When both sexual and asexual reproduction are possible, why should an individual choose to reproduce sexually?
This is a general question, but usually the possibility of a choice is not as clear
as it is in these rotifers. A priori, asexual reproduction would appear to be the
preferred strategy for maximizing the number of individuals carrying the same genes
as the parent -- which is what we expect natural selection to enforce. (In a population
of sexually reproducing animals in a steady state, the net effect of reproduction
by a female will be to produce two reproductive adults in the next generation; each
of these has only a 50% chance of carrying a particular parental gene. If one of these
females chooses to reproduce asexually, the net effect will probably still be production
of two reproductive adults in the next generation, but each of these will have a 100% chance of carrying a particular parental gene. Therefore we might expect a
gene causing asexual reproduction to increase in frequency until the entire population
was reproducing asexually.)
The most popular answer to this question is the following: (Williams & Milton,
J. Theor. Biol. 39: 545, 1973)
Sexual reproduction, which increases variability among an individual's descendents,
is advantageous when the future environment is unpredictable
These rotifers live in small ponds or pools that have a transient existence
and vary greatly in characteristics. If an individual succeeds in reaching reproductive
size, it is probably well adapted to that particular habitat, and if the habitat
remains the same the best reproductive strategy is to produce identical offspring, by
reproducing asexually. However, at the end of the season, or if the pond dries up,
the best strategy is to produce offspring with maximum variability, to increase the
chance that some of them will be successful in whatever environment they find in the future.
Mictic females are usually produced at the end of the summer, and the fertilized
eggs are usually more resistant to dessication and more likely to survive until they can develop in a new pond.
How do the rotifers know that the future will be unpredictable? In one species,
it is known that the differention of mictic females can be induced by vitamin E (a tocopherol), which is more likely to be in a diet of plants than in a diet of small
animals.
In the case of rotifers, the fact that haplodiploidy is so efficient decreases the
genetic "disadvantage" of sexual reproduction. This may be significant in maintaining
the coexistence of the two forms of reproduction. However, sexual reproduction may
be lost completely if its advantages are not continuously selected for. In fact,
there is a major group of rotifers in which only females are known. (Similarly,
there are quite a few species of parthenogenetic fish.)
The coexistence of sexually and asexually reproducing forms in a free-living
animal, as in rotifers, is unusual, and may be correlated with the determination
of sex by haplodiploidy in these animals. However, it is also encountered in the
life cycles of Hydrozoan coelenterates, where sexual reproduction produces dispersed larvae
and asexual reproduction enlarges the local population with identical individuals.
This pattern of course is also common in plants. Even more common is the presence of both sexually and asexually reproducing forms in the life cycles of parasitic
animals. In these cases, continued reproduction on or in a particular host is usually
asexual, giving the maximum rate of reproduction of individuals adapted to that host.
Sexual reproduction usually occurs when it is appropriate to produce individuals
that will find and infect new hosts. In this process, variability is probably an
advantage. Examples are: aphids, Trematoda, and many others.
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