The Natural History of Tassel-Eared Squirrels

9. Genetics

105 Genetics Upon first consideration, the various described forms of the tassel-earedsquirrelseemtobeverydistinctiveandsharplydifferentiated . Further examination, however, tends to emphasize the fact that they represent a curiously variable group, with somewhat parallel lines of variation in widely separated localities . . . —E. D. McKee, “Distribution of the Tassel-Eared Squirrels,” 1941 Introduction T he earliest genetic study, which produced the first karyotype of tassel-eared squirrels, was conducted in 1967 (ref. 1). The term “genetic polymorphism” was used to explain coat color of tasseleared squirrels in Colorado (ref. 2). Several genetic studies have examined T-cell receptor genes, phylogeny of mitochondrial DNA, and the major histocompatibility complexes of tassel-eared squirrels (ref. 3, 4, 5). The most recent study involving tassel-eared squirrel genetics was performed in 2000 and involved using ground squirrel DNA primers in Abert’s squirrel variability studies (ref. 6). Karyotypes and Chromosomes Charles Nadler and Dallas Sutton assembled and compared the karyotypes of five North American Sciuridae in 1967. They included specimens from S. a. aberti, S. niger rufiventer (fox squirrel), S. griseus griseus (California gray squirrel), and two subspecies of eastern gray squirrels, S. carolinensis carolinensis and S. carolinensis pennsylvanicus. The S. aberti (subspecies not given) karyotype was analyzed from the bone marrow of 9 106 C H A P T E R N I N E a single male specimen from an “unknown locality.” The results revealed a diploid number of forty chromosomes for all specimens examined consisting of fourteen metacentric, twenty-four submetacentric, one unpaired metacentric chromosome (X), and one unpaired acrocentric chromosome (Y) (ref. 1). Chromosomes are described based on the location of the centromere on the chromosomes. Centromeres are constriction areas and serve as attachment points for the spindle fibers during mitosis and meiosis. If the centromere is directly in the center it is referred to as “metacentric”; if near the end it is “acrocentric”; and if it is located between the center and the end it is “submetacentric” (ref. 7). The centromeres of the autosomes of S. n. rufiventer, S. c. carolinensis, and S. aberti are all in the same location, making them indistinguishable from one another. S. g. griseus differs slightly from the other three species with respect to the Y chromosome and the “presence of secondary constrictions” on several other chromosomes (ref. 1). David Forsyth conducted an analysis of the forty chromosomes of the karyotypes of S. a. aberti and S. a. kaibabensis in 1991 (figures 9.1a and 9.1b). The analysis involved fifteen specimens of S. a. aberti (five females, ten males) and ten specimens of S. a. kaibabensis (two females, eight males). He obtained samples from blood, spleen, and bone marrow . He gave an excellent description of each of the nineteen pairs of autosomes and the sex chromosomes, describing several identifying characteristics of each, such as arm lengths, arm length ratios, and size comparisons. The chromosomes of S. a. aberti and S. a. kaibabensis were described as “conserved,” meaning that no major changes have occurred between the two subspecies since the time of separation, leading to a hypothesis that neither subspecies had diverged significantly from each other and that “hybrid progeny produced by secondary contact [between the two subspecies] would most likely be fertile” (ref. 8). In a small preliminary sperm morphology study designed to differentiate the two subspecies S. a. aberti and S. a. kaibabensis taxonomically , there was not a significant difference in sperm shape or size between the two subspecies, lending support to Forsyth’s hypothesis that these subspecies have not diverged significantly (ref. 8, 9). Another examination using silver staining was conducted with sperm of the two subspecies. The various proteins on the acrosomal sheath of a sperm will bind silver differentially. Even though the silver staining of the postacrosomal sheaths of the sperm of the two subspecies was identical, the G e n e t i c s 107 respective density was clearly different between the two. This difference could have been due to the small sample size of sperm used or to the collection technique, and needs further study (ref. 9). Forsyth’s study used the Giemsa trypsin banding method to allow microscopic examination of chromosomes. Trypsin causes the chromosomes to swell, and the Giemsa stains the chromosome bands so they become visible. The Giemsa trypsin banding patterns of the chromosomes for S. a. aberti and S. a. kaibabensis were homologous with very little variation, which was attributed to the degree...