Hybridization between Schistosoma mansoni, with a diurnal cercarial emergence rhythm and S. rodhaini, with a nocturnal cercarial shedding pattern leads to F1 and F2 generations, hybrid schistosomes whose chronobiological phenotype of cercariae is characterized by two unequal emergence peaks, one diurnal and the other nocturnal. The relative importance of diurnal and nocturnal peaks depends upon which S. mansoni strain (early or late) is used for the hybridization with S. rodhaini. The results are compared and discussed with those resulting from crosses between intraspecific sympatric and allopatric chronobiological variants (early and late) of S. mansoni. The genetic determinism of the cercarial emergence of schistosomes and the significant differences observed between cercarial shedding patterns of parental species and their hybrids allow the use of this behavioural marker in biological and genetical studies in schistosome populations.
