Ovarian activity and reproductive potential in a natural population of Drosophila melanogaster

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Abstract
The ovarian state of Drosophila melanogaster females from a wild population in the south of France was studied by immediate dissection, in the field, during two different periods in the year: early in June and at the end of September. It was compared with either the reproductive capacities of aliquot females kept in laboratory conditions or the reproductive potential of the first laboratory generation. Among the wild flies, about 20% were young (less than 24 h). 85 to 90% of all females were inseminated and virgins were found among the youngest ones. In the field, the vitellogenetic activity appeared to be very low and noncontinuous. Females with active vitellogenesis were fairly rare but about 60% of the females presented eggs in retention in the ovaries and 50% had a fertilized egg waiting in the uterus. All these observations showed that fecundity in the field was certainly low. When wild females were studied in the laboratory, their daily egg production was relatively high, more than 50 eggs per day per female. This production was stimulated by insemination and decreased with adult population density. Daughter females reared in the laboratory were bigger, had more numerous ovarian tubes than their mothers (an average of 50 instead of 32), and their daily egg production was very high. However the daily overiole production in these two groups was very similar. The genetic reproductive potential of this natural French population seems to be very high but, under natural life conditions, this potential is not expressed. Wild flies do not seem to give priority to their reproductive effort but to individual survival and activity.