Evolution of sea urchin non-repetitive DNA

New methods have been applied to the determination of single copy DNA sequence differences between the sea urchin species Strongylocentrotus purpuratus, S. franciscanus, S. drobachiensis, and Lytechinus pictus. The thermal stability of interspecies DNA duplexes was measured in a solvent (2.4 M tetraethylammonium chloride) that suppresses the effect of base composition on melting temperature. The lengths of duplexes were measured after digestion with S1 nuclease and correction made for the effect of length on thermal stability. The degree of base substitution that has occurred in the single copy DNA during sea urchin evolution is significantly larger than indicated by earlier measurements. We estimate that 19% of the nucleotides of the single copy DNA are different in the genomes of the two sea urchin congeners, S. purpuratus, and S. franciscanus, which apparently diverged only 15 to 20 million years ago.