Far from being harmonious, the reproductive relationship between males and females is full of conflict. Differing optimal states for traits shared between the sexes leads to unseen conflict within the very molecular blueprint for life – the genes. Sexual conflict arising in certain alleles – alternative forms of a gene – could help to maintain genetic variation in populations. Research aiming to characterise this process of balancing selection is fundamental to our understanding of evolution. However, the theory has attracted controversy, because specific alleles producing sexual antagonistic selection have not been identified in natural populations… until now. Read More
In the first study of its kind, Professor Nina Wedell and her colleagues characterised a naturally occurring sexually antagonistic allele in fruit fly populations.
The allele increases the productivity of female flies, but reduces the reproductive success of male flies. Flies possessing this allele are also resistant to DDT and other pesticides. This allele was present in wild populations before the introduction of DDT in the 1940s, but its occurrence remained low, despite the reproductive advantage it bestows upon females.
When DDT usage became widespread, the prevalence of the allele rapidly increased in wild fruit fly populations. Although the pesticide was banned around 30 years later due to its harmful environmental effects, almost all wild fruit flies still have at least one copy of the DDT-resistant allele.
Wedell’s team had previously quantified the reproductive effects of the DDT-resistant allele. In this study, they used these results to build a theoretical model, which they applied to experimental populations of fruit flies.
Their results demonstrated that, in the absence of DDT, the frequency of the DDT-resistant allele in the population always shifts toward a stable equilibrium of 34%. This provides evidence for the maintenance of genetic diversity through sexually antagonistic selection.
The results also illustrate why the DDT-resistant allele was maintained at a low frequency in pre-1940s populations of fruit flies, despite the reproductive advantage it gives females, as it confers a cost to males. Populations with very high starting frequencies of 90% of the DDT-resistant allele took much longer to return to the equilibrium value than populations with lower starting frequencies.
The researchers explain that when DDT-resistant males are very common in the population, they are less likely to need to compete with the more successful non-resistant males for mating opportunities. As such, the spread of non-resistant alleles through the population is slow.
This groundbreaking study demonstrates that identifying naturally occurring sexually antagonistic alleles is possible, opening new avenues of research. Wedell and her colleagues have proven that genetic diversity can indeed be maintained by sexual conflict inside the genes. The ramification of this finding of how insecticide resistance alleles evolve in natural fly populations is currently being explored by Wedell and her team, supported by the Australian Research Council.
