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Similarly, in brown trout (Salmo trutta), females but not males appear to select mates that are of intermediate MHC dissimilarity to themselves (Forsberg et al., 2007).The Pacific salmon (Oncorhynchus spp.) provide ideal mating systems to examine variation in reproductive success.Mate choice, on the other hand, can select for genetic benefits including ‘good genes’ that show additive genetic variance and ‘compatible genes’ that show non-additive genetic variance (Neff and Pitcher, 2005).For example, female gray tree frogs prefer to mate with males that have long courtship calls and the offspring of long-calling males consistently have higher fitness than those of short-calling males, which suggests an additive genetic effect (Klump and Gerhardt, 1987; Welch et al., 1998).These fishes are anadromous, living part of their life in the ocean and returning to breed in freshwater streams before dying (Fleming, 1998).Because Pacific salmon are semelparous, it is relatively easy to assess lifetime reproductive success (Fleming et al., 2000; Seamons et al., 2007).

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These results indicate that sexual selection favours increased body size and perhaps integument coloration in males as well as increases genetic diversity at the MHC by female mate choice.

Most research on the reproductive success of salmon has emphasized the importance of adult body size.

Larger females can produce more and larger eggs (Beacham and Murray, 1993; Heath et al., 2003), obtain and defend high-quality nesting sites (Foote, 1990; Fleming and Gross, 1993) and bury their eggs deeper thus reducing the chance of nest superimposition (Steen and Quinn, 1999).

Indeed, one study showed that MHC heterozygous mice were more likely to clear Salmonella infections, weighed more and had higher survivorship than MHC homozygous mice (Penn et al., 2002).

Interestingly, it has also been shown in mice that peptide ligands of MHC molecules activate vomeronasal organ sensory neurons, which confirms that there is a mechanism by which female mice can evaluate MHC genotype (Leinders-Zufall et al., 2004).There has been considerable research on mate choice and MHC diversity (for reviews see Penn, 2002; Bernatchez and Landry, 2003; Ziegler et al., 2005).

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