity inflowering time, size, and growing conditions and yet the Fast Plants retained much variety.Over 150 genetically controlled traits have been recorded and are useful in research.

These Brassicas have shortened traditional breeding programs and aided in cellular and

molecular plant research. Additionally, Fast Plants have become extremely useful as ateaching tool in the classroom since all aspects of plant growth and development can beeasily demonstrated.

For more information on the development of Fast Plants and other rapid-cycling Brassicas see: Paul H. Williams, Curtis B. Hill, 1986. Rapid-Cycling Populations of Brassica. Science, New Series, vol 232 (4756), 1385-1389.

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Life Cycle of rapid cycl ing Brassica rapa

Under proper growing conditions, the life cycle is 35-45 days from planting a seed toharvesting the next generation, as summarized in Figure 2, Figure 3, and Table 1.

Figure 2. Life Cycle of rapid cycling Brassica rapa

Figure 3 (overleaf): Growth of Fast Plants

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Insert Figure 3 (overleaf): Growth of Fast Plants

Here.

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Table 1. Stages in the Life Cycle of Fast Plants: Concepts of Dependency3

Stage State Condition DependencyA.seed quiescence

(dormant embryo)suspended growth of

embryoindependent of the parent and manycomponents of the environment

B. germinating seed germination awakening of growth dependent on environment and health ofthe individual

C.vegetative growth growth anddevelopment

roots, stems, leaves growrapidly;

plant is sexually immature

dependent on environment

D. immature plant flower buddevelopment

Gametogenesis;

reproductive [male (pollen)and female (egg)] cell

production

dependent on healthy vegetative plant

E. mature plant flowering mating Pollination: attracting orcapturing pollen

dependent on pollen carriers (bees andother insects)

F. mature plant pollen growth gamete maturation;

germination and growth of

pollen tube

dependent on compatibility of pollenwith stigma and style

G.mature plant double fertilization union of gametes;

union of sperm (n) and egg(n) to produce diploid zygote(2n);

union of sperm (n) andfusion nucleus (2n) to

produce endosperm (3n)

dependent on compatibility and healthy plant

H.mature parent plant plus embryo

developing fruit,endosperm, andembryo

Embryogenesis;

growth and development ofendosperm and embryo;

growth of supporting

parental tissue of the fruit(pod)

interdependency among developingembryo, endosperm, developing podand supporting mature parental plant

I. aging parent plant plus maturingembryo

senescence of parent;

maturation of fruit;

seed development

withering of leaves of parent plant;

yellowing pods;

drying embryo;

suspension of embryogrowth;

development of seed coat

seed is becoming independent of the parent

J. dead parent plant plus seed

Death;

Desiccation;

seed quiescence

drying of all plant parts;

dry pods will disperse seeds

seed (embryo) is independent of parent, but is dependent on the pod and the

environment for dispersal

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Days 1-3: Germination and Emergence4

Germination is the awakening of a seed from a resting state. This resting state represents a pause in growth of the embryo. The resumption of growth, or germination, involves theharnessing of energy stored within the seed. Germination requires at least water, oxygen anda suitable temperature.

The radicle (embryonic root) emerges. Seedlings emerge from the soil. Two cotyledons (seedleaves) appear and the hypocotyl (embryonic stem) extends upward. Chlorophyll and

anthocyanin (green and purple pigments, respectively) can be seen. See Figure 4.

Figure 4. Germination and Emergence of Brassica rapa .

Days 4-12: Growth and Development

As a plant germinates and matures, it undergoes the processes of growth and development.Growth arises from the addition of new cells and the increase in their size. Development isthe result of cells differentiating into a diversity of tissues that makes up organs such as roots,

shoots, leaves and flowers. Each of these organs has specialized functions coordinated toenable the individual plant to complete its life cycle.

Days 4-9:

Cotyledons enlarge. True leaves appear and develop. Flower buds appear in the growing tipof the plant. See Figure 5.

Both genetics and the environment play fundamental roles in regulating growth anddevelopment, and determining the phenotype (appearance) of an organism.

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Figure 5. Development of true leaves

Days 10-12:In Fast Plants, growth and development occur rapidly and continuously throughout the lifecycle of the individual. It is most dramatic in the 10 to 12 days between seedling emergence(arrival at the soil level) and the opening of the first flowers.

The stem elongates between the nodes (points of leaf attachment), and flower buds rise abovethe leaves, as shown in Figure 6. Leaves and flower buds continue to enlarge.

Figure 6. Fast Plant, just before flowering.

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Days 13-17: Flowering and Reproduction

Flowering is the initiation of sexual reproduction by the plant. The production of male andfemale gametes (sperm and eggs) within the flower is the first step in preparing for pollination and reproduction. Each part of the flower has a specific role to play in sexualreproduction. The flower dictates the mating strategy of the species. One common strategy

involves employing animals, often insects, to carry pollen (containing male gametes) to the pistil (female reproductive organ). The structure of a Fast Plant flower is shown in Figure 6.

Figure 6. Flower Structure5

Pollination is the process of mating in plants whereby pollen grains developed in the anthersare transferred to the stigma. The pollen grains then germinate, forming pollen tubes thatcarry the sperm from the pollen to the eggs that lie within the pistil.

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Flowers can be cross-pollinated (from one plant to another) for 3-4 days. Pollen is viable for4-5 days and stigmas remain receptive to pollen for 2-3 days after a flower opens. After final pollination, the remaining unopened flower buds and side shoots should be pruned off todirect the plant's energy into developing the seed. See Figure 7.

