Heredity of specific host-finding behaviour in Schistosoma mansoni miracidia

Two strains of Schistosoma mansoni were used to investigate the hereditary basis of species-specific host recognition by analysing behavioural responses of miracidia to snail-conditioned water. An Egyptian strain of S. mansoni, capable of distinguishing its host snail Biomphalaria alexandrina from other snails was cycled repeatedly through Biomphalaria glabrata, the intermediate host of a Brazilian strain known to respond even to non-susceptible snails with high intensity. After 5 cycles in the non-natural host, miracidia of the Egyptian strain still retained their preference for the original host snail. In a second experiment, host-finding behaviour of hybrids between these two parasite strains was studied. In the F1 generation, hybrids of both parental combinations showed the same low degree of specificity as the pure-bred Brazilian strain. Approximately one quarter of F2 hybrids proved to be as discriminatory as the Egyptian strain, confirming dominant Mendelian inheritance of non-specificity in schistosome miracidial host-finding behaviour. Moreover, hybrids seem to have lost the ability to develop in B. alexandrina, possibly suggesting a link between host recognition and host compatibility. The heredity of this behavioural trait is of evolutionary and epidemiological significance, since a shift to low host-finding specificity might have been a prerequisite for S. mansoni to acquire new host snails after being introduced to South America by the slave trade.