All AbouT SEx 269
Selfing and outcrossing
Some hermaphroditic plants and animals have evolved a compromise between
sexual and asexual reproduction. They produce gametes by meiosis, but then can
fertilize themselves. Self-fertilization, or “selfing” for short, is particularly com-
mon among weedy and colonizing species. Like parthenogenesis, selfing provides
reproductive assurance—a single individual can reproduce without a partner.
Most hermaphroditic species have mechanisms that largely or completely pre-
vent self-fertilization. They outcross, that is, mate with other individuals. Animals
can avoid selfing behaviorally, but plants cannot. Most flowering plants are her-
maphrodites: each individual produces both pollen and ovules. Plants have evolved
diverse ways to prevent pollen from fertilizing the ovules of the same individual.
In some species, the pollen and ovules on a plant mature at different times. In
others, the anthers (floral structures with pollen) are physically separated from the
stigmas (structures that receive the pollen), decreasing the chance that an ovule
will be fertilized by pollen from the same flower. A remarkable scheme to prevent
self-fertilization, called self-incompatibility, has evolved several times [11]. When
pollen lands on the stigma of a flower, biochemical systems compare the geno-
types of the pollen and stigma at the SI (self-incompatibility) locus. If the stigma and
pollen share an allele at this locus, biochemical machinery is triggered that kills
the pollen (FIGURE 10.28).
Why do these hermaphrodites forego the advantage of reproductive assurance?
The major downside of self-fertilization is that offspring can suffer from inbreed-
ing depression. This is the loss in fitness shown by offspring whose parents are
close relatives compared with offspring whose parents are unrelated. (Inbreeding
depression is different than inbreeding load, discussed in Chapter 7, which is the
decline in a small population’s fitness caused by fixation of deleterious mutations.)
You saw in Chapter 4 that most mutations are deleterious, and that deleterious
mutations tend to be recessive. When an organism self-fertilizes, every deleteri-
ous mutation in the genome that is heterozygous has a 50 percent probability of
becoming homozygous in the offspring. These homozygous mutations can dra-
matically decrease the offspring’s fitness.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_10.28.ai Date 12-20-2016 01-25-17StigmaS 3 S 4 S 3 S 1 S 2 S (^1) Pollen
S 1 S 2 S 1 S 2 S 1 S 2
Style
Pollen
tube
Ovule
FIGURE 10.28 Several kinds of self-incompatibility in
flowering plants prevent ovules from being fertilized by
pollen from the same plant. The mechanism shown here
is called “gametophytic incompatibility.” When a pollen
grain lands on a stigma, it begins to grow a tube toward
the ovary, where it will fertilize an ovule. A biochemical
reaction compares the one allele carried by the pol-
len with the two alleles carried by the stigma at the SI
(self-incompatibility) locus. If there is a match, the female
tissue kills the pollen. This prevents the plant from being
fertilized by its own pollen.
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