Protein Polymorphism and Genic Heterozygosity in Two European Subspecies of the House Mouse

Electrophoretic variation in 36 proteins controlled by 41 genetic loci was analyzed in 99 house mice representing two subspecies (M. m. domesticus and M. m. musculus) from six geographic regions of the Jutland Peninsula, Denmark. Of the 36 proteins, 16 or 44% are polymorphic for two or three alleles in one or more of the six samples. In terms of the 41 controlling genetic loci, 17 or 41% are polymorphic. This is the best estimate of the proportion of polymorphic loci in the genome of the species as a whole, and is closely similar to values available for Drosophila and humans. Individual Danish mice are, on the average, heterozygous at 8.5% of their loci. At 13 of the 17 polymorphic loci there is a substantial difference in allele frequencies between the subspecies, and, at six of these loci, alternate alleles are fixed or nearly so in the two subspecies, or an allele that is almost fixed in one subspecies does not occur in the other subspecies. This degree of genetic difference, occurring in a geographic region over which there is no apparent variation in the external environment, is attributed, in major part, to intersubspecific variation in genetic environment, with identical alleles having different selective values and equilibrium frequencies in the two subspecies. In overall genetic character, as reflected by Sneath's coefficient of similarity, M. m. domesticus of the southern Jutland Peninsula is more similar to M. m. brevirostris, introduced to North America from Europe, than to M. m. musculus of the northern Jutland Peninsula. This finding, which is consistent with the taxonomic assignment of M. m. domesticus and M. m. musculus to different subspecies groups, is interpreted as evidence of cohesion of coadapted gene pools in populations of the two subspecific groups. Evidence provided by Hubby and Throckmorton (1968) that sibling species of Drosophila differ, on the average, at 50% of their loci and the present demonstration of significant intersubspecific differences at 32% of 41 loci examined in Danish house mice are advanced in support of the thesis that major reorganizations of gene pools normally accompany the speciation process.