Mating interactions of Schistosomes: Biological and epidemiological consequences

The formation of mating pairs by males and females of Schistosoma spp. in the liver or hepatic portal system of their definitive host is a requirement for male and female growth and sexual maturation, but is not always species-specific. Previous studies have shown that in mixed infections of two or more schistosome species, heterospecific pairs will form, leading either to hybridisation or parthenogenesis depending on the phylogenetic distance between the species. In certain species combinations, mating appears to be random: in others, particularly those involving species from different evolutionary lineages, there is a preponderance of homospecific pairs, demonstrating the existence of a specific mate preference system. Experiments were designed to study mating interactions between different combinations of species in mixed infections in mice and hamsters. Data from infections with S. mansoni and S. margrebowiei; S. intercalatum (Zaire strain) and S. mansoni, and S. bovis and S. margrebowiei, revealed a preference for homospecific rather than heterospecific partners. A number of homospecific and heterospecific multiple-worm pairs were also obtained. Reinforcement of assortative mating by the pre-zygotic mating barrier of heterologous immunity was indicated by worm return data from these 3 mating models. The existence of mating competition between the species was demonstrated. Males of all the species were able to actively compete for paired females by pulling them away from their partners, thus effecting a change of mate. In mixed infections of S. margrebowiei and S. mansoni, neither species appeared to be competitively dominant to the other, and the data suggest that where this is the case in sequential infections, the most successful species in terms of worm return and pairing ability will be the first species to infect the host. S. mansoni appeared to be the more competitive species in mixed infections with S. intercalatum (Zaire strain), but to a lesser extent than in previously-studied mixed infections with the Lower Guinea strain of S. intercalatum. This suggests that the Zaire strain of S. intercalatum may be more competitive than the Lower Guinea strain, highlighting an important difference between these cryptic species. There was some indication that S. bovis might be more competitive than S. margrebowiei in mixed infections of these two species. Data from hamsters infected initially with S. haematobium, and subsequently with S. intercalatum (Lower Guinea) demonstrate the competitive dominance of S. haematobium over S. intercalatum (Lower Guinea), and suggest that S. intercalatum (Lower Guinea) is unlikely to become established in areas where S. haematobium is already present. The competitive dominance of S. haematobium males over S. mansoni males is demonstrated by data from hamsters infected initially with both sexes of S. haematobium and subsequently with male S. mansoni. A number of homosexual male pairs were obtained from mixed infections of S. haematobium S. intercalatum (Lower Guinea) and S. haematobium S. mansoni, corroborating previous findings that the clasping of objects by males is neither sex- nor species-specific. An attempt to identify a female-specific RAPD (Randomly Amplified Polymorphic DNA) marker for S. haematobium resulted in the cloning and sequencing of an apparently female- specific PCR product of approximately 700 base-pairs in length. However, subsequent testing of the clone using specific primers revealed that, whilst it is present in smaller quantities in males than females, it is not specific to females, and is therefore of limited practical use as a sex-specific diagnostic tool