The Effect of Migration on the Maintenance of a Lethal Polymorphism in the House Mouse

Abstract

Most wild populations of house mice are polymorphic for alleles at the Brachury, T, locus. These alleles are either recessive lethals or viable male steriles and are maintained by an abnormal segregation (transmission) ratio in heterozygous males. The observed frequencies of t alleles in natural populations are considerably less than those expected from deterministic models. This discrepancy has been attributed to the operation of random genetic drift in the small, relatively isolated breeding units (demes) of which a mouse population is comprised. In this investigation, a Monte Carlo simulation of a geographical population has been employed to examine the range of breeding-unit sizes and interdemic migration rates for which the random drift would serve as a likely explanation for the low observed frequencies of the t alleles. In the runs made, lethal t alleles with a transmission ratio of 0.95 were examined. The results of these runs indicate that random drift would only have an important effect on t allele frequencies in populations composed of totally isolated breeding units with a genetically effective size of less than 12 individuals. With 1% migration between demes, the effective breeding-unit size would have to be less than eight. With 3% migration, random drift would only have an important effect on t allele frequencies if the breeding units had a genetically effective size of less than four. Current estimates of these parameters obtained from biochemical polymorphism data (Es-2 locus) indicate that it is quite possible that the breeding-unit sizes and migration rates of natural populations considerably exceed those in which random drift would have an important effect on t allele frequencies. From these considerations it was concluded that random genetic drift itself does not serve as an adequate explanation for the low observed frequencies of lethal t alleles. Brief consideration was given to alternative hypotheses.