Lecture 13: Plant Sex
1. The life cycles of angiosperms and other plants are characterized by an alternation of generations, in which haploid (n) and diploid (2n) generations take turns producing each other.
Plant biologists distinguish between complete flowers, those having all four organs, and incomplete flowers, those lacking one or more of the four floral parts.
A bisexual flower (in older terminology a perfect flower) is equipped with both stamens and carpals.
All complete and many incomplete flowers are bisexual.
A unisexual flower is missing either stamens (therefore, a carpellate flower) or carpels (therefore, a staminate flower).
A monoecious plant has staminate and carpellate flowers at separate locations on the same individual plant.
For example, maize and other corn varieties have ears derived from clusters of carpellate flowers, while the tassels consist of staminate flowers.
A dioecious species has staminate flowers and carpellate flowers on separate plants.
For example, date palms have carpellate individuals that produce dates and staminate individuals that produce pollen.
2. Plants have various mechanisms that prevent self-fertilization
Some flowers self-fertilize, but most angiosperms have mechanisms that make this difficult.
The various barriers that prevent self-fertilization contribute to genetic variety by ensuring that sperm and eggs come from different parents.
Dioecious plants cannot self-fertilize because they are unisexual.
In plants with bisexual flowers, a variety of mechanisms may prevent self-fertilization.
For example, in some species stamens and carpels mature at different times.
Alternatively, they may be arranged in such a way that it is unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower.
The most common anti-selfing mechanism is self-incompatibility, the ability of plant to reject its own pollen and that of closely related individuals.
If a pollen grain from an anther happens to land on a stigma of a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing an egg.
The self-incompatibility systems in plant are analogous to the immune response of animals.
The key difference is that the animal immune system rejects nonself, but self-incompatibility in plants is a rejection of self.
Recognition of self pollen is based on genes for self-incompatibility, called S-genes, with as many as 50 different alleles in a single population.
If a pollen grain and the carpels stigma have matching alleles at the S-locus, then the pollen grain fails to initiate or complete the formation of a pollen tube.
Because the pollen grain is haploid, it will be recognized as self if its one S-allele matches either of the two S-alleles of the diploid stigma.
Although self-incompatibility genes are all referred to as S-loci, such genes have evolved independently in various plant families.
As a consequence, the self-recognition blocks pollen tube growth by different molecular mechanisms.
In some cases, the block occurs in the pollen grain itself, called gametophytic self-incompatibility.
In some species, self-recognition leads to enzymatic destruction of RNA within the rudimentary pollen tube.
The RNAases are present in the style of the carpel, but they can enter the pollen tube and attack its RNA only if the pollen is of a self type.
In other cases, the block is a response by the cells of the carpels stigma, called sporophytic self-incompatibility.
In some species, self-recognition activates a signal transduction pathway in epidermal cells that prevents germination of the pollen grain.
Germination may be prevented when cells of the stigma take up additional water, preventing the stigma from hydrating the relatively dry pollen.
3. Double fertilization gives rise to the zygote and endosperm
The union of two sperm cells with different nuclei of the embryo sac is termed double fertilization.
Double fertilization is also present in a few gymosperms, probably via independent evolution.
Double fertilization ensures that the endosperm will develop only in ovules where the egg has been fertilized.
This prevents angiosperms from squandering nutrients in eggs that lack an embryo.
4. The ovary develops into a fruit adapted for seed dispersal
5. Evolutionary adaptations of seed germination contribute to seedling survival
The length of time that a dormant seed remains viable and capable of germinating varies from a few days to decades or longer.
This depends on species and environmental conditions.
Most seeds are durable enough to last for a year or two until conditions are favorable for germinating.
Thus, the soil has a pool of nongerminated seeds that may have accumulated for several years.
This is one reason that vegetation reappears so rapidly after a fire, drought, flood, or some other environmental disruption.
6. Many plants clone themselves by asexual reproduction
7. Sexual and asexual reproduction are complementary in the life histories of many plants
Many plants are capable of both sexual and asexual reproduction, and each offers advantages in certain situations.
Sex generates variation in a population, an asset in an environment where evolving pathogens and other variables affect survival and reproductive success.
Seeds produced by sexual reproduction can disperse to new locations and wait for favorable growing conditions.
One advantage of asexual reproduction is that a plant well suited to a particular environment can clone many copies of itself rapidly.
Moreover, the offspring of vegetative reproduction are not as fragile as seedlings produced by sexual reproduction.
Seeds, the product of sexual recombination, may lie dormant under an extensive clone produced by asexual reproduction.
After a major disturbance (fire, drought, for example) kills some or all of the clone, the seeds in the soil can germinate as conditions improve.
Because the seedlings will vary in their genetic traits, some plants will succeed in competition for resources and spread themselves as a new clone.
8. Vegetative propagation of plants is common in agriculture