Temperature-Regulated Changes in the Ovaries of Overwintering Females of the Queensland Fruit Fly, Dacus Tryoni.

Abstract

Changes in the ovaries of overwintering D. tryoni females at two localities in central eastern New South Wales were studied during 1969 and 1970. In the ovaries of females from both locations, new follicles were differentiated throughout the winter months but most of them were later resorbed. By the end of the winter most females had at least two and some as many as four resorbed follicles per ovariole. The most advanced follicle in the ovariole was resorbed first, except in ovarioles where mature eggs were present, when the penultimate follicle was the first to be resorbed. In a high proportion of the mated parous females the sperm disappeared from the spermathecae when all the viable mature eggs had been laid. Comparison between the changes in the median stages of ovarian development and the tempera- ture data for the same period indicated that it was the mean number of day-degrees above the developmental threshold (13.5ºC) experienced by the females which determined whether the ovaries continued to mature or not. Ovarian maturation continued while the mean number of day-degrees stayed above 1.6 and resorption occurred when it dropped below this. However, when the mean number of day-degrees dropped much below unity the ovaries showed very little change either way. The possible causes of ovarian resorption during the winter months and its possible significance for the survival of the flies is discussed.