Figure 7. Reproductively mature plant

Days 18-40: Fertilization to Seeds

Fertilization is the final event in sexual reproduction. In higher plants, two sperm from the pollen grain are involved in fertilization. One fertilizes the egg to produce the zygote and begin the new generation. The other sperm combines with the fusion nucleus to produce thespecial tissue (endosperm) that nourishes the developing embryo. In some plants endospermnourishes the germinating seedling. Fertilization also stimulates the growth of the maternaltissue (seed pod or fruit) supporting the developing seed. See Figure 8.

Within 24 hours of pollination, the sperm has fertilized the egg. Immediately followingfertilization, the pistil and other maternal structures of the flower will grow and changefunctions. Within the ovules, the embryos also grow and differentiate through a series ofdevelopmental stages known collectively as embryogenesis.

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Figure 8. Pod development

Days 18-22: Petals drop from the pollinated flowers. Pods elongate and swell. Development

of the seed and young plant has begun and will continue until approximately Day 36.

Days 23-38: Seeds mature and ripen. Lower leaves yellow and dry. Twenty days after final pollination (about Day 38) plants should be removed from water.

Days 38-45: Plants dry down and pods turn yellow.

On Day 45, pods can be removed from dried plants. Seed can be harvested. The cycle iscomplete.

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Variable Traits in B rassi ca rapaOne can observe several fairly obvious variable traits in a large population of mature FastPlants, such as: arrangement of flowers, total number of flowers, total number of leaves,arrangement of leaves, length of the lower leaf, surface area of the lower leaf, plant height,total number of seeds per plant, total mass of plant, stem length between first and second

leaves, etc. In contrast, there are other traits that don’t appear to vary at all, such as: numberof petals per flower (4); number of sepals (green petal-like structures just below the petals) per flower (4); petal color (bright yellow); and number of cotyledons or “seed-leaves” (2).

When you first have plants to observe, look carefully at them and note the variability you cansee. Remember, all your plants are almost exactly the same age, so differences you see (suchas height, leaf size, etc.) are not due to differences in age. Figure 9 shows a typical Fast Plantat about two weeks old, in which the cotyledons are distinguished from the true leaves.

Figure 9. Fast Plant, about 14 days old.

One variable trait that you might not have noted in your list is “hairiness” of leaves, petioles(leaf stalks) and stems, but if you look more closely (especially with a hand lens and desklamp) you should see these “hairs” on some plants. Technically, plant “hairs” are calledtrichomes, and they have been shown to have a specific function. What do you suppose thisfunction might be?

Fast Plants and their Butt erfly, Pieris rapae 6

Pieris rapae, the ubiquitous cabbage white butterfly, is found across North America in manynatural places and in gardens with cabbage, broccoli, and other crucifers. It hatches as a littlecaterpillar from a tiny egg laid on the surface of a leaf by the mother. It goes through 5 such

larval stages in about 18 days, finally pupating and metamorphosing into an adult, whichemerges from the pupal case about 8 days later. The adult can live up to three weeks, makingthe entire life cycle less than 2 months long.

Read: Jon Agren, Doublas W. Schemske, 1993. The cost of defense against herbivores: Anexperimental study of trichome production in Brassica rapa. American Naturalist , Vol 141(2):338-350, and Jon Agren, Doublas W. Schemske, 1994. Evolution of trichome number ina naturalized population of Brassica rapa, American Naturalist, Vol 143 (1):1-13 for moreinformation on the relationship between Pieris and Brassica.

Figure 10 (overleaf). Life cycle of Brassica rapa and Pieris rapae .

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Paste part 1 of Figure 10 (overleaf). Life cycle of Brassica rapa and Pieris rapae . Here.

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Paste part 2 of Figure 10 (overleaf). Life cycle of Brassica rapa and Pieris rapae . Here.

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Our Experiment

Sow ing Fast Plant Seed

• Label your pots, following the example of the first one.

• Put a MiraCloth wick in each pot. Make sure the pointed end comes out through thehole in the bottom of the pot, and the top extends about 1 mm above the top of the pot

• Fill each pot halfway with moist soil mix.

• Put a fertilizer pellet on the soil in each pot.

• Add more soil, up to the bottom of the pot lip.

• Position the wicks so they extend out of the soil by about 1 mm.

• Use the butt end of a dissecting needle to make a little depression in the soil for theseed.

• Touch a toothpick to a glue stick, then pick up one or a few seeds with the sticky end.

• Carefully place one seed in the little depression, flicking it off the toothpick with thedissecting needle.

• Sprinkle vermiculite over the seed to cover it.

• Put water into each pot with a plastic transfer pipet, until it drips out of the bottom.

• Decide on a watering schedule, to ensure that the soil is kept moist.

• Take the pots upstairs to the environmental chamber.

• Conscientiously follow the watering schedule over the next week, taking care not totip over any pots.

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Counting Hairs

As you have probably guessed, the variable trait that you are going to attempt to alter in thislineage is “hairiness.” In this “wild type” population, some plants should be noticeably hairy,many are slightly hairy, while others are apparently hairless. But such an observation is toogeneral, and needs further quantification.

To accomplish this, you could count all hairs on all parts of each plant, but this would be atime-consuming task, and an unnecessary one. It turns out that hairiness of one part of a plant (such as the leaf surface) is strongly correlated with hairiness on other parts (such as theleaf margin). In other words, a plant’s hairiness in general can be quantified by assessinghairiness of a specific structure.

The structure we will use for this “hairiness index” is the petiole, or leafstalk, wheretrichomes are large, conspicuous and rather easily counted, and the structure is relativelysmall with a defined starting and ending point.

For consistency, we will use the petiole of the first (lowermost) true leaf, and we will definethe limits of the petiole as follows: from its junction with the main stem (usually marked by asmall bulge or ridge, often differently colored)to its junction with the lowermost leaf vein(see Figure 11).

