78
Aging and Sex, DNA Repair in
Tab. 5
Modes of reproduction among higher plant species.
Reproduction strategies
Number
Percent
Principally cross-fertilized
Self-incompatible, dichogamous, or dioecious
Dichogamy: male and female parts mature at different times;
Dioecious: unisexual (male or female) flowers on different plants
830
55
Partially self-fertilized, partially cross-fertilized
105
7
Principally self-fertilized (autogamous)
229
15
Facultative apomicts (can be cross-fertilized)
Apomixis: vegetative methods of reproduction including propagation
by runners or bulbs and agamospermy
Agamospermy: embryos and seeds formed by asexual means
16
1
Facultative apomicts, can be self- or cross-fertilized
3
0.2
Facultative apomicts, can be self-fertilized
2
0.1
Apomicts, not known if also self- or cross-fertilized
199
13
Obligate apomicts
121
8
3.2
Frequency of Sexual Reproduction
Sexual
reproduction
is
a
widespread
strategy for reproduction. About 99.9%
of the approximately one million known
animal species are sexual. Among higher
plants, the majority of species are sex-
ual. As shown in Table 5, only 8% of
higher plants are known to be obligate
apomicts that reproduce only by vegeta-
tive means such as by runners, bulbs, or
by asexual formation of seeds. Sex is also
common among the simple eukaryotes,
including fungi, algae, and protozoa. Sex
is found among bacterial species as well,
and is common in bacterial viruses and
animal viruses.
3.3
Costs of Sex
Sex, while widespread, is very costly to
the organism using it. For example, a
sexual female lizard passes only 50% of
her genes to a particular egg, while a
comparable nonsexual (parthenogenetic)
female lizard passes 100% of her genes
to each egg. Thus, a sexual female is
only half as ef±cient in propagating her
genes as a nonsexual female (all other
factors being equal). In addition, when
two individuals must ±nd each other to
mate, there is a cost of searching out the
other party.
Another cost of sex arises from the ran-
domization of genetic information during
meiosis. A parent organism, which has
met the test of survival, has by de±ni-
tion, a well-adapted combination of genes.
The process of meiosis, which includes
recombination, generates untested new
combinations of genes to be passed on
to progeny. These new combinations, on
average, should be less successful than the
parental combinations of genes because
random changes in successful genetic
information are more likely to be dele-
terious than bene±cial.
The noted biologist E.O. Wilson de-
scribes sex in humans
as
a ‘‘gratu-
tiously consuming and risky activity’’.
Reproductive
organs
are
anatomically
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