Male-biased sex ratio: why and what consequences for the genus Schistosoma?

Schistosomes are the cause of the most significant helminth disease of humans. Their unusual sexual biology is intriguing. Instead of being hermaphroditic, as is the rule in other trematode species, they are gonochoric. Furthermore, their mating system is considered to be monogamous, a characteristic shared by only 1% of living species, and their sex ratio is male-biased. In this paper we propose an explanation of the origin of the male-biased sex ratio in schistosomes and highlight the ecological and evolutionary consequences of this bias. We argue that schistosome gonochorism, monogamy and the biased sex ratio can be integrated into a single evolutionary scheme. 2009 Elsevier Ltd. All rights reserved.