Heterochromatin variation in the Australian rodent Uromys caudimaculatus

Ten individuals of Uromys caudimaculatus sampled from Queensland gave evidence for the occurrence of two distinct chromosome races characterised by marked differences in their pattern of C-banding. In all four individuals from the north, thirteen of the twenty three chromosome which make up the standard haploid set had substantial distal C-blocks in addition to the smaller centric blocks which characterise all chromosomes other than the Y. Additionally two pairs had an interstitial block. By contrast none of the six southern individuals had fixed distal blocks though all of them except the Y carry pro-centric C-blocks and again one pair showed an interstitial block. The southern karyotype was, however, characterised by the presence of from six to nine mitotically stable supernumerary chromosomes all of which were totally C-positive despite the fact that at least five morphologically distinguishable types have been defined. While the relationship of these two types of constitutive heterochromatin remains to be clarified the large amount present in both northern and southern animals suggests that heterochromatin plays an important role in the basic biology of this species.