Asexual and Sexual Reproduction

FlowersPollination and Fertilization

SeedsPlant Development

Plant Reproduction - Ch 38

Many Plants can reproduce vegetatively. This asexual

reproduction produces genetic clones.

• Specialized roots• Specialized stems

– Beach grass example

• Specialized leaves

Aspen Clones

Sexual Reproduction Produces Genetically Diverse

Offspring• Review: sexual reproduction

involves production of haploid gametes, which then fuse to form a zygote.

• Animal gametes don’t do much.• Fern gametes, in contrast, do a lot!• Flowering plants - in between…

Alternation of generations• Multicellular haploid and diploid stages

take turns producing each other

Fig. 29.6

Sporophyte

Gametophyte

Gametophyte-sporophyte variations

Fig. 30.1

Angiosperm life cycle

Fig. 38.1

Flowers are the reproductive organs of sporophytes

• 4 whorls:– Sepals– Petals– Stamens (male;

pollen here)• Filaments• anthers

– Carpel (female; egg here)

• Stigma• Style• ovary

Monoecious vs. Dioecious Plants

• Monoecious (“one house”) - both male and female roles in the same plant.

• Includes plants with bisexual (“perfect”) and unisexual (“imperfect”) flowers

Dioecious Plants

• Diecious (“two houses”) - male and female roles on different plants – Sagittaria - in roadside ditches

Pollination is the first step in fertilization

• Fertilization is indirect in plants. • Mechanisms of pollination: how

does the pollen get to the stigma?• Adaptations for pollination.

Coevolution.

Catapulting Pollen

• Edwards et al. (2005) “A record-breaking pollen catapult” Nature 435: 164

• See also http://www.ou.edu/cas/botany-micro/ben/ben194.html

Selfing vs. Crossing

• Self-fertilization vs. cross-fertilization– 20% of plants are selfing - an evolutionary

dead end?

• How to avoid selfing:– Dioeciousness– Self rejection - self-incompatibility genes

• Analogy to animal immune system - self-recognition

– Structural and temporal adaptations in flowers

Self incompatibility

• There are various mechanisms of rejecting the pollen grain (RNAses, aquaporins…)

Formation of gametophytes

– (we need a bigger picture)

Male gametophyte• Microspores

Fig. 38.4a

Diploid

Haploid

Fig. 38.5

Pollen

Female gametophyte

• Megaspores

Fig. 38.4b

Gametophytes, continued

• Just remember this:• Male forms haploid tube cell

nucleus and generative cell (will form two sperm cells) within pollen grain

• Female forms haploid egg and two polar nuclei within embryo sac

• (three haploid cells each, to simplify)

The sequence of events leading to fertilization

• Pollination• Growth of pollen tube• Sperm cells (2) travel down tube to

ovary• Fertilization

Double Fertilization

• What is “double fertilization?”• One egg + one sperm cell = zygote

– Zygote is diploid, and develops into mature sporophyte plant

• Two polar bodies + other sperm cell = endosperm– Endosperm is triploid (!) and forms nutritive

tissue for the embryo

Seed formation

• After fertilization, ovule develops into a seed and ovary develops into a fruit

Seeds

• Double Fertilization creates the zygote and the endosperm

• The zygote divides to form an embryo• The endosperm divides and grows,

storing nutrients for the embryo (oils, proteins, starch)

• In some dicots, the nutrients are transferred from the endosperm to the cotyledons during seed formation.

Seed formation

• Seed coat• Endosperm

– Nutritive tissue – Cotyledon(s)– Seed leaves

• Hypocotyl & Radicle– Embryonic root

• Epicotyl & Plumule– Shoot tip

(Sketch)

Seed Dormancy and Germination

• Dehydration during final stages of seed formation

• Dormancy, seed banks, signals for germination

Germination

Humans and plant reproduction

• We’ve taken advantage of plants ability to reproduce asexually

• Cuttings (or fragments) from plants are used to produce MANY plants with certain desired characteristics

• At one end of a cutting is a mass of dividing, undifferentiated cells called a callus

• A callus forms adventitious roots and eventually differentiates into all parts of a plant

Carrot callus

Plant biotechnology• Using plants in

new ways to help people– Long history

• Today considered to be using genetically modified (GM) organisms in agriculture and industry– Very contentious

From teosinteto maize byartificialselection

Fig. 38.19

Modern biotechnology

• Today we’ve moved beyond artificial selection of closely related species or varieties of a single species

• Now we can transfer genes among very distantly related species through genetic engineering

• Transgenic organisms have been genetically engineered to express a foreign gene

Bt corn• Genes from the bacterium

Bacillus thuringiensis are inserted into corn plants

• The genes code for a protein (Bt toxin) that kills insect pests (especially the European corn borer)

• Using these transgenic corn plants reduces the need for pesticides, which saves money and reduces the environmental impacts associated with chemicals

Bt corn vs. monarch butterflies

• A 1999 study in Nature found high doses of pollen from Bt corn could negatively impact (including kill) monarch larvae feeding on milkweed that had been dusted with the pollen

• Later published studies have found that the concentrations in the study were unrealistically high, and that there is likely little threat to monarchs at normal levels of pollen

What’s the big deal?• We’ve been modifying species through

selective breeding for thousands of years• What’s the problem with modifying species

directly through their genes?