The evolution of mating systems in black scavenger flies (Diptera: Sepsidae)

Abstract

Black scavenger flies are characterized by sexual behaviours that are very unusual in insects. I have studied two of the most remarkable elements of their mating systems: the timing of copulations immediately after an oviposition bout (post-oviposition matings) and the males' escorting of ovipositing females. In a study of the patterns of sperm precedence in one sepsid species, I found that the sepsids' peculiar timing of matings is not associated with unusual patterns of sperm precedence: sepsid males displace rival sperm and achieve a large last male advantage, which is the most common outcome of sperm competition in insects. I discuss the potential significance of sperm transfer mechanisms for the sepsids' timing of matings, and I consider factors that may favour the maintenance of post-oviposition matings in sepsid populations. In a survey of sepsid mating patterns, I found that post-oviposition matings are typical of many black scavenger flies and that mating systems characterized by the absence of copulations with gravid females may have arisen early in the family's evolutionary history. In several black scavenger flies, ovipositing females are commonly accompanied by an escorting male, and in all but one of the species I have studied, escorting is pre-copulatory. In several species, I found pronounced geographic variation in the expression of this trait. I argue that sepsids share certain characteristics which may have facilitated multiple independent origins of escorting behaviour. In order to investigate the adaptive significance of escorting, I have conducted a comparative study of patterns of sexual size dimorphism and sex ratios at oviposition sites in conspecific populations that show great divergence in the expression of this trait. The results of this research support the pre-copulatory mate guarding hypothesis for the adaptive significance of escorting behaviour, and they suggest that conspecific populations vary significantly in the degree or nature of sexual selection acting both on morphology and behaviour of males. Furthermore, in a study of the genetic architecture of escorting behaviour, I found that the observed behavioural variation has a genetic basis: the expression of escorting behaviour is a quantitative trait with a significant sex-related component of inheritance.