Genome structure and divergence of nucleotide sequences in echinodermata.

The arrangement of repetitive and single-copy DNA sequences has been studied in DNA of some species of Echinodermata--sea urchin, starfishes and sea-cucumber. Comparison of the reassociation kinetics of short and long DNA fragments indicates that the pattern of DNA sequence organization of all these species is similar to the so-called "Xenopus pattern" characteristic of the genomes of most animals and plants. However, substantional variations have been found in the amount of repetitive nucleotide sequences in DNA of different species and in the length of DNA regions containing adjacent single-copy and repetitive sequences. Measurements of the size of S1-nuclease resistant reassociated repetitive DNA sequences show a variability of ratios between long and short repetitive DNA sequences of different species.--The degree of divergence of short and long repetitive DNA sequences and single-copy DNA was studied by molecular hybridization of the sea urchin Strongylocentrotus intermedius 3H-DNA with the DNA of other species and by determination of the thermostability of the hybridized molecules so obtained. All three fractions of S. intermedius DNA contain sequences homologous to DNA of the other echinoderm species studied. The results obtained suggest that short repetitive DNA sequences are those which have been most highly conserved throughout the evolution of Echinodermata. A new hypothesis is proposed to explain the nature of the evolutionary changes in DNA sequence interspersion patterns.