Post-copulatory sexual selection in insects: exploring new mechanisms of male and female mate choice
Sexual selection is one of the key evolutionary processes generating and maintaining biodiversity on the planet. Aspects of sexual selection have remained controversial however, especially in terms of mate choice (Rosenthal & Ryan 2022). Recent empirical work in particular has challenged the traditional roles of males and females in sexual selection. For instance, it is now clear that both males and females may compete for access to gametes of the opposite sex, and that males can be as choosy as females (Shuker & Kvarnemo 2021). One of the outstanding challenges now is to integrate these perspectives and re-interpret our understanding of mating systems and the patterns of sexual selection that emerge from them.
This PhD project will explore sexual selection in Lygaeus seed bugs. These colourful insects show counter-intuitive patterns of mating. First, both males and females mate quite frequently, even though we know matings are costly in terms of longevity for both sexes. Second, there is limited pre-copulatory sexual selection in terms of either males or females. At most, larger individuals of both sexes are more likely to copulate, but there is a lot of variation across experiments. Moreover, this limited pre-copulatory choice extends across species barriers, as heterospecific mating attempts (“reproductive interference”) are also quite common. Third, there does appear to be post-copulatory sexual selection however, as a large proportion of matings do not involve successful insemination of sperm (so-called “mating failure”). Longer copulations are more likely to lead to insemination, and indeed longer copulations lead to higher paternity for males when females are multiply mated and thus when there is sperm competition. Crucially though, alongside copulation duration, the only consistent phenotype associated with insemination success is female size, with larger females more likely to receive sperm.
Three questions stand-out: (1) Why non-random mating at the post-copulatory rather than the pre-copulatory stage? (2) Why are larger females more likely to accept/receive sperm? (3) What are the mechanisms associated with (2); for instance, is this male or female post-copulatory mate choice, or both?
The successful applicant will explore post-copulatory sexual selection in Lygaeus simulans, considering both female and male post-copulatory mate choice. We will extend the range of phenotypes studied, taking advantage of recent work exploring the chemical ecology of this aposematic species. The bugs are red-and-black, a warning signal indicating to potential predators their chemical defences. The student will test how diet influences body colour, chemical signatures such as cuticular hydrocarbons, and defensive compounds, and how these in turn influence sexual selection in both males and females. Combined with morphological and physiological studies of copulation and sperm use, this project will generate new insights into how male and female mate choice interact across the whole process of mate finding through to offspring production.