Biology 32: Evolutionary Biology Artificial selection, 2010

The power of such artificial selection is obvious, from Chihuahuas to Great Danes, all domestic

dog varieties trace their ancestry back to a common wild ancestor, the wolf (Canis lupus). One example that particularly fascinated Darwin was that of domestic pigeon varieties (e.g., tumblers, fantails, carriers, pouters, etc.), all of which descended from wild rock doves (Columba livia) over a period of ca. 5000 years.

Likewise, there are many examples with plants, including those species that humans have bred for food and beauty. One plant group, especially important to humans is Brassica, a genus in the mustard plant family (Brassicaceae). Several important crop species have been developed from Brassica, including turnips and cabbage (from B. rapa) and brussels sprouts, broccoli, cauliflower, and kale (from B. oleracea).

That plant and animal breeders have been able to dramatically change the appearance of various lineages of organisms in a relatively short period of time is an important fact. Charles Darwin noticed this and devoted the entire first chapter of The Origin of Species to the topic of artificial selection by humans. He used many examples of selection by humans to help support his argument that the mechanism of evolution in populations was natural selection.

Studies on a variety of organisms have shown that many variable traits respond to artificial selection. That is, it is usually possible to increase (or decrease) the frequency or average value of a trait in a lineage through careful selective breeding. Starting today you will attempt to accomplish this same goal.

Natural & artificial selection

Natural selection is a deceptively simple concept, relatively easy to understand at a basic level, but with profound implications that are intellectually challenging. This exercise in artificial selection will serve as an introduction to natural selection, and enable you to better understand this incredibly important evolutionary force.

Natural selection occurs when certain variants within a population of organisms experience greater reproductive success (i.e., leave more offspring) than do other variants. Generally, within a population of organisms, there is variation among individuals for any number of traits (some more obvious than others). Some of this variability is the result of genetic differences among individuals, while some is the result of different environmental conditions. Natural selection is concerned only with variability among individuals that has a genetic basis and can be passed from parent to offspring. If individuals with certain heritable characteristics have an advantage over those with other characteristics, then it follows that the next generation will be made up of offspring from those individuals with the favored characteristics.

Artificial selection is this same process with one important difference; the favored traits are those that (for one reason or another) are preferred by humans, rather than enhance the organism’s ability to leave offspring. For example, running speed in a thoroughbred horse, petal size or number in roses, egg size in chickens, pesticide resistance in crops, etc. If such traits vary in populations and if they have a genetic basis, then breeders can select the parents of the next generation, and over generations, substantial differences can be achieved between the original population and the artificially selected lines. Recall that evolution by natural selection is not goal-

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oriented, whereas the same cannot be said for artificial selection. What about the rate of evolution – would you predict it to be faster, slower, or equivalent under natural versus artificial selection?

Artificial selection using rapid cycling Brassica

Normally, selection occurs too slowly to directly observe in the course of a semester. However, in an effort to study the genetic basis for the diversity of forms found in Brassica and to incorporate more efficiently traits of economic importance (e.g., disease and pest resistance), rapid cycling Brassica plants were developed to facilitate working with these species in the laboratory. The major advantage of these plants is clear from their name – “rapid cycling” (i.e., the time it takes for a plant to go from seed to seed). Most Brassica crop species have a life cycle that takes 6 to 12 months to complete. In contrast, rapid cycling Brassica rapa completes its life cycle in ca. 35 days. These FastPlants™ are themselves the result of artificial selection for rapid flowering and maturation, high seed production, short stature, and the ability to thrive under artificial light. The result of this effort has been the production of a valuable research and educational tool. Researchers all over the world use rapid cycling Brassica in studies of specific genetic traits and for crop improvement.

The challenge in this lab is for you to act as plant breeders in an attempt to artificially select a particular trait in rapid-cycling Brassica rapa. To do so, you must quantify the variability of a particular trait and attempt to change the genetic makeup of future generations with respect to this trait via artificial selection. There are a number of fairly obvious traits in Brassica, for example, leaf length or number, plant height, trichome number, etc. In contrast, there are other traits that do not appear to vary at all (e.g., number of stamens or cotyledons). What trait(s) will you attempt to select and what assumptions must be met in order for selection to occur?