Figure 11. The limits of the petiole

An important note: the two lowermost squarish, two-lobed and rather thick leaves areactually cotyledons (“seed leaves”), not true leaves. The first true leaf is just above these twocotyledons. What you need to do now is to count the total number of trichomes on the petiole of the first true leaf of each plant in your sample. Your data will then be combined

with data from the rest of the section.

Use the hand lens and desk lamp; the trichomes are most conspicuous if strongly illuminatedagainst a dark background, such as the black table top. If present, the trichomes willgenerally be concentrated on the lower side of the petiole, but some could occur on the sidesor top as well. Be sure to rotate the plant so that you can see all aspects of its petiole. Counta second time for verification, then record your data in your data sheet.

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Reliability of hair counts

The number of hairs we count in this experiment may vary from one observer to another(inter-observer reliability), or between counts performed by the same person (intra-observerreliability). So we can never be entirely certain about the number of hairs on a given plant.Take some time to think about why the count includes this inherent variability. These kinds

of uncertainties are often called “error,” but that is not meant to imply mistakes on the part ofthe experimenter (or the equipment manufacturer).

Making multiple measurements is a way of acknowledging the uncertainty in each one, andreducing the risk of letting a single odd measurement carry too much weight. We thenrepresent our result as an average of the many repeated measurements we took. But thisglosses over the variation among measurements. We don’t have time to count each plant adozen times in this lab period, so we will compromise as follows:

• To improve the accuracy of our counting, the hairs on each plant will be counted by twostudents, and the counters will collaborate (or enlist the aid of a third counter) to come to

a fair estimate of the actual number of hairs.

• Make a data table for yourself, in which you record the plant number and number ofhairs. After you have adjudicated the number of hairs with your counting partners, enteryour data in the official data sheet.

• Enter your data in the spread sheet on the computer.

Compiling the data

When all groups have finished assessing trichome number of individual plants, your labinstructor will coordinate the effort of combining the separate data into a single data table.

After tabulating the data from all sections, your instructor will make this summary availableto you.

Select ing th e parents o f the next (second) generat ion

Only a small fraction (about 10%) of this population of plants will be selected to be parentsof the next generation. These won’t be randomly chosen, but instead will be the hairiest 10%of plants in the population, and the 10% least hairy.

Sort the data table by number of hairs. Find the top 10% in hairiness. Print out or writedown their ID numbers.

If more than 10% of the population has no hairs, then use a randomizing technique to choosethe 10% to be parents. Your instructor may have created a column in the spread sheet withrandom numbers in it; pick the 10% of the population that is bald and has the highestnumbers in this column. (You may also pick numbers out of a hat or toss dice or coins to pick the winners in this selection process.)

Indicate on the data sheet which plants have been chosen as breeders for which group.

Separate the chosen plants into hairy parents and bald parents, and dump the contents of theremaining pots into the compost pile. From now on the two sets of parents must be keptstrictly separate.

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Poll inat ion of selected plants

The plants you selected last week to be parents of the second generation should be in heavyflower. Today, you will assist these plants in the process of sexual reproduction. Brassica

rapa plants need assistance because in nature they are totally dependent on certain insects fortransferring sperm-bearing pollen from the male part of the flowers of one plant to the female

part of the flowers of another plant. In these plants, the most conspicuous floral structuresare the yellow petals. These petals enclose the male sexual structures (stamens, terminatingin the pollen-producing anthers) and female structures (pistil, with the pollen-receivingstigma, style and egg-producing ovary), as shown in Figure 6 and Figure 7. Although eachflower has both male and female parts, sperm from one plant are incapable of successfullyfertilizing eggs of the same plant. This self-incompatibility ensures out-crossing (mating between different individuals).

The insect pollinators don’t act as pollen couriers just to be nice; the plants lure and rewardthem with nutrients (nectar and edible pollen), and the insects inadvertently pick up thesticky pollen on various body parts and then carry it with them to the next flower (Figure 12).

Honeybees are common pollinators of Brassicas in the wild. We will use artificial “bees”(little pollinator wands) to help us pollinate these plants.

Figure 12. Honeybee (the usual pollinator of Brassica )

ü Designate sides of the room for hairy and bald plants. Do not mix plants or

pollinating wands! No student should pollinate more than one type!

Each student group should obtain one or two of the selected mature plants. The cross- pollination activity will be accomplished over the course of the week, so that even early- andlate-bloomers can mate with each other. The object is to transfer pollen from each plant toevery other plant. This can be done in the following way. Using a single pollinating wand,

one group will lightly rub and twirl the bristly end for several seconds onto the anthers andstigma of each open flower on their plant(s). Then pass the wand, now loaded with yellow pollen, to the next group, who will repeat the process. After the bee stick has made onecomplete round (all open flowers on all plants have been “visited” by the wand), make asecond round in the same manner. When finished, insert the wand into the soil (pointed enddown, “bee” end up) of one of the pollinated plants—don’t return the used wand to itsoriginal container, where other students might confuse it with fresh, unused ones. Finally,return all plants to the watering tray.

Fertilization will result in the development of the ovules (each containing an embryo) intomature seeds, which will be contained in the fruit or seed pod (the elongated ovary). The

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length of time from fertilization to mature seed is about 3-4 weeks. We don’t need to doanything more with these plants until it is time to harvest the seeds except to remove themfrom water in 3 weeks, which hastens the seed-maturation process. Next week, you shouldeasily be able to see the elongating fruits. In two weeks, they will have become even longer(2-5 cm) and the individual seeds inside (perhaps 5-20 per pod) will have become visible. In

three weeks, the maturing process is nearly complete. You will plant these seeds (generation2) in about 4 weeks.

