Evolutionary conservation of DNA sequences expressed in sea urchin eggs and early embryos.

DNA sequence divergence measurements indicate that Strongylocentrotus franciscanus is more distinct from S. purpuratus and S. drobachiensis than these two species are from each other, in agreement with paleontological and morphological evidence. The evolutionary divergence of several classes of expressed DNA sequences was compared with that of total single-copy DNA. Between S. franciscanus and S. purpuratus the divergence of cDNA made from gastrula cytoplasmic poly(A)+ RNA is about half that of total single-copy DNA. Similar results were obtained for cDNA made from unfertilized egg poly(A)+ RNA. In contrast, sequences expressed in gastrula nuclear RNA have diverged almost as much as total single-copy DNA.