Coadaptive Gene Complexes in Incipient Species of Hawaiian Drosophila

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Abstract
The degree of postmating isolation existing among allopatric populations belonging to 2 spp. of endemic Hawaiian [USA] Drosophila was investigated. D. grimshawi is unusual among picture-winged Hawaiian Drosophila in that it is widely distributed in the archipelago with populations on the islands of Kauai, Oahu, Lanai, Molokai and Maui. D. pullipes is very closely related and is confined to the island of Hawaii. Kauai and Oahu D. grimshawi and D. pullipes are ovipositional specialists, utilizing only 1 genus of plant. Populations of D. grimshawi from Lanai, Molokai and Maui are generalists and oviposit on several plants of a number of different families. Hybridizations among stocks representing 4 of the 6 allopatric populations (i.e., Oahu, Maui and Molokai D. grimshawi and D. pullipes) were completed in all reciprocal combinations. Kauai D. grimshawi was also crossed to Oahu D. grimshawi. The F1 hybrid progeny were used to produce an F2 and backcrosses to parental males and females. Although crosses between ecologically different D. grimshawi stocks resulted in full F1 male fertility, breakdown was observed in the F2 and backcross males. This was not observed in crosses between ecologically similar populations. Fertility breakdown in these intraspecific crosses was consistently greatest in the F2, followed by the backcross to the parental males, and least in the backcross to the parental females. This progressive breakdown, based on the assumption of no crossover recombination in Hawaiian Drosophila males, was attributed to the breakup of 2 differing sets of coadaptive gene complexes. The coadaptive gene complexes of populations with the same adaptive peaks may be identical or at least similar enough so as not to have yet evolved reproductive isolation. The D. pullipes stock produced completely sterile F1 males when hybridized with all stocks except Molokai, in which approximately 70% of the F1 males were fertile. Added to the hybrid sterility, crosses involving D. pullipes females showed a reduction in hybrid viability.