Intrageneric divergence in DNA and heterochromatin content in ground squirrels of the genus Citellus

Abstract

Until recently the comparative analysis of DNA in related eukaryotic taxa (Santiago & Rake, 1973; Straus, 1971; Vallejo-Roman et al., 1975) was carried out separately from comparative karyology by means of which the relationship between intraand interspecific karyotypic variation and heterochromatin regional variation has been shown using the chromosome C-banding techniques (Pathak et al., 1973; Prokofyeva-Belgovskaya, 1977; Lyapunova et al., 1978). Although for some species of higher organisms there are numerous data demonstrating the relationship between the repetitive DNA and constitutive heterochromatin (Yasmineh & Yunis, 1970; Pardue & Gall, 1970; Jones, 1970; Saunders et al., 1972), the comparison of chromosomal heterochromatin content variation with the variation in the number of different fractions of DNA nucleotide sequences was only made in a few studies with respect to some groups of closely related species (Yasmineh & Yunis, 1973; Mizuno & Macgregor, 1974; Narayan & Rees, 1976; Hatch et al., 1976; Ginatulina et al., 1979). Earlier we compared the data on DNA reassociation in karyologically far divergent species of the genus Ellobius widely differing in chromosome number (Ginatulin et al., 1977). In evolution, however, there are many cases of morphological divergence of species without apparent differences in chromosome number and morphology (Matthey, 1949; Vorontsov, 1958, 1966). Thus, within the ground squirrel genus (O'tellus = Spermophilus) there are species with different chromosome numbers (2n = 46, 42, 40, 38, 36, 34, 32 and 30) as well as well differentiated species with similar chromosome number (2n = 36) and morphology (Nadler, 19621966; Nadler & Sutton, 1962; Vorontsov & Lyapunova, 1969, 1970; Lyapunova, 1969; Lyapunova & Vorontsov, 1970; Matthey, 1973; Odor & Davaa, 1975). A remarkable characteristic of ground squirrels is their reproduction once a year and the onset of sexual maturity at the age of one year. This means that the geological age in years of divergence between species actually is equal to the number of generations. The genus Otellus is rather widely represented in USSR fauna. In this study Otellus species that diverged rather long ago and appear to have retained similar chromosome morphology have been used to measure the relative content and the type of C-band distribution in chromosomes, to analyse the kinetics of DNA reassociation and finally to compare the rate of intrageneric divergence in C-banded heterochromatin content and reassociation DNA characteristics.