Hawaiian Drosophila as Measured by Dna Reassociation

Four species of the Hawaiian Drosophila ph i t ib ia subgroup which are homosequential in their polytene chromosomes are resident on the islands of Molokai, Maui and Hawaii. Comparisons of DNA sequence divergence in these four have been made by hybridization of total single-copy radiolabeled tracer DNA from each of the species with excess nonlabeled DNA from each of the species, and measurement of the reduction of average melting temperature (ATma) was made in 2.4 M tetraethyl ammonium chloride. The mean ATma between either D. heteroneuro or D. silvestris and either D. planitibia or D. differens was found to be 1.06', whereas the difference between D. planitibia and D. differens in 0.65' and between D. heteroneura and D. silvestris is 0.75'. These measurements taken together with the distances calculated from isozyme studies, chromosomal relationships, as well as the island locations indicate that the ancestor of these species diverged from other planitibia subgroup flies on Molokai [age 1.8 million years before present, (My BP)]. We hypothesize that one line became the present-day D. differens and diverged probably at the time of formation of East Maui (0.8-1 My BP) to form the species D. planitibia. Flies from the other line migrated to Hawaii soon after its formation (0.7 My BP) to form the two species D. heteroneura and D. silvestris. HE four species of Hawaiian Drosophila, D. heteroneura, D. silvestris, D. T planitibia and D. differens, are close relatives from the picture-wing group of Drosophila species found in the high altitude rainforests of the newer Hawaiian Islands. The first two species are endemic to the island of Hawaii, where they are broadly sympatric. D. planitibia and D. differens are endemic to Maui and Molokai, respectively. Their evolutionary relationships have been studied using morphological characteristics (CARSON and KANESHIRO 1976), polytene chromosome banding homology (CARSON and YOON 1982), isozyme studies (SENE and CARSON 1977; CRADDOCK and JOHNSON 1979) and ethological isolation and behavior (KANESHIRO 1976; SPIETH 1981). The speciation process can also be related to the time of formation of the islands upon which they are found (CARSON and YOON 1982). Of all of these measurements, except possibly for age of the island of origin, the only one that gives a numerical measure of evolutionary distance is the Genetics 104: 353-364 June, 1983. 354 J , A. HUNT AND H. L. CARSON isozyme analysis. However, for the four species in question analysis has been limited to gene frequencies of 12 enzyme systems between D. differens (D) and D. planitibia (P) and 24 enzyme systems between D. heteroneura (H) and D. silvestris (S). A distance (b, NEI 1975) of 0.23 is found between the D-P pair and the H-S pair. Within each pair b is 0.11 for D. planitibia and D. differens and 0.02-0.08 for D. heteroneura and D. silvestris (SENE and CARSON 1977; CRADDOCK and JOHNSON 1979). Chromosomally, D. heteroneura and D. silvestris are homosequential in polytene chromosomes with D. planitibia and D. differens. The former pair, however, shares a polymorphic inversion (CARSON and KANESHIRO 1976). We have now measured the distances between these species by DNA reassociation of single-copy tracer DNA from each of the species and by measurement of the lowering of melting temperature of the hybrids. We conclude that the two pairs of species (D-P and H-S) diverged from each other at essentially the same time, and each pair derived from a common ancestral species that had colonized and diverged on the island on Molokai.