Molecular evolution of candidate genes involved in post-mating-prezygotic reproductive isolation.

Traits involved in post-copulatory interactions between the sexes may evolve rapidly as a result of sexual selection and/or sexual conflict, leading to post-mating-prezygotic (PMPZ) reproductive isolating barriers between diverging lineages. Although the importance of PMPZ isolation is recognized, the molecular basis of such incompatibilities is not well understood. Here, we investigate molecular evolution of a subset of Drosophila mojavensis and Drosophila arizonae reproductive tract genes. These include genes that are transcriptionally regulated by conspecific mating in females, many of which are misregulated in heterospecific crosses, and a set of male genes whose transcripts are transferred to females during mating. As a group, misregulated female genes are not more divergent and do not appear to evolve under different selection pressures than other female reproductive genes. Male transferred genes evolve at a higher rate than testis-expressed genes, and at a similar rate compared to accessory gland protein genes, which are known to evolve rapidly. Four of the individual male transferred genes show patterns of divergent positive selection between D. mojavensis and D. arizonae. Three of the four genes belong to the sperm-coating protein-like family, including an ortholog of antares, which influences female fertility and receptivity in Drosophila melanogaster. Synthesis of these molecular evolutionary analyses with transcriptomics and predicted functional information makes these genes candidates for involvement in PMPZ reproductive incompatibilities between D. mojavensis and D. arizonae.

Proximate and ultimate control of sex ratios in Myrmica brevispinosa colonies.

The literature on sex ratio evolution in ant colonies is dominated by inclusive fitness arguments. In general, genetic theory makes good predictions about sexual investment in ant populations, but understanding colony-level variance in sex investment ratios has proven more difficult. Recently, however, more studies have addressed ecological factors that influence colony-level sex investment ratios. Food availability, in particular, has been manipulated because larval nutrition influences female caste determination, thus implying that resource availability should be of critical importance for colony sex investment ratios. However, results from food supplementation experiments are equivocal, and it is clear that ant colony response to food supplementation is dependent on the ecological background of the population. We presented field colonies of the ant Myrmica brevispinosa with two food types (proteins and carbohydrates), and assessed their relative impact on colony-level sex investment ratios. We show that colonies receiving carbohydrate enhancement invested in more female sexuals and produced more female-biased sex allocation ratios than protein-fed or control colonies. Thus, our study is the first, to our knowledge, to demonstrate that sex ratios in social insect colonies might be sensitive to resource quality. Male investment was not influenced by food treatment, but was positively correlated with colony size. Therefore, the shift in sex ratios in our study must have been mediated through nutritional influences on female caste determination rather than male brood elimination. We also used our data to evaluate evidence for sex ratio compensation by queenright colonies in response to male production by workers from orphaned colonies.