90
Aging and Sex, DNA Repair in
the investment in sexual communication
to promote cross-fertilization, as the most
common mode of reproduction among
flowering
plants,
is
probably
due
to
the
prominence of
the
diploid
stage
and the beneFt of masking deleterious
mutations. Instances in which outcrossing
has been abandoned in favor of self-
fertilization or parthenogenesis may be
explained by the need of some plants to
survive in sparse populations in which the
costs of sexual communication would be
prohibitively high.
Sexual signals in plants promote mating,
but the subsequent meiosis and recombi-
national repair happen in the succeeding
generation when the progeny produce
germ cells. Among those flowering plants
that depend on cross-fertilization to re-
produce, sexual communication acts at
both the primary and the secondary level.
However, in plants that can undergo
self-fertilization, sexual communication is
unnecessary when this option is used.
2.
Vertebrates
. In bacteria, fungi, and
protozoa, sexual reproduction is generally
facultative and favored only under certain
conditions. In nearly all vertebrates, how-
ever, the sexual cycle is obligatory, and
thus sexual communication is ordinarily
necessary for reproduction.
In vertebrates, meiosis and concom-
mitant recombinational repair ordinarily
occur at a time separate from sexual signal-
ing. However, sexual signaling to attract a
mate is essential to continue the sexual
cycle, which includes fertilization, pro-
duction of progeny, and further meioses.
Thus, sexual signals promote mating, but
meiosis and recombinational repair occur
subsequently in the succeeding genera-
tion, when progeny form germ cells.
Among vertebrates, sexual communica-
tion for promotion of outcrossing has been
s
tud
i
edinm
an
ys
y
s
t
em
s
.Int
o
ad
s
,ad
-
vertisement vocalizations given by males
apparently serve as cues by which fe-
males
recognize
their
kin
and
thus
avoid inbreeding.
In mouse, mating preference is strongly
influenced by the major histocompatibil-
ity complex (MHC) genotype. A mouse
can distinguish close relatives from more
distantly related mice on the basis of
MHC genotype through their sense of
smell. Using this device, mice tend to out-
cross, avoiding mating with close relatives.
Inbreeding of mice derived from wild pop-
ulations has a signiFcant detrimental effect
on survivorship when the mice are rein-
troduced into a natural habitat. This effect
is even more severe than that observed in
laboratory studies of the population. In the
other vertebrates, to be described below,
the communication processes used to pro-
mote outcrossing have not been as well
deFned as in the mouse. Nevertheless, we
infer that analogous processes also exist in
these cases.
Outcrossing is promoted among birds.
The great tit (
Parus major
)i
samonog
-
amous woodland bird. In natural popu-
lations, outcrossing is promoted by the
dispersal of daughters, but not sons. ±ur-
thermore, among infrequent incestuous
matings, nestling mortality was nearly
double that of outbreeding pairs.
Among primates, avoidance of inbreed-
ing occurs in rhesus monkeys, chim-
panzees, and gorillas. Juvenile rhesus
monkey males leave the troop that they
were born into when they mature. This pat-
tern is similar to that in many mammals in
which male progeny leave the social group
into which they are born and Fnd a mate in
another group. Chimpanzees and gorillas
follow a less common pattern in which the
female, rather than the males, leave their
natal group and transfer to other groups.
previous page 90 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 92 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off