Phylogenetic Analyses of Chromosomal Banding Patterns Among the Nearctic Arvicolidae (Mammalia: Rodentia)

Numerical phylogenetic analyses of G-banded karyotypes were carried out for 22 species of arvicolids belonging to eight genera with diploid numbers among the species ranging from 18 to 64. Interspecific karyotypic comparisons indicate that 38 homologous autosomes or autosomal arms are shared by two or more species. The additional presence of 33 unique elements and 141 euchromatic rearrangements account for the extensive interspecific karyotypic variability. Ten different types of rearrangements were documented with centric fusion (27.0%), tandem fusion (14.2%) and pericentric inversion (12.8%) being the most abundant. Chromosomes and chromosomal rearrangements were quantitatively coded as present, absent, or missing in the karyotype of each species and the resulting data matrix (22 species by 214 karyotypic characters) subjected to cladistic analyses (with Peromyscus as an outgroup). Six analyses were executed where chromosomal rearrangements were weighted equal to or less than the weights assigned to entire chromosomes. Families of equally parsimonious trees were obtained and subsequently distilled into Adams-2 consensus estimates. Consistency indices ranged from 0.75 to 0.84 with reversals accounting for 2–30% of all homoplastic events. Systematically, Clethrionomys, Lemtnus, Synaptomys, Lagurus, Neofiber, and six species of Microtus share generally primitive autosomal complements; however, four derived characters suggest a relationship between M. ochrogaster and M. pinetorum. Six other species of Microtus (townsendii, pennsylvanicus, breweri, montanus, canicaudus and the more distant oregoni) share a number of derived characters. Dicrostonyx, Phenacomys, M. longicaudus, and M. oeconomus each possess a series of uniquely derived characters that tend to isolate these lineages. Chromosomal collocations support those derived from paleontology, comparative morphology, and protein electrophoresis in certain respects but not in others. Comparisons of zoogeographic distributions among four pairs of chromosomal sister taxa are interpreted to support an allopatric rather than a stasipatric (chromosomal) means of speciation in all cases except one.