Nucleolar activity and distribution of ribosomal genes in phyllotis rodent species and their laboratory hybrids

Abstract

The expression of nucleolar organizer regions (NORs), distinguished through silver staining procedures, and the distribution of ribosomal genes, detected by fluorescent in situ hybridization with a ribosomal DNA probe, were studied in 3 Phyllotis rodent species and their laboratory generated hybrids. In the hybrids between the more genetically distant species, the NORs of one parental genome were preferentially expressed. This result differs from the codominant rDNA expression previously detected in hybrids between the 2 more genetically similar of the 3 species, and suggests that nucleolar dominance is related to the compatibility of parental genomes. The mean area of each fluorescent signal was significantly larger in P. magister, which has 3 nucleolar chromosome pairs, than in P. darwini and P. xanthopygus, which have 4 nucleolar chromosome pairs each. Nevertheless, the total area of rDNA signals per cell was similar in the karyotypes of all 3 species, strongly suggesting that rDNA genes have been redistributed in these genomes rather than gained or lost during evolutionary divergence.