MAINTENANCE OF A NARROW HYBRID ZONE IN PEROMYSCUS-LEUCOPUS - A TEST OF ALTERNATIVE MODELS

This study examines the genetic characteristics of one population of t he white-footed mouse, Peromyscus leucopus, from the center of a hybri d zone between two chromosomal races characterized by pericentric inve rsions. All individuals (n = 78) were trapped in a single night and ex amined for five diagnostic genetic markers. Three individuals possesse d the pure southwestern genome and all other individuals had a mixture of parental genomes. To determine if the maintenance of this zone is best explained by either the dynamic-equilibrium, hybrid-equilibrium, or hybrid-superiority model, we proposed three independent tests. Clas sical disequilibrium statistics detected a significant association bet ween chromosomes 5 and 20. However, results from the contiguous-cluste ring analysis indicate that linkage-disequilibrium is the result of su bdivisions within the population and not due to nonrandom segregation. It is critical to distinguish between these factors as they result fr om distinctly different biological phenomena. Contiguous clustering sh owed little consistency of population subdivision among the five genet ic characters examined. Single- and multi-character distributions vary from the predictions of Hardy-Weinberg equilibrium due to an excessiv e number of southwestern parental types. However, the frequency of hyb rid classes does not vary from the expectations of Hardy-Weinberg equi librium. This, plus the high frequency of individuals homozygous for a lternative parental states, is interpreted as documentation that most individuals are the progeny of hybrids. The hybrid-equilibrium model a ppears to be an appropriate descriptor of the statistical results from this hybrid zone.