Studies Of Desiccation And Parasitism In Anciently Asexual Bdelloid Rotifers, With Notes On The Role Of Sexual Conflict In Some Human Cultural Practices

Abstract

Asexuality has major theoretical advantages over sexual reproduction. Why, then, do exclusively asexual metazoan lineages rarely endure? The Red Queen hypothesis posits that asexuals are extinguished by relentlessly co-evolving parasites and pathogens. If so, any long-lasting asexual lineage must have unusual alternative mechanisms to deal with these biological enemies. In theory, asexual host organisms could persist by dispersing rapidly among habitat patches independently of parasites. Might this scenario apply to bdelloid rotifers, a class of tiny freshwater invertebrates that abandoned sex millions of years ago? Bdelloids thrive in ephemerally moist microhabitats, withstanding periods of desiccation by contracting into tiny anhydrobiotic propagules that are carried on the wind to colonize new patches. Chapters One and Two demonstrate that lab-cultured populations of three bdelloid species can rid themselves of five deadly fungal parasites during desiccation, especially when dried and allowed to disperse in an artificial wind chamber. Chapter Three shows that natural populations of bdelloids colonize elevated habitat patches more rapidly than other microfauna, and that these wind-founded populations have significantly lower parasite incidences than control populations at ground-level. Results support the hypothesis that desiccation-tolerance and wind dispersal decouple bdelloids from coevolving enemies in space and time, allowing these asexuals to "outrun" the Red Queen. Consequences of sex include conflicts over paternity. In Chapter Four, I argue that sexual conflict underpins a puzzling human cultural phenomenon. A quarter of preindustrial societies practice some form of male genital mutilation (MGM), despite its clear costs. Substantially altering the naturally selected genital morphology is likely to reduce a man's capacity to compete with other males for fertilizations, especially outside the pair-bond. MGM could therefore reduce the ability of young men to challenge the paternity of older, married men. In societies where paternity uncertainty and reproductive conflict are high, elders may be favored to enforce MGM via sanctions and incentives. As predicted by this sexual conflict hypothesis, MGM rituals are highly public, watched mainly by men, and facilitate access to social and material benefits. MGM is more common in polygynous societies, particularly when co-wives reside separately; it is also associated with lower-than-expected incidences of extramarital sex.