HAPLOID DYNAMIC POLYMORPHISM IN A HOST WITH MATCHING PARASITES - EFFECTS OF MUTATION SUBDIVISION, LINKAGE, AND PATTERNS OF SELECTION

A deterministic genetic simulation of the coevolution of a sexual haploid host species with an asexual parasite is described in which parasites gain by matching and hosts by unmatching alleles at three linked and corresponding loci. Realistic parameter states without mutation or population subdivision can generate permanent dynamic polymorphism; however, polymorphism is preserved under wider conditions if mutation and/or migration in a metapopulation is present. Other factors favorable to polymorphism include weak selection, ''zero-sum'' patterns of fitness interaction, fast breeding of parasites, and low rather than high or zero recombination. Hard selection was most protective to polymorphism in large populations, but multiple-infection soft (truncation) selection was more protective in small populations. A protective effect of close linkage may strengthen a polygene storage concept of Mather. It may also help account for cases of ancient transpecific polymorphism involving closely linked complexes of loci, as with the major histocompatibility complex and some linkages in plant resistance to pathogens. Typical ''cyclical'' dynamics is extremely complex even when selection is moderate in strength. Genotype and gene frequency changes frequently simulate stochasticity/chaos even through wholly determined. Such states seem favorable to permanent polymorphism. This study demonstrates that, via a linkage-control locus, the dynamic states described may provide slow (frequently reversing) selection for high or low recombination. An instance of intermediate confinement of linkage under the selection is illustrated.