Heterozygosity-fitness correlations in rainbow trout: effects of allozyme loci or associative overdominance?

Previous studies with rainbow trout (Oncorhynchus mykiss) have shown that increased heterozygosity at allozyme loci is correlated with several phenotypic traits associated with fitness. We expected to find a similar effect of heterozygosity at other nuclear loci if these associations are due to loci in linkage disequilibrium with the allozyme loci (i.e., associative overdominance), rather than the allozymes themselves. We examined the association between multiple locus heterozygosity and condition factor at 10 allozyme and 10 microsatellite loci. Individuals that were more heterozygous at allozyme loci had significantly greater condition factor in two hatchery cohorts of rainbow trout (1996 P = 0.006; 1997 P < 0.001). In contrast, there was no evidence at microsatellite loci that increased heterozygosity was associated with greater condition factor. Our results suggest that the observed relationship between heterozygosity at allozyme loci and condition factor in rainbow trout appears to be due to the allozyme loci themselves, rather than associative overdominance. We cannot, however, rule out that differences in the mutation process between allozymes and microsatellites may be responsible for these observations. Regardless of the underlying mechanism, these results support the view that allozymes and microsatellites are differentially affected by natural selection.