Harvest ing seeds

The Fast Plants that you selected from the initial large population and pollinated should havemature seeds ready to be harvested. Each student group should take one or two of the plantsto your table. Remove several pods, then break open the pods into a small plastic dish,releasing the seeds. You should be able to retrieve 5-20 or so seeds from each pod. The plant from which you removed the pod is the mother of each of those seeds; the father couldhave been one or more of any of the other plants in the selected group. Similarly, your plantcould have been the father of seeds from any of the other plants in the selected group. Our

mass cross-pollination technique ensured that all individuals in the selected group had anequal opportunity to be parents.

Establ ishing the second generat ion

The goal is to plant and grow generation 2 seeds in order to assess the average hairiness ofthis new generation (the offspring of the selected generation 1 parents). Each student will beresponsible for planting a small number of seeds. (This time we will plant 2 seeds per pot,since germination rates tend to be lower in classroom-harvested seeds than seeds we get fromthe supplier.) Your lab instructor will give you further directions.

After planting, your section’s flat will be placed under the lights in the lab room, and

maintained by the staff. By next week’s lab, most of the seeds will have germinated. In twoweeks they will be ready for the final analysis of the experiment.

Counting the hairs in the second generat ion

Follow the counting procedure from the first generation, keeping separate records for theoffspring of the hairy parents and the offspring of the bald parents.

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Data Analysis

Inheritance Patterns

Is hairiness genetic?

Allele dominance and frequencyAn allele is dominant if it takes only one to express a trait. Dominant alleles are generallythose that do something: specify a working version of an enzyme, an antigen, a hormone, atranscription factor, a receptor, etc. Recessive alleles, on the other hand, are generally thosethat represent a loss-of-function. That is, they do not specify a working version of the proteinin question. It seems more likely that alleles that result in production of trichomes aredominant.

Allele frequency depends not on dominance, but on current prevalence and usefulness.Alleles that reduce the reproductive capability of their bearers tend to become less common,while those that enhance reproductive capability tend to become more common.

In a large population with random mating and no slection, allele frequencies do not change.Allele frequencies change when there is selection for one allele over the other.

When you read The cost of defense against herbivores, by Agren and Schemske, you should be thinking about how herbivores act as agents of selection on their food plants. Ifherbivorous insects prefer to eat smooth stemmed plants, then the smooth stemmed plantswill, on average, produce fewer offspring. Therefore, if hairiness is a heritable trait, thefrequency of alleles for hairiness should increase in the population, and the frequency ofalleles for smooth stems should decline*.

If there are no predatory insects to maim or kill the smooth-stemmed plants, then the smooth-

stemmed plants may actually have an advantage – they can spend their energy on making babies instead of making trichomes. In this case, the allele for smooth stems should increasein frequency in the population.

For neither scenario (selection against smooth plants by herbivorous insects or selection for smooth plants in the absence of herbivores) does it matter which allele is dominant.Whichever allele is most useful under the current conditions will become more common inthe population.

We have acted as very severe agents of selection in our population of plants. We havechosen two subsets of the original heterogeneous population to be parents, and have killed all

the rest! We reduced the reproductive capability of the non-chosen plants all the way tozero! In one subset, only smooth-stemmed plants were allowed to breed. In the other subset,only those in the hairiest 10% or so of the population were allowed to breed.

* Of course, the plants also act as agents of selection on the insects. If all the plants are hairy, the individualinsects that cannot abide trichomes do not breed as much (because they don’t get enough to eat) as those thatfind a way to eat hairy plants. Allele frequencies will change in the insects, producing a population morewilling to eat hairy plants. This will likely result in further selection by the insects against the relatively lesshairy plants, and an arms race of sorts will ensue.

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If hairiness is an inherited trait, then we should be able to create two populations by thistechnique, one in which the hairy allele(s) is(are) common, and one in which the smoothallele(s) is(are) common. No matter which allele is dominant, we can change its frequency inthe population. This should further reinforce the idea that either the dominant or therecessive allele can be more common in the population, depending on the circumstances.

Realized heritabilitySelection can act on any phenotypic variation, but can only cause evolutionary change if thevariation is genetic.

Population biologists often use an index called realized heritability, h2, to quantify the degreeto which a trait in a population can be pushed by selection. To calculate this index, we findthe response to selection by subtracting the average of the second generation from that of theentire first generation. We also find the selection differential by subtracting the average ofthe selected parents from that of the entire first generation. Realized heritability is theresponse divided by the differential.

al differenti selection

selectiontoresponsetyheritabilirealized =

parents selected avg genavg

genavg genavg tyheritabilirealized

st

nd st

−

−=

1

21

A low h2 (for example, less than 0.01) occurs when the offspring of the selected parentsdiffer little from the original population, in spite of a big difference between the populationas a whole and the selected parents.

A high h2

(for example, greater than 0.6) occurs when the offspring of the selected parentsdiffer from the original population almost as much as the selected parents do.

Is hairiness controlled by a single gene?

Complete dominanceOne of the first questions we might ask of our results is whether there is a single “on/off”type gene for hairiness. To answer such a question, we need to predict what the results of ourexperiment might be if that were the case. So here is a hypothetical inheritance pattern forhairiness in Fast Plants:

Suppose there is a single gene which determines whether or not trichomes are produced.

Suppose that there are exactly two alleles of this gene, “H”, which directs the production ofhairs, and “h”, which does not.

Since every plant gets one of these alleles from its mother and one from its father, each has a pair of alleles, and there are three possible pairs of alleles, or genotypes : HH, Hh, and hh.

