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Developmental Biology 286 (2005) 169 – 181 www.elsevier.com/locate/ydbio The spe-42 gene is required for sperm–egg interactions during C. elegans fertilization and encodes a sperm-specific transmembrane protein Tim L. Kroft, Elizabeth J. Gleason, Steven W. L’Hernault* Department of Biology, Emory University, Atlanta, GA 30322, USA Received for publication 2 April 2005, revised 15 July 2005, accepted 18 July 2005 Available online 24 August 2005 Abstract Fertilization, the union of sperm and egg to form a new organism, is a critical process that bridges generations. Although the cytological and physiological aspects of fertilization are relatively well understood, little is known about the molecular interactions that occur between gametes. C. elegans has emerged as a powerful system for the identification of genes that are necessary for fertilization. C. elegans spe-42 mutants are sterile, producing cytologically normal spermatozoa that fail to fertilize oocytes. Indeed, male mating behavior, sperm transfer to hermaphrodites, sperm migration to the spermatheca, which is the site of fertilization and sperm competition are normal in spe-42 mutants. spe-42 mutant sperm make direct contact with oocytes in the spermatheca, suggesting that SPE-42 plays a role during sperm – egg interactions just prior to fertilization. No other obvious defects were observed in spe-42 mutant worms. Cloning and sequence analysis revealed that SPE-42 is a novel predicted 7-pass integral membrane protein with homologs in many metazoan species, suggesting that its mechanism of action could be conserved. D 2005 Elsevier Inc. All rights reserved. Keywords: C. elegans; Spermatozoa; Fertilization; Gamete; Sperm – egg interactions; Spermatogenesis; Oocyte Introduction Fertilization requires that gametes meet, recognize each other, bind and fuse to form a new organism. Our current understanding of sperm – egg interactions at the plasma membrane is drawn chiefly from experiments using sea urchins, mice and, more recently, C. elegans (reviewed in Geldziler et al., 2004; Neill and Vacquier, 2004; OldsClarke, 2003; Singson, 2001; Singson et al., 2001; Stein et al., 2004; Vacquier, 1998). While it has long been known that bindin is the sea urchin sperm plasma membrane ligand (Vacquier and Moy, 1977), the cognate receptor on the oocyte vitelline envelope, EBR1, was only recently discovered (Kamei and Glabe, 2003). Many molecules have been proposed as mouse sperm and egg ligands and receptors (He et al., 2003; reviewed in Stein et al., 2004), but only the * Corresponding author. E-mail address: [email protected] (S.W. L’Hernault). 0012-1606/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.ydbio.2005.07.020 tetraspanin CD9 on the egg (Kaji et al., 2000; Le Naour et al., 2000; Miyado et al., 2000) and the Ig superfamily member Izumo on the sperm (Inoue et al., 2005) have been shown to disrupt sperm –egg fusion in mice with targeted deletions in these loci. The unusual reproductive biology of C. elegans makes it an excellent model system for the study of sperm – egg interactions during fertilization. The primary reproductive mode of C. elegans hermaphrodites is self-fertilization, but a hermaphrodite’s oocytes can also be fertilized by a male. A simple yet powerful genetic screen has been used to identify worms with sperm-specific mutations that affect fertilization (L’Hernault, 1997). Spermatogenesis-defective (Spe) hermaphrodites are self-sterile and lay only oocytes, but their sterility defect can be rescued by a male’s wild type sperm, which demonstrates that the Spe mutation affects only sperm. Despite the unusual appearance of amoeboid C. elegans spermatozoa, they must migrate to the site of fertilization, recognize the oocyte in a species-specific manner and fuse with it to form a zygote, which are tasks