Mammalian repetitive DNA sequences in a stable Robertsonian system

Abstract

The calf, sheep, and goat are members of a stable Robertsonian system, the superfamily Bovoidea, in which karyotypic evolution occurs almost exclusively by centric rearrangements. The centromeric DNA sequences in this system have been studied by buoyant density analysis and by intraspecies and interspecies in situ cytological hybridization. The results obtained with four satellite DNAs (I-IV) of the calf, satellites I and II of the sheep, and satellites I and II of the goat are as follows: 1. Satellite I of the sheep and satellite I of the goat are nearly identical by buoyant density analysis in CsCl at pH 8.4 and pH 12.5, but they differ from satellite I of the calf. Satellite II of the sheep and satellite II of the goat differ from each other and from calf satellite II in their physical properties. 2. Centromerically located satellites I and III of the calf show no homology with centromeric sequences of the sheep or goat chromosomes by in situ hybridization. Sequences homologous to calf satellite IV show limited homology to similar sequences in sheep and goat chromosomes. 3. Sheep and goat satellites I and II show no homology to centromeric DNA sequences in the calf chromosomes. 4. Sequences complementary to sheep satellites I and II hybridize efficiently to centromeric regions of goat autosomes. Sequences complementary to goat satellites I and II hybridize efficiently to centromeric regions of sheep acrocentric autosomes. 5. The large centric fusion chromosomes of the sheep contain few, if any, sequences homologous to sheep or goat satellite I but do contain sequences homol ogous to sheep and goat satellite II, indicating preferential loss of the major highly repetitive DNA (satellite I) in the fused chromosomes of the sheep and conservation of the minor sequence (satellite II). 6. Sex chromosomes of the sheep and goat, as in the calf, contain few, if any, sequences homologous to the eight satellite DNAs studied in the three animals. This may conform to a general pattern of sex chromosomes and highly repetitive DNA sequences throughout the Bovoidea. The cytogenetic implications of these findings in the chromosome evolution of the Bovoidea are discussed.