Multiple Mating and Sexual Selection

Consistent with many other species (Dewsbury 1982), sperm production is nontrivial in E. kuehniella males. Females may also incur cost from copulation, such as energy costs and disease transfer (Arnqvist & Nilsson 2000). Therefore, both sexes have evolved various strategies to choose mates and control sperm investment or fertilization for maximum reproductive success over the cost of matings (reviewed in Simmons 2001; Snook & Hosken 2004).

My study shows that E. kuehniella males prefer to mate with young, large and virgin females to gain direct benefit in terms of more offspring (Halliday 1983; Andersson 1994) as these females have higher reproductive potential. However, these

General Discussion and Conclusion

high quality (larger and younger) females also represent a greater risk of sperm competition because they are more likely to remate than lower quality ones. Sperm competition theory (Parker 1982, 1990) predicts that males experiencing higher levels of sperm competition risk are selected to increase ejaculate size to win sperm competition. On the contrary, males should produce a smaller ejaculate and conserve sperm for future matings to achieve maximum lifetime reproductive success when facing low risk of sperm competition (Galvani & Johnstone 1998). As a consequence, E. kuehniella males strategically ejaculate more sperm to females when mating with high quality females or under male-biased sex ratio than mating with low quality ones or under female-biased sex ratio.

My study indicates that E. kuehniella females prefer large and mid-aged males for mating regardless of male mating history. However, females that mate with larger and mid-aged males do not gain higher fecundity or fertility. In this species, a female choosing a large mate will have large offspring and thus she will gain indirect genetic benefit because her large sons and daughters possess higher reproductive fitness (e.g. Fisher 1958). Females’ preference for mid-aged males for mating may be because those males are more sensitive to female pheromone than younger or older ones (Calvert & Corbet 1973), rather than because females choose them for higher fecundity or ‘‘good gene’’ (Brooks & Kemp 2001). Females’ non-discrimination between virgin and non-virgin males for mating may be because male mating history generally does not affect female reproductive output in this species.

A male’s reproductive success primarily depends on the number of females he can inseminate (Simmons 2005). Females have also evolved to allow different males to fertilise their eggs from which they gain additional nutrients (reviewed in Arnqvist & Nilsson 2000) and/or genetic benefits (reviewed in Cornell & Tregenza 2007). Multiple mating in E. kuehniella females does not significantly increase their fertility, fecundity and longevity. However, females discriminate against previous mates, and adjust their oviposition patterns depending on whether they encounter new or previous mates after the first copulation and encourage multiple males to fertilize their eggs. These results suggest that E. kuehniella females may mate multiply for genetic benefit in terms of offspring diversity (Cornell & Tregenza 2007). Offspring diversity theory suggests that polyandry benefits females by reducing sib competition (e.g. Robinson 1992), disease transfer (e.g. Tooby 1982), and inbreeding cost (Cornell & Tregenza 2007). E.

General Discussion and Conclusion

kuehniella is a stored-product pest with limited dispersal ability (Rees 2003); each female produces over 300 eggs, which are laid locally within a brief period (> 80% eggs are laid in the first two scotophases), suggesting that sib competition and copulations may be very common in this species. Therefore, offspring genetic diversity should be extremely important for E. kuehniella.

Furthermore, E. kuehniella females are more likely to remate when encountering males are larger than their previous mates. Therefore, in addition to offspring diversity, polyandry may also benefits E. kuehniella females in terms of “good gene” from selecting better males in their subsequent matings (Keller & Reeve 1995).

Similar to many other species (reviewed in Xu & Wang 2010a), the last male to mate achieves a higher fertilization rate in E. kuehniella. This phenomenon has been explained as the result of males’ and/or females’ influence on sperm use for maximum reproductive success (reviewed in Xu & Wang 2010a). The last male sperm precedence in E. kuehniella may be due to sperm displacement at both sperm ejaculation and storage sites, where the second male physically displaces the first male’s spermatophore with his own in the bursa copulatrix and triggers the female to dump resident sperm in the spermatheca. Similar to other species (Drnevich et al. 2000; Takami 2007), the effect of spermatophore displacement on sperm precedence depends on the duration between the two copulations where P2 decreases with the increase of

intermating duration. However, on most occasions (> 85%) when E. kuehniella females accept a second copulation, most sperm from the first spermatophore have already moved to the ductus seminalis and reached the spermatheca. Consequently, the sperm displacement mechanism in the bursa copulatrix alone can not explain the second male precedence in this species. Just before oviposition occurs in the second scotophase about 75% of eupyrene sperm in the spermatheca of twice-mated females are from the second male. In accordance with the sperm component in the spermatheca at this time, P2 is also about 75%, suggesting that offspring paternity is directly related to the

relative number of sperm of the two males in the spermatheca and sperm displacement within spermathecae should be the primary machenism for last male precedence in E. kuehniella.

The mechanisms behind the sperm displacement in spermathecae are still poorly known. Although not observed directly, some authors (e.g. Villavaso 1975;

General Discussion and Conclusion

Hellriegel & Bernasconi 2000) suggest that the resident sperm may be physically displaced from storage by females. I have observed vigorous constricting movements of spermathecae in E. kueheniella, which appear to be able to eject sperm. Sperm displacement within spermathecae may be triggered by post-copulatory female choice, male copulatory manipulation, or both (Snook & Hosken 2004). In Drosophila, sperm dumping is believed to be under female control (Arthur et al. 1998) although males have some effect (Civetta & Clark 2000). My study shows that E. kuehniella females have some control over sperm release, probably by spermathecal contractions (Lum et al. 1981). However, how and to what extent males affect stored sperm displacement in or dumping from spermathecae is still poorly understood. E. kuehniella males might mechanically stimulate the female reproductive tract (e.g. Cordoba-Aguilar 1999) or transfer male accessory gland factors to stimulate spermathecal contractions and thus trigger ejection of stored sperm from spermathecae. Whether male accessory gland factors from the second male are associated with stored sperm ejection remains unknown and warrants further investigation.