Why Do Many Female Species Prefer Polyandry ?

by Savitha C Muppala on Nov 22 2008 6:39 PM

A new study conducted by British and Japanese researchers may explain the preference of many female species to have multiple partners.

The research team - comprising of experts from the Universities of Exeter (UK), Okayama (Japan) and Liverpool (UK) - say that "polyandry", the practice of mating with multiple male partners, could be the result of females adapting to avoid producing offspring carrying selfish genetic elements that reduce male fertility.

The researchers have revealed that their study was based on the fruitfly Drosophila pseudoobscura, which they bred over 10 generations.

Some males of this species carry a 'selfish gene' on their X chromosome that causes sperm carrying the Y-chromosome to fail, meaning that males carrying this gene can only produce daughters, all of which carry the sperm damaging gene.

In the present study, females evolved to mate with more partners when they were exposed to males carrying this selfish gene. Though there was no way for the females to tell whether or not a potential mate carried the gene, they evolved to re-mate more quickly.

After ten generations, they re-mated after an average of 2.75 days, compared with 3.25 days among the original population.

The researchers said that more frequent mating helped females ensure that sperm from different males compete, and that competition favoured males without the sperm-damaging selfish genes, allowing females to bias paternity against such males.

"Multiple mating by females has puzzled biologists for decades. It's more risky and costs precious time and energy for females. Our study suggests that these significant costs are worthwhile because the female increases her chances of producing healthy offspring of both sexes that do not carry the selfish gene," said corresponding author Dr Nina Wedell of the University of Exeter.

Selfish genes occur at random as a result of mutations, and spread quickly through populations because they subvert normal patterns of inheritance, increasing their presence in the next generation.

The researchers are of the opinion that their findings have relevance for a range of species with polyandrous females, including some primates.

Dr Nina Wedell says: "Selfish genetic elements exist in all living organisms and many compromise male fertility. Our study could provide a new explanation for why polyandry is so remarkably widespread."

The researchers say that they are presently unsure as to what implications their findings might have for understanding human reproduction, but believe that it is possible that some types of male fertility disorder are caused by the manipulation of selfish genes.

A research article on this study has been published in the journal Science.