Chromosomal Evolution

The possibility that the Peromyscus maniculatus complex of deer mice arose via the process described as "centrifugal speciation" (Brown, 1957) has been discussed by Bowers et a l . , 1973, Lawlor, 1974, Baker et al., 1975, and Greenbaum et a l . , 1978. This study was designed to yield additional da ta on the cytogenetics of three species of this complex in order to evaluate the "centrifugal speciation" model as a viable explanation of the evolutionary origin of the complex. Peromyscus melanotis and P . polionotus are believed to be peripheral isolates from the stock that evolved into the species P. maniculatus (Blair, 1950, Bowers et al., 1973). P. maniculatus is a widespread species occurring in a variety of habitats, whereas P , melanotis is a high montane s ~ e c i e s found in coniferous forest and P , polionotus is a deep sand species found in grassy fields a t lower elevations near sea level (see Hooper, 1968, for general distribution of these three species). I t is probable that P. melanotis and P. polionotus are more closely related to P. maniculatus than they are to each other. Based on gross chromosome morphology, the karyotypes of P. polionotus and P. melanotis are more like each other than either is like the karyotypes of P. nzaniculatus. We have examined the G-banding and C-banding pattern of P . polionotus a n d compared i t to the banding patterns of P. maniculatus and P . melanotis (Greenbaum et al., 1978) to determine whether this similarity in karyotype is reflected in homologous banding patterns or is a product of convergent evolution. If the chromosomal data for the species complex are to fit the centrifugal speciation model then the karyotypes of P. polionotus and P. nzelanotis must be most like the primitive condition and the karyotype of P. maniculatus must be the most derived. Additionally, the centrifugal speciation model would predict more chromosomal polymorphism a n d geographic variation in the "central stock" (P. maniculatus) and less in the peripheral isolates (P, polionotus and P , melanotis). Our study is designed to determine the chromosomal homologies in these species and thereby test the hypothesis that this group is the result of centrifugal speciation.