Author(s): Hollis, B, Kawecki TJ, Keller L, Yan Z, Houle D
Many genes have evolved sexually dimorphic expression as a consequence of divergent selection in males and females. However, the degree to which evolution can shape gene expression independently in each sex is controversial. It is possible to directly test whether genetic constraints have prevented sex-specific optima from being obtained by enforcing a monogamous mating system, which eliminates both female choice and male-male competition. This relaxed selection on males should result in evolution towards female-specific optima for any genes previously under antagonistic selection in males. Here we demonstrate broad constraints on the evolution of sexually dimorphic gene expression by maintaining populations of Drosophila melanogaster for 65 generations under strict monogamy. Compared to polygamous controls, monogamous populations evolved lower expression of male-biased transcripts and higher expression of female-biased transcripts. This pattern of feminization was present in both sexes, evident throughout the genome and across tissue types, and strongest for genes located on the X chromosome, a known hotspot for sexually antagonistic genetic variation. The breadth of the evolutionary response to monogamy indicates that sexually antagonistic selection is ubiquitous in the genome and likely to play a major role in maintaining genetic variation, driving the evolution of sex chromosomes, and reducing population-level fitness.
Predatory cannibalism involves hunting live conspecifics of size similar to (or larger than) oneself. Although this is risky and is thought to require specialized behavioural and morphological adaptations similar to those in predatory animals, instances of such predatory cannibalism have been reported in numerous non-carnivorous species. It remains unclear if such predatory cannibalism has adaptive significance, while research into its genetic and sensory basis has been hindered by the lack of a suitable model system. Here we report predatory cannibalistic behaviour in Drosophila melanogaster larvae and address its evolutionary significance. We found that groups of younger larvae regularly attack and consume larger healthy conspecific larvae. Aggregations of cannibalistic larvae are mediated by strong attraction to chemical cues from the attacked victim. The nutrition obtained through cannibalism is significant: we show that Drosophila can complete their egg-to-adult development on an exclusively cannibalistic diet, and that this diet induces phenotypic plasticity of the mouth hooks. Finally, during 118 generations of experimental evolution, replicated populations maintained under larval malnutrition evolved enhanced propensity towards cannibalism, in addition to changes in their larval-foraging path length (sitter-like behavior) and competitive ability. These results indicate that predation on conspecifics in Drosophila involves specific adaptations, has a survival value, and can rapidly evolve in response to nutritional conditions. Thus, even in non-carnivorous animals natural selection can promote cannibalistic behaviour. This discovery additionally changes the perspective on many aspects of behaviour, ecology and evolution of Drosophila and simultaneously provides an extensive model for investigating the neuronal, ecological and evolutionary aspects of cannibalistic behaviour.