Male sterile mutant casanova gives clues to mechanisms of sperm-egg interactions in Drosophila melanogaster

The plasma membrane of the spermatozoa of Drosophila melanogaster contains two integral proteins with glycosidase activity, beta -N-acetylglucosaminid ase and alpha -D-mannosidase. Biochemical analysis and ultrastructural cyto chemistry of spermatozoa of the autosomal male sterile mutant casanova reve al that at least one of these enzymes, beta -N-acetylglucosaminidase, is cr ucial for sperm-egg interactions. casanova sperm are motile, morphologicall y normal, are transferred to the female at mating, but are unable to fertil ize the eggs. The mutation was localised by deficiency mapping to the chrom osomal region 95E8-F7. Fluorimetric assays showed that the mutant's sperm h ave the same level Of alpha -D-mannosidase activity as wild-type sperm, whe reas beta -N-acetylglucosaminidase activity reaches only 51% of the wild-ty pe level. The biochemical characteristics of alpha -D-mannosidase and of th e residual beta -N-acetylglucosaminidase are the same as in wild-type males . Ultrastructural localization of the enzymes indicated that casanova sperm atozoa lacks beta -N-acetylglucosaminidase on the plasma membrane covering the acrosome, whereas the location of this glycosidase at the terminal part of the sperm tail is indistinguishable from the wild-type situation. The r esults strongly suggest that in Drosophila the beta -N-acetylglucosaminidas e of the plasma membrane covering the acrosome functions as a receptor for the glycoconjugates on the egg surface. We named the putative egg receptor EROS. This is the first evidence for an egg/sperm recognition system in ins ects. The mechanism is similar to those known from higher animals. Mol. Rep rod. Dev. 60: 248-259, 2001. (C) 2001 Wiley-Liss, Inc.