Evolutionary conservation of DNA coding for maternal RNA in sea urchins

The extent of evolutionary conservation of DNA complimentary to RNA stored in the mature oocyte of the sea urchin S. purpuratus has been assessed. To do this, such DNA was hybridized with total genomic DNA of S. purpuratus and S. franciscanus and the thermal stability of the resultant duplexes was measured by two methods. In the first method, the duplexes were bound to hydroxylapatite and thermally eluted; the difference in thermal stability between homologous and heterologous duplexes averaged 6.9 degrees C in duplicate determinations. In the second experiment, the same hybrids were thermally melted in 2.4M tetraethylammonium chloride, then assayed with S1 nuclease; the difference in thermal stability of homologous and heterologous duplexes was 4.8 degrees C. Either value is significantly lower than the divergence of total single-copy DNA among these species as measured by the same techniques. This demonstrates that DNA sequences complimentary to maternal RNA are conserved during evolution, and thus that a high fraction of them are likely to be physiologically functional.