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Evolution of Sexual Reproduction
M.Tevfik Dorak, M.D., Ph.D.
Asexual
reproduction is still used by some organisms but in general failed to pass the
test of natural selection. Sexual reproduction is the favored way of
reproducing for many organisms. In sexual reproduction, new combinations of
genes can be assembled on the same chromosomes through recombination.
Independent assortment during meiosis, which changes combinations of
chromosomes, generates endless genetic diversity. This variation enables a
species to overcome novel environmental changes by fast adaptive change. In
asexual reproduction, however, natural selection has to wait for some sort of
mutation or change due to drift to take place, to act on. Sexual reproduction
can also put two beneficial mutations together (although there is always a
possibility to break a favorable combination too), or eliminate a deleterious
one. The phenomenon that a favorable combination of genes may be broken during
meiosis is called the 'cost of recombination'. Overall, groups reproducing
sexually can evolve more quickly than those do not, because the combination of
beneficial mutations will occur more quickly and deleterious mutations will
accumulate more slowly. This is why in eukaryotic multicellular life forms
sexual reproduction is a rule. Even in bacteria, which reproduce asexually,
there are mechanisms allowing gene transfers between organisms (see conjugation
in Viral and Bacterial Genetics). It
is believed that sexual reproduction evolved as early as 2.5 to 3.5 billion
years ago (Bernstein H et al., Am
Nat 1981). Sexual reproduction is beneficial particularly when environment
changes (including changes in the number and genetic constitution of
parasites), and when offspring are dispersed widely to end up in different
places from their parents. This is why aphids produce winged offspring when
reproduced sexually, and wingless ones when reproduced asexually. Similarly,
common grass grows asexually (locally) but produces seeds by sexual
reproduction to travel away, so that in the new environments, there will be
diversity for best adaptation. A host genotype successful against parasites may
not be so in the next generation as the rate of evolution in parasites is so
fast. The only way in which an animal makes sure that its descendants will be
able to deal with different parasites is to reproduce sexually. This is
currently the most widely favored theory on the evolution of sex. An
alternative theory suggests that it may have evolved as an adaptation for
competition with other species.
The
evidence suggests that no major group of organisms (except one group of
invertebrate rotifers) has evolved and diversified over long periods using
parthenogenesis (development of eggs without fertilization 'virgin birth' as in
aphids or Daphnia (water flea), or apomixis in plants) alone. When, however,
the cost of sex (this is the cost of males, mating and recombination) outweighs
the benefits of sex, a species may switch back to asexual reproduction. This is
what is believed to have happened to dandelions. Interestingly, some species
have adapted to have both modes of reproduction depending on the environmental
conditions (see the link for
the Encyclopaedia Britannica article at the end). Aphids reproduce parthenogenetically
in the spring and summer when food supply is plenty. This enables them to
reproduce very rapidly (and produce wingless offspring). When they face the
long winter with no food around, they switch to sexual reproduction and deposit
their fertilized large eggs on plants (alternation of generations). These eggs
start a new generation of asexually reproducing (winged) females next spring.
Similarly, plants may also have alternation in generations with different modes
of reproduction. In some plants, one generation consists of diploid plants
(sporophytes). Some of their cells undergo meiosis and produce haploid spores.
These spores develop directly into haploid plants (gametophytes). Haploid
gametophytes produce gametes by mitosis which will fuse to form a diploid
zygote that develops into a diploid sporophyte plant. Most algae, fern, mosses
and some vascular plants go through these separate phases in their life cycle.
The
two important features of sexual reproduction are meiosis (production of
haploid gametes to fuse with a different haploid gamete and crossing-over
during this process) and syngamy (fusion of two haploid gametes to produce a
diploid zygote). These features result in production of offspring significantly
varied and rather different from the parents. In asexual reproduction, however,
the parent produces progeny that are exact genetic replicas of themselves.
Despite the expectation that sex should produce greater genetic diversity,
asexual species may have very high levels of heterozygosity for unknown
reasons. Asexual reproduction is generally favored when a species lives in a
stable environment. The exception of sexual reproduction resulting in haploid
offspring is what is seen in social insects such as wasps and bees. In the
situation called 'haplodiploidy', male offspring are haploid as they result
from unfertilized eggs, and females are diploid -the product of fertilization
of an egg by a sperm-. This creates a situation that sisters are genetically
75% identical to each other as they all get the same haploid set from their
fathers and either haploid set from their mothers. This has important
implications in altruism (see Hamilton's rule in the introduction).
One
consequence of sexual reproduction is anisogamy (the existence of two kinds of
gamete, eggs and sperm) which is the basis of morphological and behavioral
differences between the sexes (see sexual selection).
