Encyclopedia of Evolution.pdf - Online Reading Center

endonuclease gene is then used as a template for repair, with
the result that both chromosomes in the pair will then carry
the endonuclease gene. In this way, the endonuclease gene
can spread in these populations.
Competition of maternal and paternal alleles in animal
embryos. An embryo receives two alleles for each gene,
one from the mother and one from the father (see Mendelian
genetics). This creates a situation in which a conflict
of interest is possible. Both of the parents benefit from the
successful development, birth, and nurture of the offspring.
The interests of the mother are best served if the offspring are
successful, without impairing her ability to successfully reproduce
in the future. The interests of the father are best served
if the offspring are successful even if the mother’s ability to
successfully reproduce in the future is impaired. This would
not make any sense in a monogamous relationship, but most
animals (including humans) are not strictly monogamous.
The father’s fitness interests are therefore best served by having
more offspring either by the same female or by some
other. The father’s fitness benefits if the embryo that carries
his genes commandeers resources from the mother even at the
expense of her long-term health! In contrast, the mother’s fitness
benefits if she is in control of how much nurture to provide
to the embryo, that is, if her alleles can counteract the
selfish alleles of the father. This creates a conflict of interest
between mother and father, expressed in the development of
the embryo they create.
The only way that maternal and paternal alleles can differentially
affect fetal development is if there is some way of
distinguishing between them. There is such a way. In genetic
imprinting, either the maternal or the paternal allele is chemically
inactivated, at least temporarily, in the embryo. Therefore,
while the embryo contains alleles from both parents for
any given gene, the allele from only one of the parents may be
functional. This is an example of epigenetics (see DNA [raw
material of evolution]). The imprinting may be accomplished
by methylation of the cytosine nucleotides of the
inactivated allele. The effect of genomic imprinting on conflict
of interest between selfish maternal and paternal genetic
elements has been explored by evolutionary biologist David
Haig.
The conflict of interest between maternal and paternal
alleles has been investigated in laboratory mice, in which
alleles can be substituted in the fertilized egg cell, an experiment
that is technically (and for humans ethically) impossible
with most other animal species. One gene, Igf2 (insulin-like
growth factor 2), produces a protein that causes the mother
to devote more nutrients to the embryo but also causes higher
blood pressure. In the placenta, the maternal allele for Igf2 is
inactivated. The paternal gene for the receptor of Igf2, however,
is inactivated. The Igf2 gene acts in the interest of the
father, the receptor gene acts in the interest of the mother.
Experiments with mice indicate that the mouse embryo develops
normally if it has both the paternal Igf2 gene and the
maternal receptor, or neither—but not if both of the genes are
represented by the alleles from one parent. Igf2 and its receptor
are just two of the 40 genes in mice that are imprinted.
selfish genetic elements
Humans have most of these same genes. Some medical conditions
associated with pregnancy, such as high blood pressure,
may therefore be best interpreted from an evolutionary, conflict
of interest perspective (see evolutionary medicine).
Cytoplasmic male sterility factors in plants. Cytoplasmic
genes are found in the DNA of mitochondria and
chloroplasts (see symbiogenesis). The mitochondria and
chloroplasts cannot survive without the cell, therefore
it would seem that their genes would not harm the cell in
which they reside. However, mitochondria are passed on
only in female reproductive cells. Most plants are either hermaphroditic
or monoecious—they produce both pollen and
ovules. In dioecious species of plants, however, there are
separate male and female plants (see reproductive systems).
Nuclear genes can be passed on either through either
pollen or through ovules with equal effectiveness. However,
the mitochondria and the genes they contain are passed on
to the next generation more effectively if female reproduction
occurs much more than male reproduction. A mitochondrion
that produces a chemical that inhibits the production
of male cells, therefore, may be passed on into the next generation
more than a mitochondrion that does not produce
this chemical. Such a chemical is called a cytoplasmic male
sterility factor. Numerous such factors have been found in
plants. If a cytoplasmic male sterility factor spreads through
a population of plants, it will cause some of the plants to
be female (by failing to produce pollen). In flowering plants,
male sterility factors may also be found in chloroplasts, since
the chloroplasts are passed on only through the female cells.
This does not happen in conifers, since their chloroplasts are
passed on through the pollen.
As cytoplasmic male sterility spreads in a population,
and females become more common, the reproductive success
of the male function may increase by frequency-dependent
selection; if so, there will be an advantage to male
sexual reproduction. But what is to stop the spread of cytoplasmic
male sterility before it causes the whole population
to become female, and then extinct? Mutations occur in the
nucleus, which cause genes to produce silencing factors that
counteract the effects of the cytoplasmic sterility factors. In
some populations, sexual reproduction represents a balance
between cytoplasmic sterility factors that inhibit male function
and nuclear silencing factors that restore it. Populations
of plants may have several different cytoplasmic male sterility
factors and different nuclear silencer genes.
Flowering plants had ancestors that produced hermaphroditic
flowers. Female plants are frequently plants whose
male function has been inhibited by sterility factors. In some
cases, hermaphroditic plants come from lineages in which
cytoplasmic male sterility has never occurred; in other cases,
hermaphroditic plants are individuals in which the male function
has been lost and then restored by silencing factors. One
cannot tell by looking at a hermaphroditic plant whether or
not it is hermaphroditic because of a balance between sterility
and silencing factors. This entire phenomenon of sterility and
silencing was discovered only after DNA technology allowed
the actions of the genes to be determined.