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A potential model for early stages of chromosomal evolution via concentric Robertsonian fans: a large area of polymorphism in southern short-tailed shrews (Blarina carolinensis)

Western Tennessee contains unusually highly polymorphic populations of southern short-tailed shrews (Blarina carolinensis). We previously documented eight Robertsonian translocations (ROBs) accounting for a variation in diploid number from 46 in most of this species’ range to 34–40 in western Tennessee. We have now expanded our study to include data from adjacent areas in Tennessee and Mississippi, 10 localities in all. The new data show a variation in diploid number ranging from 31 to 41, four new ROBs (for a total of 12), and the novel finding of monobrachial translocations in this group. All animals collected from this large area (extending over 12,000 km2) had some level of ROBs, and none represented the 2n = 46 form seen in other parts of the range of this species. Because other species of shrews (genus Sorex) are not affected in the same area, the factors and/or selective forces causing this extensive polymorphism in B. carolinensis must be unique to this species and to this geographic area. Some ROBs were found throughout this large area of over 12,000 km2. Other translocations (including those with monobrachial homology) were located in one or two localities in this large area, and still other translocations were intermediate in their distribution. There was a concentric pattern to the evolution and presumed spreading of the ROBs. This allowed us to expand the concept of a Robertsonian “fan,” introduced by Matthey (1970), to that of concentric evolution of multiple fusion fans: ROBs likely arose independently, separated temporally and geographically, and radiated into surrounding populations to create this complex zone of polymorphism. This is an active process in its infancy, and it is not as mature as that seen in European studies of Mus and Sorex.