Ultrastructural variation and adaptive evolution of the ovipositor in the endemic Hawaiian Drosophilidae.

Ecological diversification of the endemic Hawaiian Drosophilidae has been accompanied by striking divergence in egg morphology, and ovarian structure and function. To determine how these flies successfully oviposit in a variety of breeding substrates, we used Scanning Electron Microscopy to examine the ultrastructure of the ovipositor of a sample of 65 Drosophila species and five Scaptomyza species of this hyperdiverse monophyletic group. The Drosophila species analyzed included representatives of the fungus-breeding haleakalae group, the leaf-breeding antopocerus and modified tarsus groups, the modified mouthparts species group, the nudidrosophila, and the picture wing clade; the latter sample of 41 species from four species groups included stem- and bark-breeders, as well as tree sap flux-breeders. Ovipositor length was found to vary more than 12-fold among Hawaiian drosophilids, with the longest ovipositors observed in the bark-breeding species and the shortest among the Scaptomyza and fungus-breeders. More noteworthy is the striking variation in overall shape and proportions of the ovipositor, in the shape of the apical region, and in the pattern of sensory structures or ovisensilla. Ultrastructural observations of the pair of long subapical sensilla on the ventral side identify these, for the first time, as taste bristles. Ovipositor form correlates strongly with the oviposition substrate used by the species, being of a distinctive shape and size in each case. We infer that the observed morphological divergence in the ovipositor is adaptive and the product of natural selection for successful reproduction in alternate microhabitats. The array of ovipositor forms that have evolved among the Hawaiian Drosophila species represent a series of ecomorphs that along with other divergent traits of the female reproductive system, have contributed to the successful adaptive radiation of this remarkable fauna.

The eggshell of hawaiianDrosophila: structural and biochemical studies inD. grimshawi and comparison toD. melanogaster.

The eggshell of the Hawaiian speciesDrosophila grimshawi was analyzed in terms of its structure and biochemical composition. It is generally similar toD. melanogaster in radial structure except that the inner endochorion is relatively compact and the outer endochorion is thick (1.9 µm) and traversed by numerous 50-nm-wide aeropyles (not present inD. melanogaster). Regionally the eggshell consists of the various specialized regions, which are: the holey posterior pole; the dorsal ridge; the operculum with very tall imprint borders; the collar with discontinuous outer endochorion; the micropyle with an external eccentric hole and an internal eccentric paracrystalline region; and the four very long respiratory appendages which exhibit tightly covered surface features for underwater plastron respiration. On the basis of the above structural data we suggest that the entireD. grimshawi eggshell (various layers and specialized regions) is the product of at least eight follicle cell subpopulations. In addition, in vitro culturing with3H-proline indicates that the eggshell is made according to a strict developmental program (analogous toD. melanogaster) and consists of several proteins ranging in molecular weight from 14,000 to 37,000 daltons.