Male extinction prevented by promiscuous females

This selfish genetic element (SGE) replicates itself by killing sperm that carry the Y chromosome, but has the side-effect that males with the gene produce fewer sperm. Female fruit flies take advantage of this weakness by having several mates, increasing the chances that SGE-free males, with a higher sperm count, will father their offspring.

The collaborative team, headed by Professor Nina Wedell from the University of Exeter, proved a correlation between the number of mates and the prevalence of the gene by collecting samples of fruit flies from seven sites in the United States.

It was already known that southern populations of the fly had a higher prevalence of the selfish sex ratio distorter compared to those further north, so the team, which also includes academics from the Universities of Liverpool, Leeds and Madrid, theorised that this was caused by the northern females being more polyandrous.

Professor Wedell, from the Centre for Ecology and Conservation at the Penryn Campus in Cornwall, said: “This fascinating research gives us new insight into how female flies have developed effective behavioural mechanisms to help ensure the continued production of males.

“We know that selfish genes exist in a number of organisms, where they can reach high frequencies. Female multiple mating appears to be a most effective behaviour to bias paternity against males carrying SGEs as they have low sperm counts and are less likely to father offspring.”

“This research shows that these populations are being kept in existence by a combination of females with a high sex drive and males with an intact sperm count.”

Evolutionary biologist Dr Tom Price from the University of Liverpool’s Institute of Integrative Biology said: “If this particular SGE was to spread to every male in a population, then no more males would be born, and the population, or even the whole species, would go extinct.”

Dr Price and Prof Wedell tested this theory in the lab with the flies captured in the USA and found that when females were only allowed to mate once, the selfish X chromosome spread rapidly through populations to drive them extinct in only nine generations due to lack of males.

However, although southern females could have multiple partners, flies captured in northern populations were much more likely to mate more often. They were also prepared to mate again much more quickly than southern flies.  Put together, this behaviour was more effective at limiting the spread of the SGE.

The paper, ‘Does polyandry control population sex ratio via regulation of a selfish gene?’ was published in the journal, Proceedings of the Royal Society B.