A controversial evolutionary feature has been demonstrated using a gene that allowed insects to survive dichlorodiphenyltrichloroethane (DDT). The discovery provides an example of something biologists have long suspected, but found surprisingly hard to demonstrate, and could help us control insect pests.
Sexual antagonism occurs when a trait is beneficial to one sex, but not the other, and it can be seen in many forms.
Wide hips help give birth safely, but can hinder running ability. For women, the advantages up to a point outweigh the disadvantages, but men get no benefit. Yet if there is a gene variation (allele) for wide hips, women will bequeath it to sons as much as daughters.
Such antagonism should generate a balance of alleles in a population. Potentially, this could benefit the species as a whole, since genetic variation provides flexibility in the face of changing circumstances. However, a paper in BMC Biology points out, “The maintenance of genetic variation through sexually antagonistic selection is controversial, partly because specific sexually-antagonistic alleles have not been identified.”
However, the paper continues, “The Drosophila DDT resistance allele (DDT-R) is an exception. This allele increases female fitness, but simultaneously decreases male fitness.” This may explain why both alleles survived.
The authors verified this idea with computer models and generations of fruit flies. The DDT-R version of the gene Cyp6g1 causes female fruit flies to lay more eggs, and increases the survival chances of their offspring, even in the absence of DDT. It decreases male mating success under normal circumstances, but becomes essential to survival when exposed to the pesticide.
Senior author Professor David Hosken of the University of Exeter said, “Our results show the potential value for insect resistance systems to not only play a part in applied pest management but also shed light on fundamental evolutionary questions."
The only previous study demonstrating the survival of sexually antagonistic alleles used an artificially engineered allele.
Prior to the invention of DDT, the disadvantages for male flies appear to have outweighed the benefits for females, with the resistant allele being fairly rare. Spraying made it nearly universal.
Mid-century DDT was used widely for pest control. Besides the now well documented environmental effects, its overuse wiped out insects that lacked the resistant allele, while resistant individuals flourished with reduced competition. And so, the gene quickly began to dominate.
In Sri Lanka, the chemical became useless against malaria-carrying mosquitoes because resistance was so widespread. Today, DDT is restricted to disease control, both to prevent the environmental damage and to slow the spread of resistance. An understanding of resistance genes could prevent a repetition.
A similar form of sexual antagonism has been proposed as the explanation for the persistence of possible “gay genes”, with evidence that women with similar genes to same-sex attracted men have more children than those who don't. This idea remains disputed, but the latest study makes it more credible.
