This article essays the implication of this unique physiological characteristic and its consequences on schistosome life history particularly their faithfulness while choosing a mate.
Adult schistosoma lives in the mammalian blood but their life cycle requires a phase of asexual reproduction within a secondary host. The life history of the parasite begins when adult female deposit eggs in the veins surrounding intestine or bladder. The mammal then continues the life cycle by transmitting those eggs through urine or feces. Once in water, the eggs hatch into marcidia, which must find an appropriate snail host. Once inside the snail each marcidium produce several hundreds of carcariea which when released in water seeks the skin of suitable mammals to burrow into. There are many different species of schistosomes of which Schistosoma haematobium, S. mansoni, and S. japonicum clinically important parasites that infect humans. Schistosoma haematobium , commonly called urinary schistosomiasis, dwells in the vesical veins surrounding the urinary tract and therefore mammal host usually excretes its eggs in urine. It is found throughout most of Africa and in parts of Western Asia. Two species, S. mansoni and S. japonicum cause intestinal schistosomiasis. S. mansoni usually occupies the mesenteric veins around the large intestine,while S. japonicum usually occupies the mesenteric veins around the small intestine. The mammalian host excretes the eggs in feces and continues the life cycle.
Figure 1: Life cycle of Schistosoma (Machen, Rogers n.d.)
Gonochorism is a reproductive strategy that describes a sexually reproducing species in which there are two distinct sexes. It is a very unique characteristic exhibited by schistosomes and thus this unusual physiological state managed to raise considerable amount of curiosity amongst parasitologists over the past two decades. Loker (2006) in his most recent study has put forward a hypothesis to address the foundattion of schistosome gonochorism. He says that first, schistosomes were derived from hermaphroditic ancestors; second, the potential for gonochorism appeared in the spirorchiids, most notably with Griphobilharzia; and third, 'true gonochorism' appeared when schistosomes invaded their endothermic hosts (birds and mammals). The evolution of separate genders was supposedly beneficial in providing optimal genetic diversity against the sophisticated immune system of warm-blooded vertebrate hosts. Loker (2006) also suggest that the segregation of two genders was determined by the specialization of each gender for a limited set of 'domestic task' and this was further backed by the gender associated gene expression profiling study by (Hoffman et al. 2002). It was concluded that males primarily looks after the transportation while females concentrate on egg production.
In such an interaction schistosomes, once paired, the male and the female worms remains in this state for a long time and the mating is assumed to be monogamous (Reed 1990). In most of the Schistosomes, pairing is essential for complete female growth and reproductive morphogenesis (Popiel 1986).
Figure 2: A typical schistosoma pairing (Beltran 2008, p.2)
Kunz (2001) demonstrated that virgin females (one sex females) are considerably smaller than paired females and it is already established that egg laying female