80
Aging and Sex, DNA Repair in
the same mutation (becoming homozy-
gous recessive) in each progeny. If an
average of one to several deleterious mu-
tations are present in each individual, the
cost of inbreeding in terms of defective
progeny would be high. If a hypotheti-
cal outcrossing individual should arise in
such an otherwise inbreeding population,
any mutations in this outcrosser would
very likely be complemented by the wild-
type alleles from its mating partner and
thus defective progeny would be avoided.
Complementation would occur because
the homologous chromosomes from the
two parents are not likely to carry the
same mutations. Thus, loss of progeny
due to expression of mutations would be
greatly reduced. This gives a strong imme-
diate selective advantage to switching to
outcrossing from inbreeding.
This advantage would not last indeF-
nitely in our hypothetical example. Be-
cause of the ability of outcrossers to mask
deleterious mutations by complementa-
tion, mutations would not be weeded
out by natural selection as efFciently as
in inbreeding individuals. Eventually, the
mutations that build up in the population
will cause as much lethality to progeny
of the outcrosser as that in the origi-
nal inbreeding population. However, if
the outcrosser tried to switch back to in-
breeding, there would be a great loss of
progeny due to the larger number of mu-
tations now present. In summary, there
is a large immediate beneFt in switch-
ing from inbreeding to outcrossing and a
large immediate disadvantage in switch-
ing back. Therefore, mutation provides
a selective pressure to maintain the out-
crossing feature of sexual reproduction
among diploids.
Overall, meiosis, with its promotion
of recombinational repair, may be the
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nous double-strand damages and leftover
single-strand damages in the diploid cells
that produce germ cells. On this view,
the recombination aspect of sex is an
adaptation for dealing with DNA damage,
a major type of genetic noise. ±urther-
more, outcrossing allows the masking
of mutations through complementation.
The outcrossing aspect of sex deals with
mutation, the second major type of ge-
netic noise.
3.5
Repair of DNA Damage of the Germ Line
in Nonmeiotic Cell Divisions
The germ line is characterized by peri-
odic events of meiosis, but during the
intervals between such events, cell divi-
sions are ordinarily by mitosis. Cells of the
germ line are presumably capable of the
same types of repair processes that occur
in somatic cells, discussed in Section 2.
±or instance, there is a relatively high
level of PARP activity in premeiotic and
meiotic spermatocytes, an indication of
BER. ±raga and coworkers in 1990 mea-
sured the accumulation of one type of
oxidatively damaged DNA base, 8-hydroxy-
2
0
-deoxyguanosine, in various tissues of
the rat. Although the average accumula-
tion in the rat kidney was 80 residues
per cell per day, there was no detectable
accumulation in the testes. This lack of
accumulation in the testes of normal indi-
viduals can be interpreted as a reflection of
efFcient BER in the germ line of normal
individuals.
It is notable, in Table 2, that where life
span is extended, fertility is either normal
or extended as well, where it has been
tested. In Table 3, where defects in DNA
repair cause early aging, fertility is always
reduced, where it has been tested.
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