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