Evolutionary distances in Hawaiian Drosophila measured by DNA reassociation

Comparisons of the sequence divergence of three species of Hawaiian Drosophila have been made by hybridization of single-copy tracer DNA of each of the species with driver DNA from each species, and measurement of the average melting temperature (Tma) in a chaotropic solvent (2.4 M tetraethylammonium chloride) which minimizes differences due to base composition. Correction was made for the length of hybrid duplex regions to obtain the reduction in thermal stability due to divergence. An accuracy of +/- 0.2 degrees C was achieved and the mean reduction in Tm for hybridization between D. heteroneura and D. silvestris (found only on the island of Hawaii) was 0.55 degrees C and between D. picticornis, found only on the island of Kauai, and the other two species was 2.13 degrees C. The rate of DNA change is estimated to be between 0.2 and 0.4%/My by assuming that the D. heteroneura-D. silvestris divergence occurred 0.8 My ago and the divergence between these species and D. picticornis between 4 and 6 My ago. The general single copy DNA sequence divergence appears to be very much greater than the minimal coding region sequence divergence previously estimated from allozyme studies.