Modes of reproduction
Mitotic
parthenogenesis
Sexual
parthenogenesis
Self-fertilizing
hermaphroditism [simultaneous or sequential]
Sex
with polyembryony
Inbreeding
sex
Outbreeding
sex
Inbreeding: Despite sexual reproduction, inbreeding
lowers offspring fitness. This is called inbreeding depression. This occurs
because of the presence of deleterious recessive mutations in populations.
Inbreeding depression is due to homozygous offspring expressing deleterious
alleles. In humans, for example, it is estimated that each individual caries
three to five recessive lethal genes. In humans, >40% of progeny born to
full sib matings either die before reproductive age or suffer severe
disabilities (May RM. When to be incestuous. Nature 1979;279:192-4;
see also other references). One theoretically possible
advantage of inbreeding is that it reduces the cost of recombination. Mechanisms
evolved to prevent inbreeding include differential dispersal by two sexes,
similarly for plants, differential seed dispersal by the two sexes and the
self-incompatibility (SI) system, and MHC-disassortative
mating preferences (Penn DJ & Potts WK,
1999). The most extreme form of inbreeding, self-fertilization, however, still
occurs in many plants. This usually occurs in monoecious plants in which both
male and female flowers exist on the same plant. A mechanism evolved to prevent
inbreeding in plants is dioecious plants which have only one kind of flower
(male or female) on each plant. Other mechanisms have evolved to prevent
self-fertilization in monoecious plants: differences in flowering times of male
and female flowers, in hermaphrodites with perfect flowers, male and female
parts of the flower are physically separated, and the self-incompatibility
system which reduces the viability of self-fertilized embryos.
Self-incompatibility, is a genetically controlled mechanism to prevent
inbreeding in plants. The SI loci prevent matings with self and also reduce
matings with close kin; when pollen and maternal tissue share an
incompatibility allele, fertilization is prevented, thereby enforcing
disassortative matings (Haring V, 1990). The secreted glycoproteins encoded by
the SI loci are envisaged to interact with a pollen component to cause arrest
of pollen tube growth. This finding that some plants avoid inbreeding through
disassortative mating preferences controlled by a highly polymorphic
self-incompatibility system provides a striking precedent for the evolution of
MHC-disassortative mating to avoid inbreeding in vertebrates (Jordan WC &
Bruford MW, 1998a & 1988b; Penn DJ & Potts WK,
1999). There seems to be no mechanism to counteract outbreeding depression.
Inbreeding
leads to an increase in homozygosity at all loci because the breeding pairs are
initially genetically more similar to one other than would be the case if a
pair of individuals had been taken at random from the population. Inbreeding
distributes genes from the heterozygous to homozygous state. Thus, homozygosity
increases without any change in allele frequencies.
Although
inbreeding may be harmful to a species, sometimes outbreeding may also cause
depression. It appears that there is an optimum amount of inbreeding and
outbreeding. Very close relatives (including self) and totally unrelated
individuals are unsuitable as mates, but medium to close relatives should be
preferred. In plants, fertilization by donors of plants about 10m distant from
them generally yields higher numbers of seeds than self-fertilization,
fertilization with near-neighbors, or fertilization with plants further away
than 10m.
Sexual reproduction in plants: The advantages
of cross-pollination are such that plants have evolved elaborate mechanisms to
prevent self-pollination and to have their pollen carried to distant plants.
Many plants avoid self-pollination by producing chemicals (products of the SI
system) that prevent pollen from growing on the stigma of the same flower, or
from developing pollen tubes in the style. Other plants, such as date palms,
some orchard trees, and stinging nettles, have become dioecious, producing only
male (staminate) flowers on some plants and female (pistillate) flowers on
others. Some plants are dichogamous - that is, the pistil ripens before or
after the stigma in the same flower becomes receptive. See also Plant Genetics.
Summary
Advantages of sex: slower rate of reproduction but faster
evolution, lower extinction rates, fast removal of deleterious mutations and
better adaptation to host-parasite arms race.
Disadvantage of sex: cost of recombination, cost of mating
(competition, no mating, wrong mating), cost of males.
Asexual reproduction: generates genetically identical progeny,
conservation of harmful mutations (but also favorable genotypes), new genotypes
are generated only through mutation.
Please update your
bookmark: http://www.dorak.info/evolution/sreprod.html
The abstract of a review on Advantages of Sexual Reproduction
A full-text book by H-R Gregorius on Mating Systems
Matt Ridley's book: The Red Queen: Sex and the Evolution of Human Nature
PBS Programs: Nature
of Sex Evolution: Sex
Encyclopedia Britannica article on Sexual Reproduction (subscribers only)
M.Tevfik Dorak, MD, PhD
Last updated 9 January 2007
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Genetics HLA MHC Inf & Imm Genetic
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