Today’s lab

Prior to the start of the semester, we established a population of plants, all from the basic wild type stock rapid cycling Brassica rapa seed. Seed was germinated in January and kept in the walk-in growth chamber (next to room #105) in the Life Sciences Building. This population will serve as the initial generation for our laboratory. The plants should be just beginning to flower (see Fig. 1 for Brassica life cycle).

Study the plants at your table & make some observations. Note those traits that do not appear to vary, and those traits that are variable among plants on your bench. How might you measure these traits accurately and with high repeatability? Speculate on the importance of the particular trait in nature. Talk with your classmates about the traits that interest you and what you want to attempt to select. As a class, we will discuss various traits, consider the ways in which you might standardize measurements (this is very important), and choose traits for our analyses.

Measuring plants

Be careful! Handle plants with care as they are a bit fragile, not to mention they are the only plants available. To make measurements, various equipment is available (rulers, calipers, counters, hand lenses, microscopes, etc.).

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For each plant in the initial generation, record your trait of interest and at least one other character that you think might be correlated (or uncorrelated) with your chosen trait. For example, if you are attempting to select for short plants, you might also want to count the number of leaves, as it might be the case that shorter plants cannot produce as many leaves since they have short stems. In contrast, there may not be such an expectation for leaf length. That is, plants selected to be shorter may have leaves that are just as long as the taller, initial generation plants. Recording the number of leaves and leaf length will allow you to see if these traits are affected when you select for shorter plants.

Choosing parents

After you have measured the initial population, make a list of plants that could serve as parental plants for the next generation. For example, if you are interested in making a hairless plant, you would select the individuals that had the fewest number of trichomes from this initial population, and use these as the parental plants for the next generation. As a class, we will negotiate which group gets which parent plants.

Pollinations

In nature, Brassica rapa plants are completely dependent on insects for pollination (i.e., the transfer of sperm-bearing pollen from the male part of the flowers to the female part of flowers). Although each flower has both male and female parts, pollen from one plant is incapable of fertilizing ovules from the same plant. This self-incompatibility system ensures that individual plants mate with other plants. Because of this, we must act as pollinators and cross-pollinate the plants today (& over the next few days) until enough seed is generated for the offspring generation.

Each of your parental plants can be used as both a male pollen donor and a female seed parent. From the finalized list of parents, assign several different crossing combinations. It will be easiest (in terms of bookkeeping and plant labeling) to assign a single male (pollen) donor for each female (seed) parent. Following convention, we will list the female parent first when writing out crossing combinations; thus, 56 x 210 means that the female parent was 56 and the male parent 210. Note also that a single plant can serve as a pollen

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donor for more than one female plant. I recommend pollinating at least ca. 10 flowers on each of your selected female parents. Take

pollen from the male plant by touching the anthers with a toothpick. If the anthers are mature, yellow pollen will stick to the toothpick. Now rub the toothpick gently onto a mature stigma of the maternal parent (Fig. 2). Using a magnifying hand lens, you should be able to see pollen on the stigma surface.

Harvesting fruit Successful fertilization will result in the development of ovules into mature seeds, which will be

contained within the fruit. The length of time from fertilization to mature seed is ca. 28 days (Fig. 1). Any fruits following your crosses will be collected, labeled, and stored for your group. Seed from these crosses will be germinated for the second (offspring) generation.

Once fruit is harvested, record which plants were successfully used as parents for the next generation and calculate the mean value for your trait of interest (& any correlated traits) among the parents used to generate the second generation. How much larger/smaller, etc. were your selected parents compared to the initial generation?

The Next Generation... We will meet again over Brassica on Thursday, 1 April to measure the offspring generation.

Male parent (pollen donor)

Female parent (pollen recipient)

Fig. 2. Pollinating Brassica. Using a toothpick, rub the anthers of an open flower to collect pollen. Next, transfer the pollen to the stigma of the appropriate maternal parent. Remember to use a new (pristine) toothpick for each crossing combination.