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'.

Experimental trypanosomiasis of natural hybrids between house mouse subspecies.

This study characterises the extent of the susceptibility to parasites (first demonstrated with helminths) of hybrids between Mus musculus domesticus and Mus musculus musculus. Experimental infections with Trypanosoma musculi of M. m. domesticus, M. m. musculus and their natural hybrids have been performed to compare their level of resistance/susceptibility. It appears that contrary to the results with helminths, hybrid mice present the same level of resistance/susceptibility to the trypanosome as M. m. musculus and M. m. domesticus individuals. This result is interpreted in the light of the modalities of host parasite interactions and leads us to hypothesise on the role of parasitism in the evolution of the house mouse hybrid zone.

Comparison between patterns of pinworm infection (Aspiculuris tetraptera) in wild and laboratory strains of mice, Mus musculus.

Sixteen laboratory and 7 wild-derived strains of mice were infected with the pinworm Aspiculuris tetraptera in order to compare their resistance levels estimated by the intestinal parasite loads. It appears that (i) in 4 strains out of 23, females and males harbour different parasite loads; (ii) wild and laboratory mice display a broad range of infection levels when compared independently; (iii) the laboratory strains are more resistant than the wild ones. We suggest that (i) compared to sex, the strain (i.e. genetic) effect is the main parameter which determines the levels of infection; (ii) resistance was selected in laboratory strains during their breeding because of the parasite pressure present in captivity.

Assortative pairing in Gammarus insensibilis (Amphipoda) infected by a trematode parasite.

We have investigated the influence of Microphallus papillorobustus (Trematoda) on the reproductive biology and mating patterns of its intermediate host Gammarus insensibilis (Amphipoda). Infected Gammarus species show altered behaviour which renders them more susceptible to predation by Charadriiform birds, the parasite's definitive hosts. In a natural population of G. insensibilis, mean parasite intensity was higher for unpaired individuals than for paired individuals. Fecundity was reduced in infected amphipods. Size-assortative pairing was significant, although infected males were found with smaller females compared to uninfected males of the same size. There was also a positive assortative pairing by parasitic prevalence. Vertical segregation between infected and uninfected individuals, male-male competition for access to uninfected females, and female choice may explain assortative mating for prevalence. This study provides the first empirical evidence that parasites can have a direct effect on patterns of mating in gammarids.