Host - parasite interactions: : the relationship between encystment load of the freshwater pearl mussel (Margaritifera margaritifera) and genetic diversity of its host (Salmo trutta)

The freshwater pearl mussel (FPM, Margaritifera margaritifera) is an endangered bivalve mollusc with an obligatory larval parasitic phase on brown trout (Salmo trutta). The FPM has declined throughout its entire range due to causes such as habitat degradation, eutrophication, acidification, changed hydrology and lack of host fish. This study aimed to investigate if heterozygosity, allelic richness, number of alleles, inbreeding and differentiation of brown trout (Salmo trutta) are related to the encystment load of glochidia larvae of the FPM. The results showed that the observed heterozygosity was negatively related to encystment load. This may be a result of the fact that high heterozygosity is generally associated with a strong immune system that wards off parasities. High heterozygosity may also be an advantage for the host, because it results in a larger potential for selection against the parasite in the host-parasite arms race. There was also a positive relationship between the inbreeding coefficient and glochidia encystment. This makes sense since high inbreeding is negatively related to heterozygosity. High inbreeding of trout should thus reduce the ability of inbred trout populations to ward off parasites. The results may have implications for conservation management. Introductions of foreign trout strains that alter the genetic interactions between the host fish and the parasitic mussel should be managed. Small and fragmented trout mussel populations, with typically high inbreeding coefficients, may have a relatively high probability to reproduce and survive and it may therefore be worthwhile to protect these trout populations. However, inbred trout populations should be managed with care, since inbreeding increases the risk of extinction of the trout. If a trout population is extinct, their sympatric mussel population may thus face extinction as well.