Causes and consequences of mating failure in the Lygaeidae

Natural selection should favour fertile individuals who are able to successfully copulate and pass or receive sperm. However a non-trivial number of copulations fail to result in offspring production across a wide range of taxa, despite investment in securing access to mates and their gametes. Over the course of this thesis, I investigate both the causes and consequences of this phenomenon in the seed bug species Lygaeus simulans, in which mating failure rates of up to 60% have been documented. Lygaeids are highly polyandrous and engage in prolonged matings and males possess elongate genitalia which display high levels of inter-specific morphological diversity. Despite this variation, after performing comparative behavioural trials across five related lygaeid species, I found mating failure rates were ubiquitously high. Upon dissection, mating failure was shown to be predominantly caused by sperm transfer failure. Rather than occurring randomly, these mating failures were significantly repeatable within individual males, constituting a male-associated phenotype. Subsequent quantitative genetic analysis revealed genetic variation underlying failure and associated behavioural traits, but no heritability, implying high levels of environmental variation determine this phenotype. Despite the non-random occurrence of mating failure, ‘L. simulans’ females were found to show no pre-copulatory discrimination between previously successful and previously unsuccessful mates. In an effort to establish if cryptic female choice is partially responsible for the rates of mating failure observed I then experimentally manipulated female muscular control during mating using anaesthesia. Female anaesthesia greatly shortened mating duration, but I found little conclusive evidence that females either actively prevent or enable successful male intromission. Uncovering and characterising the presence of such counter-intuitively high and potentially adaptive levels of mating failure challenges out understanding of how natural and sexual selection shape primary sexual function and fertility, and highlights the importance of post-copulatory interactions in successful reproduction