Phylogeny of mitochondrial DNA clones in tassel-eared squirrels Sciurus aberti

The tassel-eared squirrel, Sciurus aberti, includes six subspecies which occupy restrictive and apparently identical habitats in Ponderosa pine forests in the south-western United States and Mexico; the strict habitat requirement of this species is based on dietary requirements which are only fulfilled in these forests. To examine evolutionary relationships among certain subspecies of S. aberti, we obtained estimates of nucleotide diversity within subspecies as well as nucleotide divergence between subspecies using mitochondrial DNA (mtDNA) analysis. Restriction site polymorphisms were identified in samples of the four US subspecies: S. a. aberti (Abert), S. a. kaibabensis (Kaibab), S. a. ferreus (Ferreus), and S. a. chuscensis (Chuska) Fourteen mtDNA clones were resolved that were, with one exception, uniquely subspecific. Dendrograms constructed by neighbour-joining and maximum parsimony methods revealed two major assemblages: (1) an Abert/Kaibab group; and (2) a Ferreus/Chuska group. The Abert vs. Ferreus clones exhibited the greatest net nucleotide divergence, with a lineage separation estimate approximating 572,000 years ago assuming a nucleotide substitution rate of 7.15 x 10(-9)/year/site. Five out of ten Chuska squirrels shared a clone with one Abert sample; the relative sizes of these two populations and their respective ranges as well as their close proximity support the proposal for relatively recent intermixing of Abert and Chuska populations resulting in what appears to be Abert-->Chuska migration. Nucleotide diversity within subspecies ranked as Kaibab < Ferreus < Abert < Chuska; the relatively high diversity for the Chuska sample is based on the apparent introgression of Abert mtDNA. The relative diversity exhibited by Kaibab, Ferreus and Aberti samples corresponds to the range size of the respective subspecies.