The Effects of Mating System and Genetic Variability on Susceptibility to Trematode Parasites in a Freshwater Snail, Lymnaea Stagnalis

Abstract

The amount and distribution of genetic variability in host populations can have significant effects on the outcome of host-parasite interactions. We studied the effect of mating system and genetic variability on susceptibility of Lymnaea stagnalis snails to trematode parasites. Mating system of snails from eight populations differing in the amount of genetic variability was manipulated, and self- and cross-fertilized offspring were exposed to naturally occurring trematode parasites in a controlled lake experiment. Susceptibility of snails varied between populations, but mating-system treatment did not have a significant effect. Heterozygosity of snails was negatively correlated with the probability of trematode infection, however, suggesting that parasitic diseases may pose a serious threat to populations lacking genetic variability.

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“…Similar results (i.e. lack of an effect) have been obtained in a number of experimental studies, primarily in invertebrate host taxa (Strauss & Karban, 1994;Stevens et al, 1997;Ouborg et al, 2000;Haag et al, 2003;Trouv´e et al, 2003;Puurtinen et al, 2004). In a study similar to ours, Trouv´e et al (2003) investigated 13 subpopulations of the hermaphroditic snail Lymnaea truncatula and failed to find a relationship between selfing rate and parasite prevalence among studied populations.…”

“…Moreover, as in our study, they failed to find a relationship between infection with digenetic parasites and genetic diversity (microsatellite H e ). Furthermore, Puurtinen et al (2004) failed to find an effect of mating system, self-or cross-fertilization, on the susceptibility to infection by trematode parasites in the freshwater snail Lymnaea stagnalis. Unlike our study (and that of Trouv´e et al, 2003) however, they did find a strong negative correlation between probability of infection and genetic diversity (microsatellite H e ), which leads to the conclusion that populations lacking genetic diversity represent an elevated extinction risk.…”

“…Several studies have addressed the putative link between inbreeding and parasitism (Reid, Arcese & Keller, 2003; table 1 therein), yet empirical evidence from wild populations is unfortunately limited or contradictory. Several studies, particularly in vertebrates, have reported a relationship between inbreeding and infection (Coltman et al, 1999;Cassinello, Gomendio & Roldan, 2001;Carr & Eubanks, 2002;Penn, Damjanovich & Potts, 2002;Wiehn et al, 2002;Spielman et al, 2004); however various other studies, especially in invertebrates, failed to discover an effect (Stevens, Yan & Pray, 1997;Ouborg, Biere & Mudde, 2000;Haag, Sakwin´ska & Ebert, 2003;Trouv´e et al, 2003;Puurtinen et al, 2004).…”

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“…The parasite sterilises the snail and in a later stage of infection it enhances growth (de Jong-Brink, 1995). Susceptibility of snails to parasites varied between populations in L. stagnalis (Puurtinen et al, 2004). Parasites have a limited effect on inbreeding depression in L. stagnalis (Puurtinen et al, 2004), as snails produced by self-fertilisation and snails from cross-fertilisation treatments were equally likely to become infected by trematode parasites.…”

“…Susceptibility of snails to parasites varied between populations in L. stagnalis (Puurtinen et al, 2004). Parasites have a limited effect on inbreeding depression in L. stagnalis (Puurtinen et al, 2004), as snails produced by self-fertilisation and snails from cross-fertilisation treatments were equally likely to become infected by trematode parasites. However, only a single generation of selfing was followed so that infection probability after sustained self-fertilisation may change.…”