Sexual selection and the evolution of sex-role reversal in honeylocust beetles

Sammanfattning: Sexual selection is the prime evolutionary force that makes males and females different. This process has long been viewed as one where male compete with one another and where females choose. However, since the discovery that multiple mating by females is common in animals, sexual selection theory has been expanded to include mate competition between females and mate choice by males. However, empirical studies addressing these themes are scarce. In my thesis, I explore the evolution of sex role reversed mating systems using the honey locust beetles (Megabruchidius dorsalis and M. tonkineus). I used these species to shed light on (1) how closely sexual selection in females resembles its better?studied male counterpart, (2) the implications of male mating costs for mating system evolution and (3) the effects of reproductive competition between females on the evolution of female courtship behaviour. By manipulating male mating rate, I found that males that mated more lived shorter lives, showing that mating is costly for males. I also demonstrated that males are choosy about whom they mate with and prefer vigorously courting females (Paper II). In contrast to males, previous studies suggested that female honey locust beetles benefit nutritionally from mating due to the large ejaculates provided by males. I manipulated male condition to show that male adult feeding had significant effects on female reproduction. Females that mated with males of good condition lived longer and produced more offspring than females whose mates were in poor condition (Paper III). When mating is costly for males, theory predicts that sexual selection in females can be strong. I compared sexual selection in honey locust beetles to that in two other species of seed beetles with conventional sex roles. I found substantial sexual selection in honey locust beetle females, which was comparable in strength to that in males (Paper I). I also measured the evolutionary effects of altered sex ratios on mating system parameters in both honey locust beetle species, using an experimental evolution design. Under female-biased sex ratios, representing strong sexual selection in females, females of M. dorsalis rapidly evolved elevated courtship intensity, thereby intensifying the reversal of sex roles (Paper V). In M. tonkineus, males evolved under male-biased sex ratios to transfer larger ejaculates, demonstrating the role of male-male reproductive competition for the evolution of male provisioning (Paper IV). My thesis highlights the essential, and often overlooked, role that females play in mating system evolution and that their contribution cannot simply be reduced to mate choice.