Ultrastructural diversity in the egg chorion of Hawaiian Drosophila and Scaptomyza: ecological and phylogenetic considerations

Abstract

Formation of the egg shell (chorion) in Drosophila and Scaptomyza (Diptera : Drosophilidae) is a complex developmental process involving coordinated synthesis and secretion of multiple proteins by the monolayer of follicle cells surrounding the egg. Using scanning electron microscopy, the ultrastructure of the chorion in 37 endemic Hawaiian drosophilids, representing the genera Drosophila and Scaptomyza, were analyzed and compared with 7 representative species of continental Drosophila. The detailed structure of the chorion was described for 8 chorionic regions: the respiratory filaments, follicle imprints, operculum, micropyle, dorsal ridge, ventral rim, posterior pole, and the chorion cross-section. The morphology of each region is similar among related species, but strikingly different among groups. The main functions of the chorion are to protect the developing embryo from the vicissitudes of the environment and to provide channels for gas exchange during embryogenesis. Adaptation to the diverse ovipositional substrates used by Drosophila in general, and the Hawaiian species in particular, has resulted in extraordinary diversity in the various chorionic structures. The respiratory filaments differ in number and have evolved to different lengths and degrees of porosity. Furthermore, other regions also involved in respiratory exchange (the operculum, follicle imprints, the pole region, and the dorsal ridge) have diverged in parallel to the ecological divergence. The thickness and complexity of the outer endochorion are dramatically different in various groups, providing varying degrees of mechanical strength to the eggshell, which promotes embryonic survival in the diverse microenvironments. These varied chorionic structures have been found to provide useful morphological characters for phylogenetic analyses of the drosophilids.