On the mechanism of amplification of satellite II DNA sequences of the domestic goat Capra hircus.

A restriction enzyme analysis of the satellite II DNAs of the domestic goat Capra hircus, sheep Ovis aries and ox Bos taurus (p = 1.720, 1.723 and 1.722 g/cm3, respectively) has been carried out and shows that, although all three are composed of repeat units of 700 base-pairs, goat satellite II is present in the genome primarily in the form of 2100 base-pair trimers. Unequal crossing-over between repeat units has produced an oligomer series, whose oligomers gradually decrease in copy number the further they are in molecular weight from the trimer. The trimer population is much more uniform than the monomer population, as most trimers have similar restriction patterns, whereas their component monomers differ considerably in their restriction properties. This heterogeneity was confirmed by cross-hybridization of the different monomers of a cloned trimer. Here, heterologous hybrids were much less stable than the homologous hybrids. Attempts were made to simulate such an oligomer series by computer, using a longitudinal (saltatory), and two horizontal (unequal crossover and drive) models. Simulations of both the saltatory and drive mechanisms could produce the oligomer series in approximately the observed ratios, but only the former could simultaneously produce other restriction properties of this sequence family. This was because the other restriction sites were in a different (monomer) register, and it is difficult for a drive model promoting traits in only one register to fix properties in different registers. The unequal crossover model proposed by Smith (1973, 1976) generally produced homogeneous arrays from heterogeneous arrays, but higher-order repeat structures could be produced when the efficiency of crossing-over between different monomer types was reduced. In most of these arrays, the dimer was the predominant oligomer, but in approximately 10% the trimer was predominant. Since the unequal crossover model produces dimeric arrays with high frequency given appropriate conditions, it is an attractive model for explaining the production of satellite DNAs whose structure has evolved through a series of doublings, such as mouse major satellite DNA and the "alphoid" satellite sequences of primates. Other factors necessary for this model to work are generally considered to be natural components of the speciation process. It is therefore suggested that, although the saltatory model conforms most closely to the observed structure of goat satellite II, this particular satellite DNA may represent one of the few cases when the unequal crossover mechanism does not give rise to a dimeric structure.