SummaryThe Y chromosomes of most Drosophila species are necessary for male fertility but they are not involved in sex determination. They have many puzzling properties that resemble the effects caused by B chromosomes. Classical genetic and molecular studies reveal substantial affinities between Y and B chromosomes and suggest that the Y chromosomes of Drosophila are not degenerated homologues of the X chromosomes, but rather that their Y chromosomes evolved Accepted 7 December 1995 as specialized supernumeraries similar to classical B chromosomes.

Drosophila subobscura has 128 spermatids per cyst, enclosed by two cyst cells. At beginning of elongation in control males the spermatid nuclei surround the head cyst cell nucleus, in sex ratio males nuclei are found throughout the cyst. Spermatid nuclei can elongate in any position in the cyst. Nuclei can be eliminated during individualization or degenerate after individualization. The number of sperm in any wrong position in the cyst varies in control males from 0 to about ten, in sex ratio males from 0 to more than 50. Two cyst sizes are distinguishable. At beginning of elongation small cysts have homogeneously stained spherical nuclei which later on are rod like. Large cysts have granulated nuclei which at first become spindle shaped and then slender. The length of the DNA containing part of elongated sperm heads of the long class is about 33 ~zm in sex ratio and control males. The small sperm heads are 15 ~zm in sex ratio but 20 ~zm in control males. The complete DNAcontaining-sperm-length is about 10K less in short sperm and 5~ less in long sperm of sex ratio males than in those of control. Sex ratio males have more cysts per testis than control males. In sex ratio we counted 53.8~, in control males 49.4~ short cysts.

A male Drosophila melanogaster deposits many more sperm in a female's bursa copulatrix than are stored in her ventral receptacle or paired spermathecae soon after copula has ended. The remaining sperm are expelled by the female. These observations suggest a sexual con£ict over the processes involved in sperm storage. We used genetically manipulated £ies to study the role of the central nervous system in sperm storage. Flies with female bodies but masculinized nervous systems, or isolated female abdomens, stored signi¢cantly fewer sperm than did control females. Furthermore, compared with control £ies, there were relatively more sperm in the ventral receptacle and relatively fewer in the spermathecae. These results suggest that the female nervous input counteracts the male's attempts to force sperm into the ventral receptacle during copula and promotes active transport of sperm to the spermathecae during and after copula. The female is clearly a very active partner in in£uencing processes involved in sperm competition, especially as only stored sperm can be used later to fertilize eggs. To our knowledge, this is the ¢rst study to show directly the involvement of the female nervous system in sperm storage.

No abstract