GENETIC VARIATION FOR HOST PREFERENCE WITHIN AND AMONG POPULATIONS OF DROSOPHILA TRIPUNCTATA

Some time ago, perhaps in the Miocene, a number of flies dispersed from the Old World to the New World tropics, multiplied, and thus initiated the tripunctata radiation of the genus Drosophila (Throckmorton, 1975). The descendant species diversified ecologically, and they now breed variously in fungi, rotting fruits, sap flows, flowers, and decaying vegetation-a diversity of larval feeding niches almost as broad as that of the genus itself. A sole member of this radiation, D. tripunctata, has colonized the Nearctic, and it is presently abundant in many forests of the central and eastern United States (Wheeler, 1981; Fig. 1). This species is polyphagous in an unusual and interesting way: it commonly utilizes both decaying fruits and mushrooms as breeding sites (Sturtevant, 1921; Carson and Stalker, 1951). Since Drosophila larvae are not very vagile, the primary basis for the polyphagy of D. tripunctata and indeed for the ecological diversification of the group as a whole must ultimately reside in the oviposition site preference of female flies. The diversification of descendant species, in particular, would seem to depend critically on the existence of genetic variation for host preference among females. Of course, physiological adaptations that allow larvae to exploit these various resources are also important and should, in general, evolve in concert with changes in oviposition site preference (for examples in