Asexual and Sexual Reproduction Flowers Pollination and Fertilization Seeds Plant 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 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

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 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 teosinte to maize by artificial selection Fig

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?