For male fruit flies, sperm is not enough in the battle to reproduce – at each mating a cocktail of proteins is transferred to the female, many of which directly influence female behaviours in ways that benefit the male. In a new paper, published in this week's PLoS Biology, the online open access journal, a team of biologists from Willie Swanson's lab have investigated these proteins using a new method that is able to identify male proteins in mated females. The work examines the complex seminal soup and shows that these proteins are evolutionarily labile – they change quickly and frequently, a sign of genes under intense selection.
Seminal fluid could be dismissed by the uninitiated as simply a transport medium for sperm, not as interesting the precious genetic information that it helps to ferry. In the 1960's scientists began to examine the seminal fluid of Drosophila – the fruit fly favoured by geneticists - and found that it has an important role in the mating process in its own right. Proteins transferred at mating are important not only for the chaperoning of sperm but also in altering the behaviour of the recipient female in ways that would promote the success of mating. For example, proteins increase the female's rate of egg-laying, and even reduce her interest in mating with other males. Previous work has identified 19 different proteins by examining the reproductive tract of male flies.
This new study takes a different approach – Findlay et al. fed female drosophila food labelled with identifiable carbon atoms. As carbon is incorporated into all proteins, females fed this diet produce labelled proteins. The scientists could then identify, post mating, any unlabelled proteins in the female as having been acquired from the male's sperm. This new method sheds light a larger range of transferred proteins, and also allows researchers to measure their relative abundances. The paper shows that the proteins have evolved swiftly – with closely related Drosophila species having different seminal proteins and with some seminal protein genes having undergone duplication – the potential precursor to the formation of a new gene.
This study opens up exciting new areas for research for scientists studying sexual selection and sexual conflict. Seminal fluid is the battleground between male and female – with the male under a selection pressure to produce proteins that can manipulate the female and the female under selection to resist. Findlay et al reveal a whole new collection of unknown proteins ripe for further study.
Source: Public Library of Science