PARASITE-HOST COEVOLUTION AND GEOGRAPHIC PATTERNS OF PARASITE INFECTIVITY AND HOST SUSCEPTIBILITY

Ebert (1994) has proposed the rule that parasites are, with few except ions, more infective to sympatric hosts than to allopatric hosts. We t est this rule using field data for schistosome infections of planorbid snails and find that, although sympatric parasite-host combinations d o tend to be more compatible, there are exceptions where particular al lopatric parasite-host populations are significantly more compatible. We develop a mathematical model of the dynamics of the parasite-host i nteraction where parasite infectivity and host susceptibility are defi ned by the matching of genotypes in a diploid system. The model predic ts dynamic polymorphisms where parasite allele frequencies track host allele frequencies but with a lag. Because of this lag it is possible for allopatric combinations to be more compatible than sympatric combi nations. Any 'rule' that precludes this possibility is unlikely to pro ve robust.