Heterochromatin Variation and DNA Content Conservatism in Geomys attwateri and G. breviceps (Rodentia: Geomyidae)

Pocket gophers from two chromosomal races (F and G) of Geomys attwateri, G. breviceps (race E), F × G hybrids, and an E × G hybrid were karyotyped and examined for variation in heterochromatin and DNA content. Most specimens were collected from a hybrid zone in East Texas described in detail by Tucker and Schmidly (1981) and Dowler (1989). C-bands and fluorescent dyes (chromomycin A3 and Hoechst 33258) revealed two types of heterochromatin in C-band regions. Distribution of heterochromatin and amount of chromomycin-bright (guaninecytosine rich) heterochromatin differed markedly between the species, but the mean nuclear DNA content did not differ significantly. This may have resulted from selection for maintenance of a restricted range in genome size, whereas the distribution and base composition of heterochromatin was free to change.