84
Aging and Sex, DNA Repair in
aspect of sex, this should be consistent with
general biological observations as well. In-
deed, both in animals and in plants, it is
usually seen that when hybrids are formed
from the crossing of two genetically dis-
tinct inbred lines, these hybrids are more
vigorous than either of their two parental
lines. This hybrid vigor is responsible for
much of the crop improvement that has
been achieved in modern agriculture. The
opposite side of this observation is the
fact that consanguineous marriages, in hu-
mans, result in an increased frequency of
impaired offspring. Observations in other
animals and in plants suggest that close in-
breeding results in the production of less
vigorous progeny. This inbreeding depres-
sion appears to be due largely to expression
of deleterious recessive mutations and
reflects in part the cumulative effect of
numerous mildly deleterious mutations.
4
Vegetative Survival Strategies
4.1
Survival of Vegetative Cell Populations
Vegetatively growing populations of bacte-
ria can be regarded as potentially immortal
as long as nutrient resources are abundant.
However, even in such populations there
appears to be constant attrition due to DNA
damage and deleterious mutation.
Flowering plants generally reproduce
sexually, with gamete formation by a mei-
otic process followed by gamete fusion as
the prelude to embryogenesis and seed
formation. Plants, unlike most animals,
are also able to generate complete new
individuals of similar genetic constitution
from vegetative parts. Meristematic buds
or excised pieces of tissue can propagate in
the appropriate environment. Apparently,
plant vegetative cell lines can be main-
tained inde±nitely under appropriate con-
ditions. These lines probably maintain
themselves by a strategy of replacement,
where cells with lethal unrepaired DNA
damage or expressed deleterious muta-
tions die and are replaced by replication of
nondefective cells. Nevertheless, in some
plant tissues DNA damage may accumu-
late. For instance, in dry seeds, fragmenta-
tion of nuclear DNA occurs with time.
Most
forest
trees
live
for
at
least
100 years, many of them for more than
300 years, and a few survive for more
than 1,000 years. Clonal tree species may
occupy a location for several thousand
years. Most of the tree is dead, and only
a thin shell of dividing cells (cambium)
occurs around the trunk and in the leaves.
A tree actually represents a free-living
clone of cells in which selective removal
of cells with irreversible accumulated
damage
is
constantly
occurring.
One
would not expect to ±nd old cells in a
tree any more than one would ±nd old
cells in a growing culture of bacteria. The
evidence discussed in this section suggests
that some proliferating cell populations
can cope with unrepaired DNA damage
by a replacement strategy, which can
be maintained inde±nitely as long as
nutrient
resources
are
abundant
and
the level of unrepaired DNA damage is
not excessive.
4.2
Vegetative Survival Strategy for
Mitochondria (and Chloroplasts) in the
Germ Line
An analog of cellular replacement at
the molecular level (‘‘molecular replace-
ment’’) facilitates the purging of both
DNA damage and deleterious mutations in
cytoplasmic genomes (mitochondria and
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