Since, in our hypothetical situation, “H” allows hairs to be made, any plant with at least one“H” will have trichomes, that is, plants with either the genotype HH or Hh will produce hairsand plants with the genotype hh will not. The appearance of the plant with respect to thegenes in question is called its phenotype .

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According to this scheme, we cannot tell the genotype of a hairy plant by looking at it, but allthe hairless plants have the genotype hh. So when two hairless plants are mated, they each put one “h” into their respective gametes, and their offspring would all be hh, as well.

On the other hand, some of the hairy plants might have the genotype HH and some, Hh, sothere are three possible matings: HH x HH, HH x Hh, Hh x Hh. It is possible in this thirdmating for an offspring to be produced with the genotype hh, making it bald. It is left as anexercise for the reader to figure out the probabilities of the various genotypes in the offspringof these matings.

We can examine our data to find out whether all hairless parents had hairless offspring, andsome hairy parents had hairless offspring and thus answer the question of the presence of asingle on/off gene.

We can also examine our data for the converse: is there a single on/off gene whose dominantallele prevents trichome production? Again, the reader is left to construct the relevantdiagrams and use the data to answer this question.

Incomplete dominanceSuppose the hairiness gene we’ve been discussing isn’t a simple on/off switch, but amodulator of some sort. Suppose there is a “T” gene (for trichome), which influences thenumber of trichomes, rather than the mere presence or absence of them. Suppose that thereare still two alleles, “T”, and “t”, but that “T” is incompletely dominant to “t”. This meansthat the heterozygote, Tt, would have a phenotype (a number of hairs) intermediate betweenthat of the two homozygotes, TT and tt. Plants with the genotype TT would have many hairs,those with the genotype Tt would have some hairs, and those with the genotype tt wouldhave very few hairs. Each “T” can be thought of as contributing one “phenotypic unit”,which in this case would be some number of hairs. This is illustrated in Table 2.

Table 2. Genotypic and phenotypic categories for a single-

gene, incomplete dominance model of trichome number

inheritance.

# of “phenotypic units” 2 1 0

phenotype HairiestIntermediately

hairy smoothestGenotype TT Tt tt

Matings between similar homozygous parents (TT x TT, or tt x tt) should produce onlyoffspring just like the parents. Matings in which at least one of the parents is heterozygous(Tt) would produce offspring with greater variation. Calculating the results of such matingsis left as an exercise for the reader.

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Cross Results:

T t Genotype:

TPhenotype:

t Proportion (fraction) ofoffspring of a Tt x Tt cross:

We can examine our data for evidence that a single gene with incomplete dominance controlstrichome number in our plants. Are there offspring of two very hairy parents that have asfew hairs as the least hairy parents? Are there offspring of two bald parents that have asmany hairs as one of the hairy parents?

Is hairiness controlled by multiple genes?

Hairiness in Fast Plants most likely differs from these simple traits in that there are not just

two or three phenotypes, but a whole range (from zero to “many”). Explaining theinheritance of such traits involves a more complicated model.

Traits such as body size or skin pigmentation in humans, or milk production in cattle, forwhich there exists a continuum of phenotypes between two extreme values, are known togeneticists as quantitative (or continuous) traits. Traits such as petiole trichome number inFast Plants, number of bristles on fruit fly abdomens, or number of eggs laid by house flyfemales, are in a related category called meristic traits. These often exhibit wide variabilityamong individuals, but the variability is incremental, not continuous (e.g., an individualmight have 5 or 6 hairs, but not 6.3 hairs).

Despite the difference between these two kinds of traits, geneticists feel their mechanism ofinheritance is similar. Such traits are very important—most traits in most organisms arequantitative or meristic, and relatively few are simple two-phenotype traits such as exhibited by Mendel’s peas (red/white flowers) or the Macintosh rabbits (straight/floppy ears).Quantitative and meristic traits are also often called polygenic traits, because the model that best explains their inheritance involves multiple gene loci, with two or more alleles at eachlocus.

Two loci. Let’s imagine a situation in which there two genes contribute to the hairiness ofthe petiole in Fast Plants. As in the previous example, each locus has two alleles (“A” and“a” for one locus; “B” and “b” for the other), and each “uppercase” allele contributes one“phenotypic unit”. Thus, an individual with genotype AABB would exhibit a phenotype of 4

units; individuals with genotype AABb or AaBB would each have a phenotype of 3 units;etc., as shown in Table 3.

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Table 3. Genotypic and phenotypic categories for a 2-gene model of trichome number

inheritance.

Number of “phenotypicunits”

4 3 2 1 0

phenotypes

Very many

hairs

Many

hairs

Some

hairs

A few

hairs

Almost no

hairs

Possible genotypes AABBAABbAaBB

AAbbAaBbaaBB

AabbaaBb

aabb

As before, when both parents have only the dominant alleles (AABB x AABB) or only therecessive alleles (aabb x aabb), the offspring will have the same genotypes as their parents.

Suppose, however, that our hairiest parents were in category 3, that is, with genotypes AABband AaBB. There are three possible matings in this population: AABb x AABb, AABb xAaBB, and AaBB x AaBB. What are the possible genotypes and phenotypes in the offspringof these matings? What are the highest and lowest phenotypic categories that might result

from matings between parents with category 3 phenotype?

We will have to consider whether our data are consistent with a 2-gene system such as this.

Three loci. If there were 3 genes, “A”, “B”, and “C”, there would be 7 phenotypiccategories. The reader should fill in Table 4 like that for two loci, showing how manydifferent genotypes would there would be for each category.

Table 4. Genotypic and phenotypic categories for a 3-gene model of trichome number

inheritance.

