DROSOPHILA HETERONEURA AND DROSOPHILA SILVESTRIS: HEAD SHAPES, BEHAVIOR AND EVOLUTION.

Abstract

The planitibia subgroup of the picturewinged Drosophila is represented on Hawaii, the easternmost island of the chain, by two closely related species, D. heteroneura and D. silvestris (Carson, 1979). Hawaii consists of seven sequentially aged volcanoes that are fused into a single large land mass (Fig. 1, Table 1). The flies dwell in the rain forests on the slopes of these volcanoes at elevations of circa 900-1,500 m. Drosophila heteroneura essentially is restricted to the 900-1,200 m range and thus is fully sympatric with silvestris but populations of the latter extend above the areas occupied by heteroneura. Further, silvestris is present on the northernmost volcano Kohala where heteroneura is lacking. Drosophila heteroneura prefers areas of the forest where the density of the vegetation, especially the trees that form the forest canopy, is lower than that preferred by silvestris. As a result the light intensity in the understory portions of the forests where heteroneura lives is higher than in the areas typically selected by silvestris. Thus, although the species broadly overlap in their distributions, there is partial geographical and ecological isolation between them. The prime larval substrates of both species are the fermenting parts of plants, especially the bark of Clermontia spp (Montgomery, 1975). Clermontia is an understory shrub widely but not uniformly scattered throughout the forests. In areas where a number of individuals of Clermontia are present, both species of flies can be found in close association and both species have been reared from the same fermenting mass of Clermontia (Kaneshiro and Val, 1977). Morphologically the two species are distinctly different and readily separable (Hardy, 1965; Carson, 1979). Sexual isolation under laboratory conditions is strong but not complete (Kaneshiro, 1976). However, when heterogamic mating does occur the F1 hybrids produced are viable and fertile (Carson, 1979). Further, fertile hybrids have been recovered in the field (Kaneshiro and Val, 1977; Carson, 1979). The following discussion is concerned with the probable roles that behavior and the geological history of the island of Hawaii have served in the evolution of these two species and is based on behavioral data from laboratory and field studies conducted during the past decade. The laboratory studies used both field captured and laboratory reared individuals. Conant (1978) conducted intensive field studies on both species in the Pauahi area, which is located in the Kona district 8.6 km east of Captain Cook, where the two species live in sympatry. His findings parallel my field data.