(1) Sexual selection and plasticity in Drosophila flies: male olfactory displays as a multivariate trait.
I am currently using quantitative genetic techniques, as well as gas chromatography to examine sexual selection in Drosophila serrata flies, in collaboration with Howard Rundle at the University of Ottawa. In Drosophila flies, males produce olfactory signals that attract females. Males create these complex displays by varying the production of different cuticular hydrocarbons (CHCs). Previous research has identified a specific multivariate combination of CHCs that is most attractive to females. As a signal of male quality that is costly for males to produce, it is predicted that males will produce CHCs in an efficient way: males should produce the most attractive CHCs when there is the highest probability of attracting a female. I have been studying male plasticity in CHC production, and female plasticity in preference for male CHCs (Gershman et al. J Evol Biol 27(6):1279-1286). So far, we have found that male CHC attractiveness is affected by time of day and social environment, with males becoming more attractive during the day than at night. The presence of females increases male attractiveness, while the presence of other males decreases male attractiveness. Further, females are least attractive to males during the times of day that males are the most attractive, suggesting a conflict of interests between males and females (Gershman et al. 2014 Proc R Soc B in press). Experimental results in prep indicate that social environment affects the development of male CHC attractiveness, and female accessibility mediates male CHC plasticity.
(2) Sexual conflict over courtship feeding gifts: spermatophylax composition as a multivariate trait
Males of many taxa are required to contribute courtship gifts to females for successful copulation and sperm transfer. In orthopterans, males often produce large nutritive courtship gifts that they transfer to females, in addition to small packets that contain sperm. In G. sigillatus, these gifts have not been found to help offspring success. Instead, the spermatophylax (edible courtship gift) prevents the female from removing the male sperm packet before most of his sperm has been transferred to her sperm storage organ. Further, due to accessory gland products in the spermatophylax, consumption of the spermatophylax causes females to delay mating again (Gordon et al. 2012 Anim Behav 83:369-375). Therefore the spermatophylax both serves as ejaculate protection, and reduces sperm competition for the male. Females, however, may gain genetic benefits for their offspring from mating with more than one male, thus consuming the spermatophylax may not be in her best interest.
In work with the Sakaluk lab at Illinois State University, I examined the amino acid composition of the spermatophylax using gas chromatography, to study relationships between different amino acids and female willingness to accept and consume a spermatophylax. We found that the amino acid composition of the spermatophylax is highly heritable. Further, female resistance to consuming spermatophylaxes is also highly heritable. This information suggests that there is the potential for natural and sexual selection to act on these traits. Using selection analysis to determine which multivariate combinations of amino acids are most attractive to females, we found that the fitness surface is saddle-shaped, indicating that there is more than one combination of amino acids that results in an attractive spermatophylax. This selection analysis also provides an index of “spermatophylax attractiveness” to females. The heritability of spermatophylax attractiveness is highly significant, and the genetic correlation between spermatophylax attractiveness and female resistance to consuming spermatophylaxes is significant, indicating potential sexual conflict between males and females over the amino acid composition of the spermatophylax (Gershman et al. 2012 Proc B 279:2531-2538; Gershman et al. 2013 J Evol Biol 26(4):693–704).