DROSOPHILA-SUBOBSCURA FEMALES PREFERENTIALLY SELECT LONG SPERM FOR STORAGE AND USE

Drosophila subobscura males produce short and long sperm. From a total of approx. 20,000 sperm transferred per copulation, 66% were short an d 34% long. Thirty hours after copulation females had stored approx. 9 00 sperm in the receptacle and spermathecae. The remaining 19,000 sper m were eliminated from the uterus. Females preferentially selected the long sperm from the ejaculate for storage, but the percentages of lon g sperm were different in the two storage organs. In the receptacle 89 % were long sperm and in the spermathecae 80%. While the ratio of shor t to long sperm in the receptacle was constant, the ratio varied in th e spermathecae. After seven days of egg laying only one third of the i nitial amount of sperm remained in the storage organs. At this time th e percentage of short sperm was higher than just after storage, indica ting that long sperm were preferentially used for fertilization. We mo nitored the entire egg laying periods of females which mated once, cou nting all inseminated eggs; the number of hatched eggs each resulted f rom a fertilizing sperm. Additionally, we recorded those sperm which d id not penetrate the egg but were found outside of the egg on the chor ion. The sum of fertilizing sperm plus the sperm on the chorion (916) matched perfectly the mean number of stored sperm per female (915).