# of“phenotypic

units” 6 5 4 3 2 1 0

phenotype Hairiest smoothest

Possiblegenotype(s)

The more genes involved in adding to the number of trichomes, the more phenotypiccategories there are, and the more closely the theoretical distribution can match the realdistribution, which after all, had 24 different numbers of trichomes on their petioles. Anotherfactor is how many trichomes on average are added by each dominant allele. Biologicalthings being the way they are, we should expect a rather loose correlation between thenumber of dominant alleles and the number of trichomes, as opposed to a precise one-to-onecorrespondence.

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What happens in the second generation depends on the nature of the inheritance of trichomenumber in fast plants. What if, for example, the hairiest individuals in our first generationdid not happen to have all possible trichome-generating alleles? (I.e., What if our hairiestindividuals were in fact heterozygous for one or more trichome genes?) How would thataffect the possible range of trichome number in their offspring?

What if there is more than one genotype that results in a smooth petiole, so that some of thesmooth individuals were heterozygous? What might we see in their offspring?

Analyzing the data to describe the variat ion in the pop ulat ion

In addition to the variation in counting, of course, there is variation between plants. Our goalin this semester long experiment will be to see how much of this variation can be accountedfor by genetics. We will compare the hairiness of the original population with the nextgeneration, after selection. We need measures not only of hairiness, but also of variation inhairiness.

The mean (average)Calculating the average number of hairs per plant is one way to indicate hairiness. Supposethe 6 plants planted by one student had the following number of hairs (arranged indescending order): 15, 13, 10, 8, 5, 0. The average number of hairs is 8.5, calculated asfollows:

5.86

51

6

058101315==

+++++=average

The Excel file has been set up to calculate this automatically as the data are entered, sinceadding up hundreds of numbers is tedious. You can use the function wizard (f x ) to calculate

the average: pick a cell to contain the average, click on f x , choose AVERAGE, highlight thecolumn containing the number of hairs, and click OK.

Frequency distribution

One way to look at the hairiness in the population is to construct a histogram, or bar graph, toillustrate the variation. To do this, we need to lump some of our data together in categories.A convenient starting point would be to count how many plants have between 0 and 4 hairs,how many between 5 and 9, how many between 10 and 14, etc. This effectively createshairiness categories.

The student at the computer highlights just those cells whose entries range from 0 through 4,

and counts them. If she keeps her finger down on the mouse, Excel will tell her, in the upperleft corner of the spread sheet, how many rows she has highlighted. If she lets her finger up,she has to count them by hand.

A student at the board constructs a table showing the frequency distribution. Table 5 is sucha table for our 6 plant data given in the section on averages, above.

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Table 5. Frequency distribution of number of

hairs on the first petiole of 6 week-old plants

number of hairs number of plants0-4 15-9 2

10-14 215-19 1

Each student should construct a histogram illustrating the frequency distribution of hairs onall the plants. The histogram illustrating the data in Table 5 is given in Figure 13. Figure 14illustrates the distribution of trichome number in the first generation of Fast Plants grown inspring 2001 for an experiment similar to ours. The height of each bar represents the numberof plants whose number of hairs fell into the category given below the bar. Talk with yourinstructor about learning how to construct such a histogram with Excel.

0

1

2

3

0 to 4 5 to 9 10 to 14 15 to 19number of trichomes per petiole

n u m b e r o f p l a n t s

0

100

200

300

400

500

600

20 to 2415 to1910 to 145 to 90 to 4number of trichomes per petiole

# o f f i r s t g e n e r a t i o n p l a n t s

unselected

selected smooth

selected hairy

Figure 13. Distribution of number of

hairs per petiole

Figure 14. Frequency distribution of

trichome number in first generation Fast

Plants, Spring 2001.

The standard deviation

One common method of expressing the variation within a population is to calculate one ofseveral conventional indices of uncertainty. An especially common such index is thestandard deviation. It is a statistical measure of the precision in a series of repetitivemeasurements. It is calculated this way:

1)(

2

−

−= ∑

n

x x sd i

Wheresd is standard deviation,n is the number of data,xi is each individual measurement, ¯ x is the mean of all measurements, and

Σ means the sum of

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The standard deviation is something like the average deviation from the mean. We subtracteach measurement from the mean of the measurements. Since some of these are positive andsome are negative, and will cancel each other out when we add them together, we squareeach deviation to make it positive. Then we add all these squares together, and divide by thenumber of measurements (actually, we fudge it here, using n-1 instead of n, because this

gives a better result—sorry) to get a sort of average. Finally, we correct for having squaredthe deviations by taking the square root of the whole thing.

The standard deviation has become popular because of an interesting feature. For a largesample, two thirds of the values can be found within one standard deviation of the mean,95% within two standard deviations, and 99% within three.

In the example above, the standard deviation would be 5.5, calculated as follows:

16

)5.80()5.85()5.88()5.810()5.813()5.815( 222222

−

−+−+−+−+−+−= sd

5

)5.8()5.3()5.0(5.15.45.6 222222−+−+−+++

=

47.59.295

5.149

5

25.7225.1225.025.225.2025.42===

+++++=

A greater spread among my values would give a higher standard deviation; a smaller spread,a lower one. Table 6 illustrates this: the data set from the example above is listed betweentwo other data sets, each with the same average and different standard deviations.

Table 6. Three data sets with the same average and different standard deviations.

raw data average standard deviation

12, 11, 9, 8, 7, 4 8.5 2.88

15, 13, 10, 8, 5, 0 8.5 5.47

19, 17, 11, 3, 1, 0 8.5 8.34

It is a good thing we have computers to do the calculations for us, but for the answer to have

meaning, we need to know how it is done.

