IAP2-22-390

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.

Methodology

The student will use a range of behavioural experiments, combined with study of the reproductive morphology and physiology of the bugs, to better understand whether males or females shape overall insemination success. Further, the student will use chemical ecology approaches to understand how defensive compounds, mediated by diet, shape sexual attractiveness across male and female interactions. There will also be scope for fieldwork to test our findings in natural settings.

Project Timeline

Year 1

Year 1: The chemical ecology of sexual selection in Lygaeus simulans. Introduction to the study system and laboratory experiments. The student will then design and perform experiments mapping CHC profiles to sexual success across pre- and post-copulatory episodes in both males and females. Preliminary work developing techniques for studying male-female genitalic interactions within the female reproductive tract. Identification of relevant training requirements and course attendance.

Year 2

Year 2: Diet and sexual selection in Lygaeus simulans. Building on Year 1, the student will design and develop experiments manipulating diet – including access to natural and artifically-derived toxins – to explore how diet shapes both the chemical signatures and sexual success of males and females. Fieldwork to exlore reproductive success and chemical defences in the wild. Manuscript(s) preparation. On-going professional development courses.

Year 3

Year 3: Combine previous work with experiments linking observed outcomes of mating interactions with internal processes of post-copulatory sexual selection, both in terms of morphology but also physiology, including chemical composition of male ejaculates and female reproductive tract fluid. Manuscript and thesis preparation. On-going professional development courses.

Year 3.5

Manuscript and thesis preparation.

Training
& Skills

The project will provide a number of key training opportunities for the successful candidate: (1) training in experimental design and behavioural techniques, in both the field and the laboratory; (2) chemical ecology techniques; (3) quantitative skills, from data management through to statistical analysis; (4) communication skills, in terms of presenting work to both academic and non-technical audiences through a variety of media. Experience of fieldwork and the ability to drive would be useful for the project.

References & further reading

Balfour, V.L., Black, D. & Shuker, D.M. (2020) Mating failure shapes the patterns of sperm precedence in an insect. Behavioural Ecology and Sociobiology, 74: 25.

Rosenthal, G.G. (2017) Mate choice. Princeton University Press.

Rosenthal, G.G. & Ryan, M.J. (2022) Sexual selection and the ascent of women: Mate choice research since Darwin. Science, 375: p.eabi6308.

Shuker, D.M. & Kvarnemo, C. (2021) The definition of sexual selection. Behavorial Ecology, 32: 781-794.

Shuker, D.M. & Simmons, L.W. (eds) (2014) The Evolution of Insect Mating Systems. Oxford University Press.

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