Fertilization-induced modification of chorion N-acetylglucosamine groups blocks polyspermy in ascidian eggs

Abstract

Sperm bind to the chorion (vitelline envelope) of Ascidia nigra, which has many exposed N-acetylglucosamine (Glc Nac) groups. The Glc Nac-binding lectin wheat germ agglutinin (WGA), and p-nitrophenyl N-acetylglucosaminide block sperm binding as does N-acetylglucosaminidase digestion. Thus chorionic Glc Nac is an important component of the sperm receptor. Sperm bind to the chorion in increasing numbers during the first minute after fertilization, then bound sperm fall off to reach a low level by 2 min. Sperm unbinding is blocked by Glc Nac but not other sugars. Chorion binding of fluorescent WGA is also depressed after fertilization. Supernumerary sperm which have unbound from chorions gain a WGA binding site at the tip of the head, demonstrating the transfer of a Glc Nac group from chorion to sperm upon release. Glycerol-extracted eggs bind sperm during the first minute but fail to show the decline seen with living eggs. Supernatant obtained after fertilization of a dense egg suspension contains a trypsin-sensitive factor which modifies glycerinated eggs so that sperm binding is greatly inhibited. The block to polyspermy forms with the same kinetics as sperm unbinding, and is also sensitive to Glc Nac. This suggests that release of a chorion modifying factor removes Glc Nac residues from the chorion, releasing extra sperm and preventing polyspermy.