Studies of oogenesis in natural populations of Drosophilidae. II. Significance of microclimatic changes of oogenesis of Drosophila mimica [in Hawaii]

The reproductive mode of Drosophila mimica, a species endemic to the island of Hawaii, was determined by analyzing the ovarian development of three natural populations during a 9-mo period. The developmental profile of the ovaries remained qualitatively the same in all the populations and for the entire collecting period. Each fly had comparatively few ovarioles (23.8 ± 4.21) and each developmental stage was present in only a fraction of the ovarioles. Mature eggs were usually found in half of the ovarioles and some vitellogenic oocytes were always degenerated. Quantitative differences were found between populations and between collections, and these differences were correlated with the environmental conditions. Low relative humidity was found to be the most important factor in disrupting ovarian development by terminating, directly or indirectly, the growth of oocytes at the stage of RNA-yolk synthesis. A 7-day period of constant low relative humidity causes the degeneration of growing oocytes, while a constant high humidity for the same period reinitiates normal development. This mechanism assures the presence of not more than one mature egg per ovariole, and thus avoids the overpopulation of the natural breeding substrates. The adaptive significance and the theoretical implications of such a reproductive strategy are discussed.