Investigating the causes of mating failure in Lygaeus simulans seed bugs

Abstract

We should expect mating systems to evolve to maximise the relative fitness of individuals, i.e. through sexual selection acting on individuals to maximise the number of offspring they produce. However, some mating systems can appear to ‘go wrong’. This is when apparently non-adaptive behavioural phenotypes, which cause fitness losses during reproduction, arise, such as same-sex sexual behaviours, reproductive interference, and mating failure. In this thesis I focus on the phenomenon of mating failure – when copulations fail to result in the production of offspring. Recorded levels of mating failure across taxa are non-trivial, and I here investigate the causes of mating failure in Lygaeus simulans seed bugs, a species known to suffer from high levels of failure (40-60%). I first investigated the genetics behind a pale mutant colour morph, which was inherited in a Mendelian fashion, providing a visible genetic marker for the investigation of post-copulatory selection processes. I found that mating failure caused extreme bimodality in second-male paternity, which could lead to misinterpretations of sperm competition mechanisms. I also found a strong link between copulation duration and mating failure, with longer copulations less likely to fail. Additionally, mating failure decreased when pairs were allowed to copulate for longer, or multiple times, indicating it is not primarily due to infertility or mechanical failures. Instead, evidence points towards mating failure being caused by cryptic male choice (CMC). In terms of post-copulatory outcomes, larger females were consistently more likely to lay eggs, have offspring (and hence not experience mating failure) and have more offspring. Males preferred to copulate, and copulate for longer, with larger females, suggesting CMC. Unfortunately, I was unsuccessful at teasing apart male and female effects to definitively demonstrate that mating failure is caused by CMC, but I believe the methods used could be useful in investigating CMC in the future.