Diversity and evolution of reproductive traits among cactophilic Drosophila.

Abstract

Interspecific diversity of reproductive traits among cactophilic Drosophila is described at different levels. The insemination reaction concept is reexamined using ultrastructural techniques. Examination of the vaginal contents of five Drosophila species suggested three distinct, more precise terms: the sperm sac, the mating plug and the true insemination reaction. Behavioral and physiological factors associated with the expulsion of the sperm sac in D. mettleri are explored in detail. Timing of the expulsion of the sperm sac appears to be correlated with the nutritional state of the female; the aggressive post-mating courtship of the males also has an effect. D. nigrospiracula and D. mettleri are compared in diverse aspects of their reproductive biology. Chemically, the sperm sacs are different. D. mettleri females retain the sperm sac longer than D. nigrospiracula females. In D. mettleri the sperm takes longer to move into the ventral receptacle and females absorb materials from the male ejaculate; females mate before the eggs are fully mature. In D. nigrospiracula sperm moves rapidly into the ventral receptacle and females do not integrate materials from the male ejaculate. Field work reveals that the developmental stage of the eggs in D. mettleri is associated with the presence of suitable oviposition substrate. There is a differential distribution of sexes and species with respect to the feeding areas and a temporal succession of the two species. Bias sex ratio in the field is discussed. The courtship song of the six species in the eremophila and anceps complexes is a phylogenetically conservative trait. This pattern is interpreted in terms of interspecific hybridization and distribution data for each of the complexes. A new technique for recording female sounds during courtship is proposed. The use of discriminant analysis helped to interpret differences among behavioral patterns, and allowed identification of courtship components which may be responsible for the mating asymmetry between two populations of D. mojavensis. The diversity described is interpreted from two perspectives: a proximal interpretation, integrating results from the different levels of organization, and a historical interpretation of the patterns of similarities and differences among species.