59
PLANT REPRODUCTION
PLANTREPRODUCTION
Reproduction
Until now, the focus has been primarily on survival and growth of the individual plant – converting resources into self. Some resources are also converted into offspring, which could be thought of as another form of self
There are two main forms of reproduction in plants1. Asexual – the offspring is genetically identical to
the parent. Cells divide exclusively by mitosis.2. Sexual – the offspring result from the fusion of two
gametes each produced by meiosis. This is a strange process whose purpose (function) remains one of the great mysteries in biology.
Asexual Reproduction
Asexual reproduction is widespread in plants, and exists in an array of modes that form a continuum with growth of the plant body.
plantlets
Runners - stolons
KalanchoeStrawberry
Root sprouts
These asexual modes of reproduction often produce conspicuous “clones” of identical genotypes
Runners - rhizomes
GoldenrodAspen
38.12
Winter creeperKudzu
Some introduced species with this potential have “escaped”
Using this potential for asexual propagation has been a major activity in agriculture – both for ornamental and crop plants.
The key is that once a desired genotype is discovered, it can be replicated identically indefinitely.
This has been done for a long time with plants, but new techniques are always emerging. Progress has been much slower in cloning animals.
38.16/38.14
http://members.optusnet.com.au/~purdiejj/Grafting/grafting.htm
RootstockScion
Grafting
The key to successful plant biotechnology is the ability to replicate desired genotypes.
But the field is being revolutionized by new techniques of genetic engineering that create new genotypes (that then need to be cloned)
38.14/38.12
38.17/38.15
38.18/38.16
Resistant Papaya
Golden Rice
Read about plant biotechnology in your textSection 38.3 pp 815-819 (38.4 pp 783-786 7th)
Sexual ReproductionNatural genetic engineering?
The seeds of seed plants contain embryos (sexual offspring) that result from meiosis – two haploid gametes fuse to form the zygote. The zygote is a single cell which then replicates mitotically to produce the multicellular embryo.
In this section we look at this process in some detail
38.8
Alternation of Generations (review)
The plant life cycle is more complicated than the typical animal.
Both the haploid and diploid phases begin as a single cell, and both cells then undergo mitosis to form a multicellular phase
haploid (n) – gametophytediploid (2n) - sporophyte
The diploid sporophyteundergoes meiosis to produce haploid spores
What is meiosis?
The zygote is “diploid” because it contains two complete sets of genes, one “haploid” set from each parent. This means two copies of genes of each gene type (“locus”). The two copies need not be identical (alternate “alleles” are common at each locus)
What is “diploid”?
The diploid zygote replicates mitotically to form the multicellular diploid sporophyte. To create the spores, particular cells in the sporophyte undergo meiosis, each replicating once then dividing twice to produce four haploid spores. These spores then replicate mitotically to form multicellular gametophytes, which produce the single-celled haploid gametes (which form the zygote by fusion).
13.7
Recombination – the real point?
At least as important as the production of haploid spores, is the recombination of alleles in meiosis. Before making spores, the “mother cells” undergoing meiosis essentially “shuffle” the two parental genotypes, creating new genotypes that are mixtures of the parents. Often, these recombinant gametes are fused with gametes from other individuals (“outcrossing”) to make the zygotes. This creates new genetic variation. The offspring are differentfrom each other and from either parent.
13.11/13.1013.12/13.11
Independent assortment
Crossing over
14.12/14.11
The paradox of sex - why meiosis?
We have emphasized that evolution favors organisms that are good at transforming resources into more self – but here is a widespread mode of reproduction that seems designed to create offspring that are different from the parents, that are deliberately NOT self.
Why? It is assumed there are good reasons, we just don’t know what they are for sure. But of course there have been many proposed explanations (hypotheses). (see p998)
This is another good example of the difficulty of verifying functional explanations.
1. Offspring are more different from each othera. in an unpredictable world, at least some are more
likely to survive (the “lottery” hypothesis)b. reduced competition between offspring, greater
average success2. Offspring are different from the parent
a. more resistant to natural enemies of the parent, especially parasites
b. more likely to be “average” (nearer the mean of the population)than parent. “regression to the mean”
3. Higher variation increases the rate of evolution –brings together favorable alleles
4. Recombination speeds the elimination of bad traits, preventing extinction
5. Errors in genes (DNA) can be repaired by comparisons between alleles from different parents
The Main Contenders
Flowers
Meiosis occurs in only a few cells of the millions of cells in a mature plant. In angiosperms, meiosis happens in cells inside flowers.
Flowers are also devices for facilitating the fusion of the haploid gametes, particularly as pollinator attractors.
megaspores => female gametophyte => egg (make seeds)
microspores => male gametophyte => sperm (gamete fusion)
Fusion occurs between two types of haploid gametes, the sperm and the egg. These are produced by gametophytes that develop from spores also of two types (heterospory)
These two types of spores and gametes are produced in different floral structures.
