Metazoan parasite species richness and genetic variation among freshwater fish species: cause or consequence?

The factors responsible for the maintenance of genetic variation among natu ral populations remain a mystery. Recent models of host-parasite co-evoluti on assume that parasites exert frequency-dependent selection on their hosts by favouring rare alleles that may confer resistance against infection. We tested this prediction in a comparative analysis that sought relationships between levels of genetic variation and the number of metazoan parasite sp ecies exploiting each host species. We used data on 40 species of North Ame rican freshwater fishes. After controlling for sampling effort and phylogen etic influences, we found no relationship between genetic polymorphism and parasite species richness among fish species. However, we found a marginal negative correlation between parasite species richness and heterozygosity. This result goes against the prediction that increased selective pressure b y parasites should be associated with higher levels of genetic variation. I nstead, it suggests that parasites may be colonising host species showing l ow levels of genetic variation with greater success than genetically more v ariable host species. (C) 2000 Australian Society for Parasitology Inc. Pub lished by Elsevier Science Ltd. All rights reserved.