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

This study examines the genetic characteristics of one population of the white-footed mouse, Peromyscus leucopus, from the center of a hybrid zone between two chromosomal races characterized by pericentric inversions. All individuals (n = 78) were trapped in a single night and examined for five diagnostic genetic markers. Three individuals possessed 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. Classical disequilibrium statistics detected a significant association between chromosomes 5 and 20. However, results from the contiguous-clustering analysis indicate that linkage-disequilibrium is the result of subdivisions within the population and not due to nonrandom segregation. It is critical to distinguish between these factors as they result from distinctly different biological phenomena. Contiguous clustering showed little consistency of population subdivision among the five genetic characters examined. Single- and multi-character distributions vary from the predictions of Hardy-Weinberg equilibrium due to an excessive number of southwestern parental types. However, the frequency of hybrid classes does not vary from the expectations of Hardy-Weinberg equilibrium. This, plus the high frequency of individuals homozygous for alternative parental states, is interpreted as documentation that most individuals are the progeny of hybrids. The hybrid-equilibrium model appears to be an appropriate descriptor of the statistical results from this hybrid zone.