The standard deviation is often used in histograms, to illustrate the spread of values in thedata, as well as to help compare one group of data with another.

Figure 15 llustrates the use of means (the bars) and standard deviations (the lines) to comparethe number of trichomes in the various groups of Fast Plants from a previous semester’sexperiment. The heights of the bars tell us that the two sets of offspring were different fromthe first generation of plants, and the standard deviation marks tell us that there was morevariation among the offspring than among the selected parents.

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0

5

10

15

20

25

1st generation selected hairy selectedsmooth

2nd hairy 2nd smooth

n u m b e r o f t r i c h o m e s

( m e a n + S D )

Figure 15. Comparison of number of trichomes on the petiole of the first true

leaf of: all plants in the first generation, the selected hairy parents, the selected

smooth parents, and the 2 second generations.

Thinking about variat ion

Think about what would be expected to change about the variation in hair number from thefirst to the second generation, after selection. Consider not only the mean number of hairs, but also the standard deviation. Will overall variability stay the same? Will the averagechange?

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Writing the report

General Guidelines

A research report, in communicating to your peers, should accomplish several things. Inaddition to telling the story of what you did and what you found, it should also justify yourwork. By this I mean that it should provide enough background information to provideappropriate context, connect your findings to a larger body of knowledge, and make thequestion seem intrinsically interesting or practically valuable. It should argue persuasivelyfor your conclusions. And it should do all this in the conventional style of scientificcommunication.

As with other expository writing, a scientific report has an introduction, a middle, and aconclusion. First you set the stage and try to get the reader’s interest, then you explain whatyou did and what you found, then you sum up the significance of your results, returning tothe context established at the beginning. Write in your own voice; don’t aim for anartificially stilted, formal style, but avoid slang. Use the active voice when possible (e.g., We

randomized the order of presentation, as opposed to The stimuli were presented randomly).

Be as explicit as possible. For example, when you say something like “increased hairiness inthe second generation”, be sure it is clear to the reader whether you mean more hairs per plant, or more plants with hairs. Also, distinguish between “gene”, “allele”, and “trait”.

Scientific names

The genus name for an organism is a proper noun, so it is capitalized. The species name isan adjective, and is not capitalized. Both are Latin, a foreign language, so the entire name isitalicized, like this:

Homo sapiens Brassica rapa

Pieris rapae.

Because it is a proper name, it usually doesn’t act as an adjective. Say “ Brassica rapa has been studied”, or “plants of Brassica rapa have been studied”, rather than “the Brassica rapa plants have been studied.” [This last sounds rather like “the Jane Smith person”.]

The second time you refer to a species, you may abbreviate its name by using the first initialof the genus: B. rapa. [Obviously this only works when the name is unambiguous. In a paper about human gut organisms, for example, you’d have to distinguish between

Eschericia coli (a bacterium) and Entamoeba coli (an ameba).]

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The Conventio nal Form at

Introduction

In this section, you present your background material and establish the context for your work.Review previous work on this subject. Try to give enough background material so that your project can be seen as part of a larger endeavor. What common knowledge or scientific fact

or particular observation led you to ask the question your experiment was designed toanswer? What does the reader need to know to follow your reasoning? E.g.,

In “Basic Color Terms”, Berlin and Kay (1999) give the terms white, black,red, green, yellow, blue, brown, purple, pink, orange, and grey as the basic

color terms of English. …

In fall 1999, the Unity of Science class asked students to mark on the visible

light spectrum the location of the “best” red, orange, yellow, green, and blue.

There was very poor agreement in the placement of the “best” orange.

After setting the stage, tell the reader what you were trying to find out (not what you weretrying to prove), and how you came to that question from the background information. E.g.,

From the color naming data collected in class, we know that subjects place

light with a wavelength 580 nm in the red, the orange, or the yellow, so we

decided to ask our subjects to mark the red/orange and orange/yellowboundaries, as well as the “best” red, orange, and red. We hoped to compare

the boundary results with the “best color” results.

Don’t give away the punch line, but do tell whether you had a particular expectation, andwhy. E.g.,

We expected subjects to put the “best” color about halfway between its

boundaries with the neighboring colors, even when asked to do so out oforder. Further, we thought that there would be as much disagreement as to

the boundaries as there had been to the “best” colors.

Methods

What did you do and how did you do it? This section should be written as a description ofwhat you did, not a recipe or set of instructions. (A recipe sounds like this: Tell the subject

to sit in front of the color naming spectrometer, cover her head with black cloth, and look in.

Set the wavelength to 580 nm, and ask her to name the color . A report sounds like this: The

subject sat in front of the color naming spectrometer, covered her head with black cloth, andlooked at the spectrum inside. We set the wavelength to 580 nm, and asked her to name the

color.)

Be sure to give the reader information that cannot be obtained otherwise, e.g., whichtrichomes you counted. You can leave out some organizational or administrative details(such as the organization of your data sheet or the numbering of pots), unless you think this bears directly on the data or your interpretation.

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Results

What happened in your experiment? Write about what you observed. Refer to each of yourfigures and tables by their numbers in writing (because if you don’t, they are not really partof your paper). Summarize the main features of your results, and point the reader, in writing,to the appropriate table or figure for the details. Don’t interpret in this section. For example,

Although there was some variation, all of the subjects placed the “bestorange” somewhere between the orange/red and yellow/orange boundaries.

Table 1 summarizes our results.

or

As shown in Figure 1, subjects who placed the orange/red boundary farthestto toward the red end of the spectrum also placed the “best orange” farthest

toward the red.