Developmentally, flowers are highly modified leaves, in a series of “whorls” (like nodes). The generalized flower has four whorls. Not all flowers have all whorls functional. Multiple flowers on the same growing tip are called “inflorescences”.
Whorl unit1. Calyx sepals2. Corolla petals3. Androecium stamens => microspores4. Gynoecium carpels => megaspores
38.2
Perianth
Androecium – the “male house”
The stamens produce the pollen grains that transport the recombinant haploids from one plant to another. Each stamen consists of an anther supported by a filament.
The anther contains the microsporocytes(‘cyte’ = cell, a.k.a. the micropsoremother cells) that undergo meiosis to form 4 haploid microspores. Each microspore divides once mitotically to form a 2-celled pollen grain, containing the tube cell and generative cell.
Pollen grain = Male gametophyte38.3/38.4
Pollen wall very tough, often with a complex and ornate surface, distinct between species. It resists decay and is abundant in the fossil record and sediments. This has made it valuable in reconstructing past climates. “Palenology”
Pollen must survive in the environment so it can be carried to another plant where it can germinate if it lands on the stigma of the right flower. This is called out-crossing.
Gynoecium (“female house”, aka pistil)
Consists of one or more carpels in various patterns (text illustrates the simplest possible pistil – one carpel/one ovule)Each carpel consists of
1. Ovary – the base of the carpel, containing one or more ovules where meiosis takes place to form the megaspores
2. Stigma – receives the arriving pollen
3. Style – supports the stigma and connects it to the ovary.
The ovule is the structure that will become the seed. It contains the megasporocyte(meiosis) surrounded by protective integument layers, with one opening – the micropyle.
Meiosis produces 4 megaspores, of which 3 usually degenerate. The remaining spore divides mitotically to produce the embryo sac (often 3 divisions = 8 cells – genetically identical)
Embryo sac = female gametophyteOne cell becomes the egg cell.Other cells become polar nuclei – involved in fusion, synergid cells (help attract pollen tube), and antipodal cells.
38.3/38.4
Fertilization (‘double fertilization’)If pollen germinates on a stigma, it forms a pollen tube that grows down the tissue of the style until it reaches the ovule. Only one pollen tube will enter any given embryo sac.
While the tube grows, the generative cell divides to form 2 sperm cells.
Tube enters micropyle & embryo sac and 2 sperm released – 2 fusions
1. Egg + sperm => zygote2. Sperm + polar nuclei
=> primary endosperm
38.5/38.6
Post-fertilization
1. Zygote becomes embryo in seed2. Primary endosperm becomes seed endosperm3. Integuments become seed coat4. Fruit (containing seeds) develops from the ovary and
other flower parts
38.7
Completing the life cycle
38.2
Perianth (Calyx + Corolla)
Solution: The two remaining whorls (calyx and corolla), the main reason for the conspicuous beauty and variety of flowersMain function: the attraction and control of pollinators. Flowers are best viewed as an array of devices to manipulate these animal ‘vectors’.
Problem: pollen grains are very small, and may have to go a long distance to hit a very small target – a receptive stigma.
Calyx (sepals) - lowest whorl, often green, support and protection, sometimes colorful, petal-likeCorolla (petals) – often colorful, conspicuous, sometimes highly modified
Flowering plants owe their success at least partly to the ‘domestication’ of a variety of animals for pollen transport.
Pollen is high in protein, and ancestors of angiosperms were wind pollinated, producing large amounts of pollen. Early vectors were probably pollen predators, possibly beetles.
Movement of these consumers presumably increased pollen transfer between plants – which could have led in small steps to modern angiosperms.
Now many animals serve as pollinators, including bees, moths and birds. Each vector has its own characteristics, and plants have evolved to take advantage of those characteristics.
Note: many of these photos were lifted from Meeuse, B. and S. Morris.1984. The Sex Life of Flowers.Facts On File, NY.
BeesBees are the single most important group (>20,000 species)
Well developed sight and smellVision insensitive to red, see ultravioletLearn quickly which flowers are rewarding – pollinator fidelityAdults live on nectar, raise larvae on pollenSome bees are social, most solitary
Bee flowers use these characteristicsBrightly colored – often yellow or blueDistinct patterns – “nectar guides”, sometimes in ultravioletHigh nectar concentration, 50-60% sugar
Nectaries are special epidermal glands that secrete nectarStructure often a flat landing platform, or more elaborate
chamber to force bee to contact anthers and stigma.
Wasps. Many wasps are pollinators, including fig wasps.
Figs (genus Ficus) are a diverse group of tropical trees, with highly specialized wasp pollinators.Fertilized female wasps enter the fig floral structure (‘synconium’) carrying pollen and actively pollinate the flowers, then lays eggs in some of the ovules. Those ovules will die, but emerging female wasps will collect pollen, mate, then fly to another tree to repeat the process.
