[Divergence of repetitive DNA sequences in echinoderms. II. Comparison of sequences with a low degree of intragenome divergence].

The DNA-DNA hybridization method was used to estimate the value of intergenomic divergence of sequences of middle and thermostable fractions among representatives of three classes of echinodermata (Echinoidea, Asterodea and Holothurioidea), and also representative of another type, ascidian Halocynthia aurantium. The results obtained were compared with analogous measurements carried out with a fraction of thermolabile reiterating sequences. It was found that in each of the three fractions there are sequences that are inherent in DNA of all the species compared. The proportion of these sequences in the thermolabile fraction is larger than that in the other two. The degree of divergence of the common sequences were estimated by the temperature stability of hybrid duplexes. Comparison of the related species shows that it is higher for the sequences of the thermostable fraction, whereas the sequences that are inherent in distant species diverged more markedly in the thermolabile fraction. Part of the sequences in the thermostable fraction form hybrid duplexes with a high melting temperature, even if one of the DNAs is from distant species. No such sequences were found in the thermolabile fraction. The results obtained are discussed from the point of view of the functional role of various groups of reiterating sequences are of the significance of these data for comprehension of the evolutional history of species.