Adult Age and Breeding Structure of a Hawaiian Drosophila silvestris (Diptera: Drosophilidae) Population Assessed via Female Reproductive Status

The Upper 'Ola'a Forest population of Drosophila silvestris, a dipteran species endemic to the island of Hawai'i, was studied to investigate adult age and breeding structure of this natural population. Analyses of insemination status and ovarian developmental stage were carried out for both laboratory-reared and field-collected females, including a sample of F1 individuals that had been marked and released into the field population shortly after adult eclosion. Marked females were recaptured from 7 days old to more than 4 months after· release; this sample included representatives of all seven ovarian developmental stages scored (from early previtellogenesis to fully mature ovaries). The profile of female reproductive maturation in the field flies was similar to that in laboratory-reared flies, except that developmental rates were substantially slower and more variable in the natural population, largely because of lower field temperatures. Using information on ages and ovarian condition of the marked females, an independent population sample of wildcaught adult females was estimated to include 28% young flies approximately 2 to 3 weeks old (ovaries previtellogenic), 37% maturing flies from 2 to 4 or more weeks old (vitellogenic ovaries), and 35% reproductively mature flies from 1 to more than 4 months old. The unexpected excess of young flies in the adult population up to 4 or 5 weeks old (65%) can be interpreted by several alternative hypotheses (e.g., age-related dispersal, predation, location of suitable breeding substrates, baiting effects), but further studies are required to confirm whether this age pattern is typical. Earliest onset of female receptivity occurred at mid vitellogenesis in both field and laboratory flies, with insemination frequencies increasing as ovaries matured. It is surprising that field females showed higher mating success at all competent ovarian stages than females reared in the continuous presence of males. Further, all reproductively mature field females, both marked and unmarked, were inseminated. In this species, sexual selection acts primarily on males, with the lack of female mating failure in the field providing no evidence of sexual selection among adult females