Variety ‘may be the catalyst behind sexual evolution’

LONDON - Variety may well be the spice of sexual evolution that encourages a move away from asexual reproduction, according to a study from the University of Toronto in Ontario, Canada.

The study shows that a species of rotifer that reproduces both sexually and asexually, Brachionus calyciflorus, opts for sexual reproduction more often in varied habitats than in homogeneous settings, suggesting that varied surroundings may have contributed to the evolution of sex.

“We could actually see how the investment in sex changed over time, so we could test predictions about the evolution of sex, giving us an idea of which conditions favoured it,” Nature quoted Lutz Becks as saying.

Rotifers are tiny freshwater planktonic animals, and B. calyciflorus generally favours asexual reproduction. But sometimes, when conditions are right, the species will partake in sexual reproduction.

The researchers found that after 12 weeks, 7percent of eggs in the homogeneous environment were the dark-coloured eggs produced by sexual reproduction, instead of the opaque eggs produced when the organisms reproduce asexually. But 15 percent of the eggs produced by rotifers in the heterogeneous environment were of sexual origin, suggesting that the varied environment increases the propensity for sexual reproduction.

However, the rate of sexual reproduction seems to be dropping. The researchers say the results indicate that the benefits of sexual reproduction outweigh its costs when the surroundings are varied, but in constant environments sex is not advantageous enough to be a viable strategy - reproducing clonally makes more sense.

Brian Charlesworth, an evolutionary geneticist at the University of Edinburgh, UK, said that it is difficult to tell whether this study explains why sexual reproduction evolved in the first place and why it remains so widespread.

“This may just be a contributory factor - there are many other postulated processes that are not addressed in these experiments. It is suggestive, but I don’t think people are going to go around saying ‘Ah, we now know why sex evolved,’” he said.

Rufus Johnston, a reader in behaviour and evolution at the University of Cambridge, UK, agreed.

“This could just reflect rotifer breeding behaviour rather than telling us anything about the evolution of sex.” (ANI)