FiguresA drawing, graph, diagram, or photograph (in short, any kind of picture) is a Figure. Put its

label, number, and title (which should be as explicit as you can make it) beneath it, as shownin this lab manual. (See, for example, Figure 1 on page 1, or Figure 15 on page 29.) Refer toit as Figure __ at the appropriate place in your written account.

Color is used primarily to distinguish between two or more sets of data. For example, to puttwo sets of data (e.g., hairiness of the entire first generation and that of the two sets ofselected parents in Figure 14) on the same graph in order to show the differences betweenthem, you must give the reader an easy way to distinguish between them. (Shades of gray orhatch marks may stand in for color in a black-and-white printed sheet.)

You can copy and paste an Excel “chart” into a Word file, as I have done in this manual.

Then you can place your illustration wherever you think it best fits into your paper.

Excel doesn’t like to put titles below the graph, so I usually type them with Word. Thisallows me to use Word’s automatic numbering capability. (Insert/caption lets you put infigure and table numbers; Insert/cross-reference lets you refer to them later.) If you use thisautomatic numbering, you can change the order of your figures and tables as you edit your paper and they will renumber themselves, as well as all references to them. (You may haveto save, close, and reopen the file to see the renumbering occur, however.)

TablesA list of numbers or words is a table , and gets numbered separately from the figures. Tablelabels and titles go above the table, as illustrated in throughout this manual. (See forexample, Table 5on page 27, or Table 6, on page 28). Refer to tables by number as you dofigures.

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NumbersReading a long list of numbers isn’t very interesting, so you need to come up with a way to point out the salient features of a graph or table without telling every single value. Thinkabout what you find striking or instructive about the data, and point it out.

Avoid unnecessary decimal places. For example, it seems more efficient (and moreemphatic) to say that none of the plants selected for smooth petioles had any trichomes thanto say they had an average of 0.00 trichomes. (Since you count hairs, you should be usingwhole numbers in any case.)

If you weigh it, or measure its volume, you have an amount ; if you count them, you have anumber , e.g., an amount of dirt; a number of trichomes.

Discussion

This is where you interpret and explain your results. What do you think your results mean?Are they as expected? Do they make sense? Are they interpretable? If not, why not?(Don’t fall back on “experimental error.” This explains nothing.) Be explicit, e.g.,

The variability from one subject to the next suggests that naming of colors is

imperfect, at best. We think this variability is “real,” that is, it results fromdifferences between subjects rather than from inconsistencies in the

measuring device, since no subject placed the “best orange” outside the

boundaries she set at a separate time between orange and red or yellow.

or

The spectrum, as viewed in our device, was slightly skewed, and although wehad told subjects to use the top of the indicator line, perhaps getting better

alignment between indicator and spectrum would reduce some of the

variability from person to person.

References

It is important to give credit appropriately to the authors whose work you refer to (or borrowfrom, or quote, or criticize, or otherwise make use of). This includes material you find on theInternet as well as published articles in the scientific literature. Web citations should includenot only the URL, but also a title and as much as you can figure out as to the author orsponsoring organization. The American Psychological Association publishes the APA StyleGuide at http://www.apastyle.org/elecref.html; this is a useful resource.

There are several citation formats in use in the scientific literature. One of the clearest and

most widespread is to put embedded references in the text and complete citations inalphabetical order at the end of the paper. The embedded references look like this:

Williams and Hill (1986) describe the development of Fast Plants.

or

Fast Plants were bred for both research and teaching purposes (Williams and

Hill, 1986).

If there are more than 2 authors, you can abbreviate the citation like this: Jones, et al.

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Page numbers are usually omitted when you cite the article in the body of your paper, because most scientific articles are short. The complete citation at the end of the paper lookslike this:

Author, initial, year. Article title. Journal name, Volume (#): pages.

Williams, P.H., Hill, C.B., 1986. Rapid-Cycling Populations of Brassica. Science, New Series, 232 (4756):1385-1389.

It’s not necessary to summarize your references one at a time. It might be more concise (andmore useful to the reader) if you combined the results by topic, something like this:

Although the number of trichomes was not correlated with the total number of

flowers or seeds, plants with more trichomes flowered later than those with fewer (Agren & Schemske, 1994, 1995).

1 All references are from the Wisconsin Fast Plants web site: http://www.fastplants.org., retrieved 1/23/02.

The lab exercise is adapted from: Evolution By Artificial Selection: A 9-Week Classroom Investigationusing Rapid-Cycling Brassica rapa, Bruce A. Fall, Steve Fifield, and Mark Decker, University of Minnesota,Minneapolis, MN 55455: http://www.fastplants.org/TeachingMaterials/Labs/FALL.pdf

2 Adapted from Research Origins of Fast Plants, http://www.fastplants.org/Introduction/Science.htm

3 Life stages material (Table 1, and Figure 10) adapted from Fast Plants Life Cycle: Seed to Seed in 35 Days,http://www.fastplants.org/PDFs/PDF's%20for%20WFPID's/Growth%20and%20Development/Fast%20Plants%20Life%20Cycle.pdf

4 Day-by-day developmental material adapted from Fast Plants Life Cyclehttp://www.fastplants.org/Introduction/LifeCycle.htm

5 Photograph from View a Fast Plant, http://www.fastplants.org/Introduction/ViewFP.htm, diagram from Fall

et al. (note 1).6 Pieris material adapted from The Fast Plant and its Butterfly: Investigating the Life Cycles of Wisconsin

Fast Plants and the Brassica Butterfly, Wisconsin Fast Plants Notes, 2001, Volume 14 (1)http://www.fastplants.org/PDFs/2001_Newsletter.pdf

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Fast Plants 2002