Hidden costs of sex

Media Release
March 31, 2005

Hidden costs of sex

An evolutionary biologist at The University of Auckland has answered one of biology’s most puzzling questions, “Why did sex evolve and persist?”

Dr Matthew Goddard at the School of Biological Sciences, within the Faculty of Science, says the question has been a problem for evolutionary biology because, unlike asexual reproduction, sex requires the expenditure of time and energy.

In addition, he says, sexual reproduction disrupts favourable gene combinations and has a “two-fold cost”. This is because in organisms with unequal-sized gametes or reproductive cells, the female transmits genes at only half the rate of an asexual equivalent.

“On the face of it, this means it is much easier for organisms to reproduce themselves asexually,” he said.

Dr Goddard, who recently joined the University from Imperial College, London, set out to uncover the hard evidence to test a 100-year-old hypothesis – the Weismann effect - that sex allows more effective natural selection because it increases genetic variation.

In a paper, published today in “Nature”, Dr Goddard says his research on yeast populations, which can reproduce both asexually and sexually, has shown that sex does in fact increase the rate of a species’ adaptation to a harsh environment, but has no measurable effect on fitness in a benign environment.

Dr Goddard says when yeast cells are supplied with sufficient nutrients they reproduce asexually, but deprivation of nutrients triggers sexual reproduction.

But it has been difficult to study the net effect of sex on the organism’s rate of adaptation, he says, because the yeast cells have been subject to different environmental conditions.

Advances in knowledge about the molecular mechanisms of cell division in yeasts enabled Dr Goddard to develop a new asexual strain of yeast, which continues to reproduce asexually even when deprived of nutrients.

In turn, this enabled Dr Goddard to directly compare the rate of adaptation of the new asexual yeast population with a wild yeast population placed under the same environmental conditions.

Dr Goddard says his results indicate that sexual reproduction provides a selective advantage for adaptation to new environments and support Weismann’s ideas.

“They are an advance on earlier studies because by manipulating the sexual status of the yeast, we have mimicked the natural situation more closely and excluded a variety of possible confounding factors.”

Population experiments with fast-replicating micro-organisms have been very valuable in testing different ideas about the maintenance of sex, he said.

“A challenge now is to understand the nature of the mutations that underlie adaptation and to extend these techniques to larger plants and animals.”

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