Comparative structure and evolution of goat and sheep satellite I DNAs

The satellite I DNAs of domestic goat (Capra hircus) and domestic sheep (Ovis aries) have been studied using molecular hybridisation and restriction enzyme analysis. Both satellite DNAs are composed of repeat units of 820 base pairs in length, but their restriction maps, although similar, differ in certain respects. Thus the majority of sheep satellite I repeat units have two EcoRI sites and one AluI site, whereas the majority of goat satellite I repeat units have one EcoRI site and two AluI sites. The sheep satellite I repeat units with the two EcoRI sites are much more homogeneous than the repeats forming the remainder of the satellite, as judged by the difference in the melting temperatures of native and reassociated molecules. DNAs from species of wild sheep and goats were screened for the presence of these repeat units, and they appear to have been amplified during the radiation of the Ovis genus. Goat satellite I is composed of a single sequence type which has changed through base substitution until the sequence now shows considerable heterogeneity. It is proposed that the major sequence types of these two satellite DNAs were amplified by different saltatory replication events at different times in the evolution of the group.