Robertsonian chromosomal rearrangements in the shout-tailed shrew, Blarina carolinensis, in western Tennessee

We report significant heterozygosity for numerous Robertsonian translocations in the southern short-tailed shrew (Blarina carolinensis) in western Tennessee. Eight Robertsonian rearrangements were documented using G-banding techniques that explain the variability in diploid numbers from 46 throughout most of the range of the species to 34-40 in western Tennessee. These fusions resulted in the loss of telomere sequences and were not associated with nucleolar organizer regions. When heterozygocity is considered, the lowest diploid number possibly present would be 30. Four localities with distances of over 180 km apart were sampled, and 80-90% of the collected animals were heterozygous for at least one rearrangement. No putative parental type was found in western Tennessee. Heterozygosity for the same rearrangements was found in these different localities, and no monobrachial fusions were noted. Thus, this is a very wide hybrid zone with rare or absent parental types in the areas sampled or is an evolutionary stage preceding establishment of Robertsonian races. Selective forces, if any, were minimal, as evidenced by the wide area of polymorphism, significant heterozygosity, and the fact that the Robertsonian translocations were in Hardy-Weinberg equilibrium. The origin of such extensive polymorphism in western Tennessee is discussed, especially in light of putative effects of the New Madrid seismic activity. Similarities and differences are noted between the Blarina model and the well-documented variation in the European common shrew (Sorer araneus) and Mus musculus groups.