Contributions of Chromosomal Analysis to the Systematics of North American Chipmunks

Abstract

Chromosomes from the bone marrow of 32 chipmunks belonging to the genera Eutamias and Tamias were analyzed using a colcemide, hypotonic citrate, acetic orcein squash technique. The modal diploid chromosome number was 38 in Eutamias minimus caryi, E. m. consobrinus, E. m. neglectus, E. q. quadrivittatus, E. speciosus frater, E. a. amoenus, E. t. townsendii, E. t. cooperi, E. t. senex, E. d. dorsalis and Tamias striatus griseus. Three distinctive chromosomal patterns were observed among these species. The first pattern, found only among specimens of Eutamias minimus was designated Eutamias Karyotype A. Chromosomal patterns of E. quadrivittatus, E. speciosus, E. amoenus, E. townsendii and E. dorsalis were similar and were collectively designated Eutamias Karyotype B. A third chromosomal pattern, found in Tamias striatus, was named the Tamias Karyotype. The sex chromosomes of all species were of the X-Y type. Correlation of chromosomal characters with present concepts of chipmunk systematics suggests that chromosomal data is most meaningful at the generic and infrageneric levels. A close chromosomal relationship was found between Eutamias and Tamias. The observation of two distinct karyotypes within Eutamias suggests there may be two species groups in this genus and zoogeographic data support this new concept. Evolution of chipmunks is discussed and it is postulated that the Nearctic Region was populated by two or possibly three separate eastward invasions of chipmunks from the Palearctic Region. The systematics of the presently recognized chipmunk genera, Eutamias and Tamias, have undergone a number of revisions, each based on a variety of characters. The first extensive revision of the chipmunk genera was presented by Howell (1929) and was based on examination of cranial and external characters from a large series of specimens representing 16 species of Eutamias and one species of Tamias. Howell reviewed the earlier works of Allen (1877, 1890) and others, and relegated all chipmunks to two genera: Tamias for the eastern North American chipmunks; and Eutamias subgenus Eutamias for Asiatic chipmunks, and subgenus Neotamias for the chipmunks of western North America. Ellerman (1940), proposed a different classification of chipmunks in which Eutamias, Neotamias and Tamias were all given subgeneric rank within the single genus Tamias. Cranial morphology, dentition, and external features formed the basis for this decision. In an extensive phylogenetic study of Sciuridae, Bryant (1945) re-evaluated the problem using not only cranial and external characters but other skeletal and dental characters, myology and paleontology as well. He agreed with the classification proposed by Ellerman (1940) and concluded that the differences between eastern and western chipmunks were not of sufficient phylogenetic importance to warrant retention of the two groups as separate genera. More recently, White