Butterflies and Moths
Flowers using day flying butterflies and moths are much like bee flowers, including high sugar concentration. They can see red, so flowers often red as well as yellow and blue.
Flowers using night fliers are often white and fragrant.
hawkmoth
Yucca moth – highly specialized to it host plant (Yucca). Like fig wasp, it actively pollinates and for the same reason – to lay eggs.
Note: most pollinators make no effort to actively pollinate flowers, pollination is a byproduct of their feeding (or mating) activity.
Birds. Hummingbirds (new world) and sunbirds (old world) are important pollinators. They are highly visual day fliers with a poor sense of smell. Want nectar, taken up by capillary action (grooved tongue).
Bird flowers are brightly colored, often yellow or red, have no odor, and high volume of dilute nectar
Corollas are often deep fused tubes that make nectariesaccessible only to the long-billed birds – with anthers and stigma extended to touch the head of the bird.
Bats and other mammals.Bats are important pollinators in the tropics. They are night fliers with a good sense of smell, and consume both pollen and nectar.Bat flowers – fragrant, white or dull colored, copious nectar and pollen, very strong structures.
Various small mammals
Giraffe pollination?
Acacia
Traps. Some flowers contain chambers that hold insects temporarily, often during a switch from female to male.
Other tricks.
Water Lily
Carrion mimics. Many beetles and flies are ‘fooled’ by plants that smell like rotting flesh. Flowers are generally odd looking, dull colored, and putrid smelling.
Female mimics
Nectar robbing
But plants aren’t always in control
Wind pollination – many species of angiosperms are wind pollinated (e.g. grasses, many temperate trees). All evolved from insect pollinated ancestors.
Flowers inconspicuous, green, no nectar, abundant pollen
Female flowersMale inflorescence
Some species have even returned to water pollination
Outcrossing and Inbreeding
All these mechanisms are seen as ways to promote the efficient transfer of pollen to other plants (outcrossing) – which must be a good thing since it would be much easier to self-fertilize.
In fact, many species (>50%) are self-incompatible, that is they have a programmed self-recognition mechanism that prevents their pollen from fertilizing their own ovules.
Despite this, selfing is also quite common. It turns out that most species are either mostly outcrossing or mostly selfing. It is thought that this reflects the relative costs and benefits of outcrossingversus selfing (increased efficiency if inbreeding can be overcome)
(b) Oxalis alpina flowersThrum flower Pin flower
Stamens
Styles
Styles
Stamens
(b) Oxalis alpina flowersThrum flower Pin flower
Stamens
Styles
Styles
Stamens
Fruits
Seeds develop from the ovules inside the ovary, and fruits develop from the ovary of the gynoecium (and sometimes other flower parts). The fruit structure is often closely determined by the structure of the carpels.
3 types of fruit1. Simple fruits – develop from one or
several united carpels. 2 types.A. Fleshy – includes grapes,
peaches, applesB. Dry – some split open
(‘dehiscent’, e.g. pea pods), others not (‘indehiscent’, e.g. acorns, walnuts)
2. Aggregate fruits - multiple separate carpels from one flower (e.g. raspberries, strawberries)
3. Multiple fruits – multiple flowers -inflorescences -(e.g. pineapple, skunk cabbage)
38.10/38.9
Fruit dispersal
Function of the fruit is to aid in offspring dispersal – moving the seeds away from the parent. Protection of the seeds can be an important component of this, but many seeds use ‘vectors’ to carry the fruits/seeds, like pollination.
1. Wind. Some seeds very small, can blow around like pollen. Some have wings with aerodynamic properties (maple, dandelion, milkweed)
30.9, 38.11
2. Water. Some plants growing near (or in) water make buoyant water dispersed fruits ( coconut, mangrove, skunk cabbage)
3. Animals. Many fruits are designed to be dispersed by birds and mammals (including primates). Either by attaching to them or feeding them. Edible fruits high in sugars, fats and/or proteins.
.
Some fruits have hooks or spines that attach to animals.
Often they are conspicuously colored and displayed when ripe. Usually fruit is eaten but the deed dropped or not digested. Some seeds require gut passage for germination
Germination and Dormancy
Seeds require particular environmental conditions to germinate (e.g., levels of water, O2, temperature, and light), and these are often conditions that signal good conditions for a growing seedling.
Some seeds will not germinate even when conditions are favorable – called “dormancy”. Release from dormancy requires special pre-conditions. E.g., some seeds require a cold period, “stratification”, before they will germinate. Why?
Dormant seeds may remain viable for a few years, or sometimes many years. Record is an arctic lupine, where seeds found in a frozen lemming burrow were 10,000 years old, still viable.
Dormancy is sometimes described as a kind of dispersal in time. What is meant by that?
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