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Evolutionary biologist studies one of nature’s



By Kelly Foss

When Dr. David Innes, a professor of biology in the Faculty of Science, teaches students about the costs associated with sex he’s not talking about paying for dinner and a movie. Instead he’s referring to an important question in evolutionary biology – why most organisms continue to reproduce sexually, given that there appear to be more advantages for species that reproduce asexually.

For more than 25 years, Dr. Innes’ lab has been looking at this question through his work on the reproductive variation in a species of freshwater crustacean, Daphina pulex, also known as the water flea. This species has both sexual and completely asexual forms and in studying them he has been able to observe both in natural populations and experiments in the laboratory to determine exactly what, if any, advantages there are in sexual versus asexual reproduction.

“Most people are familiar with organisms that reproduce sexually,” said Dr. Innes. “If you tally up plants and animals you will find that in most cases most species are sexual. But there are some plants and animals that have ‘given up’ sex and now reproduce completely asexually.”

There are many reasons why such a “choice” could be made, according to the professor. He says there are many costs, or disadvantages, associated with sex when you compare the two types of reproduction.

“A typical asexual organism is simply a female producing copies of herself, so there is no need to find a mate,” said Dr. Innes. “You can see that would have an advantage. There are lots of risks and energy expended on mating. If a female didn’t have to do that, all of that energy could be reallocated to producing more offspring.”

Added to that is the cost of producing males. Dr. Innes explains that a female which reproduces sexually will have half of her eggs develop into males, leaving only the other half of her offspring, the females, to continue reproducing. Whereas with a completely asexual reproducing female, since she is only copying herself, all of her eggs will also be female and all of those offspring can now go on to reproduce.

“You can see from a demographic point of view, or a population growth point of view, the female that has abandoned sexual reproduction will have an immediate advantage,” said Dr. Innes. “That’s an enormous cost to the sexual organisms and one matter continues to puzzle evolutionary biologists – How can sexual reproduction be maintained if it’s in competition with a female that reproduces asexually?”

The answer may lie in the advantages to sexual reproduction, which are more long term. The major difference between the two forms of reproduction is that a sexual female produces genetically diverse offspring, whereas an asexual female produces a genetically uniform offspring.

“Some people feel that’s a key to solving the riddle,” said Dr. Innes. “There are lots of theories and hypotheses but what it boils down to is that genetically diverse offspring can adapt to a changing environment.

“One model, called the Red Queen model, proposes that organisms are constantly threatened by competitors, predators, parasites and diseases, because they’re evolving as well. There’s sort of an evolutionary arms race which is what the Red Queen is about. As a host evolves a resistance, the parasite evolves to overcome those resistances. So you have this biotic environment that’s evolving, and if you don’t evolve, then you’re doomed.”

Another risk for organisms that reproduce asexually, according to Dr. Innes, is that they are in danger of accumulating mutations. Sex and recombination has been viewed as a repair mechanism for such mutations.

Though there are lots of theories about the advantages and disadvantages of sexual and asexual reproduction, there is little empirical evidence looking at real organisms. That’s where Dr. Innes’ work comes in.

The water flea has both sexual and asexual reproduction and he’s studied the populations in areas where both forms live side by side in the same pond. Over the years he’s watched a particular pond in Southern Ontario to see what happens when an asexual clone invades a sexual population.

Though much can still be learned, the work of Dr. Innes and his students is bringing evolutionary biologists that much closer to answering one of nature’s greatest mysteries.
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