asexual and sexual reproduction flowers pollination and fertilization seeds plant development plant...
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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?