Susceptibility of natural hybrids between house mouse subspecies to Sarcocystis muris.

A previous study showed that the susceptibility of hybrids between two Mus musculus musculus and Mus musculus domesticus did not apply to every parasite. The authors proposed that only parasites which exerted enough constraints would induce the selection of poorly compatible systems of resistance in the subspecies. This study completes the previous work. Experimental infections of mice of the two subspecies and their hybrids with the tissue-dwelling protozoan Sarcocystis muris show that hybrids are more susceptible to the coccidian than M. m. musculus and M. m. domesticus individuals. This result demonstrates that the hybrids are not only susceptible to intestinal helminths, and confirms the 'constraint hypothesis'.