The evolutionary causes and consequences of polyandry in Nasonia vitripennis

Abstract

Explaining the ubiquity of and variation in female multiple mating remains a key problem in evolutionary ecology. Much of the empirical work regarding the costs and benefits of mating has for obvious reasons relied on species that are already polyandrous. In this thesis I investigate the costs and benefits of polyandry in the parasitoid wasp Nasonia vitripennis, which is mostly monandrous in the wild, but that evolves polyandry under laboratory culture conditions. This has given me the rare opportunity to explore what initiated the evolution of polyandry, rather than what maintains it. In N. vitripennis polyandrous females gain a fecundity benefit when mating with virgin males. However mating also results in a sex allocation cost. Females produce more male biased sex ratios immediately after mating resulting in reduced fitness. I found no evidence that this sex allocation cost occurred because males block, displace or incapacitate the sperm of their rivals, instead it appears that this cost represents a female physiological constraint. Additionally, N. vitripennis females are more likely to re-mate when hosts are available and male density is high. However, mating under these conditions does not mitigate the costs of harassment, arguing against convenience polyandry. My results demonstrate that changing the local mating structure alters the economics of mating. In the wild females oviposit alone and maximise their fitness by laying mostly daughters. In laboratory culture many females oviposit together which reduces selection on adaptive sex allocation so the ‘sex allocation cost’ of polyandry is lifted. Furthermore, less female-biased sex ratios in the laboratory increase the opportunity for polyandrous females to benefit from re-mating with virgin males. My work highlights how studying monandrous species could be key to understanding why polyandry is so ubiquitous in nature, studying the exceptions may in this case help us to understand the rule.