The evolutionary biologist Olivia Judson wrote, “The battle of the sexes is an eternal war.”
In most animal species, the costs associated with reproduction differ between the sexes: females often benefit most from producing high-quality offspring, while males often benefit from mating with as many females as possible. As a result, males and females have evolved profoundly different adaptations to suit their own reproductive needs. So, how does selection act on the nervous system to produce adaptive sex-differences in behaviour within the bounds set by physical constraints, including both size and energy, and a largely shared genome?
A new study from the Goodwin group led by Dr Tetsuya Nojima and Dr Annika Rings, offers a solution to this long-standing question by uncovering a novel circuit architecture principle that allows deployment of completely different behavioural repertoires in males and females, with minimal circuit changes.
In the vinegar fly, Drosophila melanogaster, males compete for a mate through courtship displays; thus, the ability to chase other flies is adaptive to males, but of little use to females. A female’s investment is focused on the success of their offspring; thus, the ability to choose the best sites to lay eggs is adaptive to females. When investigating the different role of only four neurons found in the brain of both male and female flies, the researchers found that sex differences in neuronal connectivity routed sensory information in a sex-specific manner, with males receiving visual inputs and females receiving primarily olfactory (odour) inputs. Importantly, they demonstrated that this dimorphism leads to sex-specific behavioural roles for these neurons: visually guided courtship pursuit in males and communal egg-laying in females. These small changes in connectivity between the sexes allowed for the performance of sex-specific adaptive behaviour most suited to these reproductive needs through minimal modifications of shared neuronal networks. This circuit principle may increase the evolvability of brain circuitry, as sexual circuits become less constrained by different optima in male and females.
This work has been supported by funding from the Wellcome Trust and Biotechnology and Biological Sciences Research Council (BBSRC).
The full paper, “A sex-specific switch between visual and olfactory inputs underlies adaptive sex differences in behavior”, joint-first authored by Dr Tetsuya Nojima and Dr Annika Rings, is available to read in Current Biology.
Read more on the Oxford Science Blog: "Males and females are programmed differently in terms of sex."