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Can promiscuity save a species?

Feb. 25, 2010
Courtesy of University of Exeter
and World Science staff

Pro­mis­cu­ous fe­males may be key to a spe­cies’ sur­viv­al, at least among cer­tain fruit flies, ac­cord­ing to a study pub­lished in the Feb. 25 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy.

The study could solve the mys­tery of why fe­males of most spe­cies have mul­ti­ple mates, de­spite this be­ing more risky for the in­di­vid­ual, sci­en­tists said. Known as pol­y­an­dry among sci­en­tists, fe­male promiscu­ity is nor­mal across the an­i­mal king­dom, from in­sects to mam­mals. 

Dro­so­phila pseudo­ob­s­cu­ra mat­ing. (Cour­tesy U. of Ex­e­ter)


In the fruit flies, re­search­ers found, pol­y­an­dry helps pre­vent ex­tinc­tion be­cause with­out it, in­creas­ing num­bers of all-fe­male broods are born. This oc­curs as a re­sult of a chro­mo­some called the “sex-ratio dis­tor­tion chro­mo­some,” said the stu­dy’s au­thors, Ni­na Wed­ell of the Uni­vers­ity of Ex­e­ter, U.K. and col­leagues.

The chro­mo­some, when car­ried by a ma­le, causes all of its sperm with male genes to die be­fore they can fer­ti­lize an egg, Wed­ell ex­plained. The all-fe­male off­spring will car­ry the same chro­mo­some and pass them on to their sons, who will pro­duce more all-fe­male broods. Even­tu­ally all ma­les will die out, and with them the popula­t­ion.

Sev­er­al fruit fly spe­cies have the chro­mo­some in some form.

Wedell and col­leagues worked with the spe­cies Dro­soph­i­la pseu­do­ob­scura. They gave some popula­t­ions the op­por­tun­ity to mate nat­u­ral­ly, so that fe­males had mul­ti­ple part­ners. The oth­ers were re­strict­ed to hav­ing one mate each. They bred sev­er­al genera­t­ions of these popula­t­ions. 

Over 15 genera­t­ions, five of the 12 popula­t­ions that had been mo­nog­a­mous be­came ex­tinct af­ter males died out, Wed­ell and col­leagues re­ported. But popula­t­ions with free-loving fe­ma­les avoid­ed this fate; sex-ratio dis­tor­tion chro­mo­somes were far less com­mon among them.

Female promiscu­ity saves the day be­cause males car­rying the dan­ger­ous chro­mo­some pro­duce only half as many sperm as nor­mal ma­les, the sci­en­tists not­ed. When a fe­male mates with mul­ti­ple ma­les, their sperm will com­pete to fer­ti­lize her eggs. The few sperm pro­duced by males car­rying the sex-ratio dis­tor­tion chro­mo­some are out-com­peted by the sperm from nor­mal ma­les, and the dis­tor­tion chro­mo­some can­not spread.

“We were sur­prised by how quickly – with­in nine genera­t­ions – a popula­t­ion could die out as a re­sult of fe­males only mat­ing with one part­ner,” Wed­ell said. “Polyandry is such a wide­spread phe­nom­e­non in na­ture but it re­mains some­thing of an enig­ma for sci­en­tists. This study is the first to sug­gest that it could ac­tu­ally save a popula­t­ion from ex­tinc­tion.”


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Promiscuous females may be key to a species’ survival, at least among certain fruit flies, according to a study published in the Feb. 25 issue of the research journal Current Biology. The study could solve the mystery of why females of most species have multiple mates, despite this being more risky for the individual, scientists said. Known as polyandry among scientists, female promiscuity is normal across the animal kingdom, from insects to mammals. In the fruit flies, researchers found, polyandry helps prevent extinction because without it, increasing numbers of all-female broods are born. This occurs as a result of a chromosome called the “sex-ratio distortion chromosome,” said the study’s authors, Nina Wedell of the University of Exeter, U.K. and colleagues. The chromosome, when carried by a male, causes all of its sperm with male genes to die before they can fertilize an egg, according to the group. The all-female offspring will carry the same chromosome and pass them on to their sons, who will produce more all-female broods. Eventually all males, and the population, die out. Several fruit fly species have the chromosome in some form. Wedell and colleagues worked with the species Drosophila pseudoobscura. They gave some populations the opportunity to mate naturally, so that females had multiple partners. The others were restricted to having one mate each. They bred several generations of these populations, so they could see how each fared over time. Over 15 generations, five of the 12 populations that had been monogamous became extinct after males died out, Wedell and colleagues reported. No populations with free-loving females, where sex-ratio distortion chromosome chromosomes were far less prevalent, met this fate. Female promiscuity saves the day because males carrying the dangerous chromosome produce only half as many sperm as normal males, the scientists noted. When a female mates with multiple males, their sperm will compete to fertilise her eggs. The few sperm produced by males carrying the sex-ratio distortion chromosome are out-competed by the sperm from normal males, and the distortion chromosome cannot spread. “We were surprised by how quickly – within nine generations – a population could die out as a result of females only mating with one partner,” Wedell said. “Polyandry is such a widespread phenomenon in nature but it remains something of an enigma for scientists. This study is the first to suggest that it could actually save a population from extinction.”