Abstract
Schistosomiasis is a disease of great medical and veterinary importance in tropical and subtropical regions, caused by parasitic flatworms of the genus Schistosoma (subclass Digenea). Following major water development schemes in the 1980s, schistosomiasis has become an important parasitic disease of children living in the Senegal River Basin (SRB). During molecular parasitological surveys, nuclear and mitochondrial markers revealed unexpected natural interactions between a bovine and human Schistosoma species: S. bovis and S. haematobium, respectively. Hybrid schistosomes recovered from the urine and faeces of children and the intermediate snail hosts of both parental species, Bulinus truncatus and B. globosus, presented a nuclear ITS rRNA sequence identical to S. haematobium, while the partial mitochondrial cox1 sequence was identified as S. bovis. Molecular data suggest that the hybrids are not 1st generation and are a result of parental and/or hybrid backcrosses, indicating a stable hybrid zone. Larval stages with the reverse genetic profile were also found and are suggested to be F1 progeny. The data provide indisputable evidence for the occurrence of bidirectional introgressive hybridization between a bovine and a human Schistosoma species. Hybrid species have been found infecting B. truncatus, a snail species that is now very abundant throughout the SRB. The recent increase in urinary schistosomiasis in the villages along the SRB could therefore be a direct effect of the increased transmission through B. truncatus. Hybridization between schistosomes under laboratory conditions has been shown to result in heterosis (higher fecundity, faster maturation time, wider intermediate host spectrum), having important implications on disease prevalence, pathology and treatment. If this new hybrid exhibits the same hybrid vigour, it could develop into an emerging pathogen, necessitating further control strategies in zones where both parental species overlap.
Highlights
The increasing use of molecular techniques in ecological studies has revealed many cases of hybridization and introgression in plants and animals [1,2,3], but examples in metazoan parasites are rare [2]
No sequence variation was found within the S. haematobium sequences. 0.3%
Back-crossing of the hybrid progeny with one of the parental species resulted in the observed introgression
Summary
The increasing use of molecular techniques in ecological studies has revealed many cases of hybridization and introgression in plants and animals [1,2,3], but examples in metazoan parasites are rare [2]. Hybridization can have a major impact on adaptive radiation and diversification of the species under study [2,4], and in the case of parasites, this may have an impact on the host and the epidemiology of disease. Schistosoma species have a two-host life cycle with an asexual stage within an intermediate freshwater snail host and a sexual stage within the definitive mammalian host; parasite eggs are voided in the urine or faeces depending on the species. Depending on the phylogenetic distance between the two species involved, crossing may lead to parthenogenesis or hybridization, with certain combinations being more viable than others. Experimental hybrids tend to show heterosis: they have a higher fecundity, faster maturation time, higher infectivity, and wider intermediate host spectrum compared to their parental species Experimental hybrids tend to show heterosis: they have a higher fecundity, faster maturation time, higher infectivity, and wider intermediate host spectrum compared to their parental species (e.g. [7,8])
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