Phylogenetic Analysis of Karyological Variation in Three Genera of Peromyscine Rodents

Yates, T. L., R. J. Baker, and R. K. Barnett (Department of Biological Sciences and The Museum, Texas Tech University, Lubbock, Texas 79409) 1979. Phylogenetic analysis of karyological variation in three genera of peromyscine rodents. Syst. Zool. 28:40-48.-Chromosomal homologies between Neotomodon alstoni, Baiomys taylori and species of Peromyscus for which G-band chromosomal data are available in the literature were identified using Gand C-banding techniques. The primitive karyotype for the peromyscine genera is hypothesized to have a 2n = 48 composed of two or three pairs of biarmed and 20 to 21 pairs of acrocentric autosomes, with heterochromatin restricted to the centromeric regions. Patterns of chromosomal change are presented in the form of a cladogram. The G-banded karyotype of Baiomys taylori is composed of all acrocentric autosomes and is most like that of Peromyscus crinitus, differing only by pericentric inversions in pairs 1, 22, and 23. Neotomodon alstoni shares synapomorphic inversions with several species of Peromyscus. Based on these data, we believe Neotomodon is best considered a congener of Peromyscus. [Peromyscus; Neotomodon; Baiomys; karyology; phylogenetic relationships.] Certain types of chromosomal data are particularly well suited to a cladistic approach for the following reasons. Chromosomal changes occur as distinct events and, by using G-band analysis, it is possible to determine which chromosomes are homologous among closely related taxa and which chromosomes have changed. By observing the overall pattern, plus outgroup comparisons, one can often determine which chromosomal condition is primitive (plesiomorphic) and which is derived (apomorphic). Knowledge of the primitive and derived karyotypic conditions allows the inference of phylogenetic relationships and construction of a phylogeny based on synapomorphic (derived character states shared between species) chromosomal changes. Certain types of chromosomal changes are rare (for instance, pericentric inversions) and the probability that the same event would occur twice (convergent evolution) is unlikely. Thus, species which sharederived conditions probably evolved from a common ancestor which contained the derived conditions. The genus Peromyscus is an excellent taxon for chro-mosomal studies because: 1) there are many species (57 recognized by Hooper, 1968) from which data can be gathered; 2) several species are represented by numerous geographic races, a condition which permits correlation with low-level chromosomal differentiation; 3) individuals are small and relatively easy to collect; 4) they readily breed in the laboratory and, in some cases, fertile hybrids can be produced; 5) all species thus far examined have a diploid number of 48, yet there is considerable variation in the gross morphology of the karyotype (intraspecific and interspecific); 6) there are enough pericentric inversions to establish synapomorphy patterns; 7) there is considerable variation in heterochromatin between species and populations, and 8) cell lines grow well in tissue culture and detailed cytogenetic studies can be made. For these and other reasons, numerous authors have examined chromosomal variation in Peromyscus (Hsu and Arrighi, 1968; Duffey, 1972; Lee et al., 1972; Pathak et al., 1973; Schmidly and Schroeter, 1974; Arrighi et al., 1976; Murray and Kitchin, 1976; Greenbaum and Baker, 1978; Greenbaum et al., 1978a and 1978b). Baker and Mascarello (1969) suggested that the primitive karyotypic condition