Germ-cell warfare in ascidians: Sperm from one species can interfere with the fertilization of a second species

Ascidians (invertebrate chordates) are very abundant in many marine subtida l areas. They often live in dense multispecies clumps; thus, interspecific competition for space may be intense. Although most noncolonial species are broadcast spawners, their eggs can be fertilized only by sperm of the same species (1). Multiple fertilization is lethal and all animals have evolved blocks to polyspermy. Ascidian eggs block polyspermy by enzymatic (2) and electrical mechanisms (3). Sperm bind to N-acetylglucosamine groups on the vitelline coat (4, 5, 6, 7). Follicle cells surrounding the vitelline coat release N-acetylglucosaminidase during egg activation (8), preventing the b inding of all sperm but a few (2). 1 show here that this interaction is not species-specific; sperm from one species can cause glycosidase release fro m follicle cells of a second species. Furthermore, once glycosidase release has been induced the subsequent addition of sperm from the egg-producing s pecies fails to fertilize a substantial proportion of these eggs. This lead s to the hypothesis that sperm from one species of ascidian can interfere w ith fertilization of a second species. While intraspecific sperm competitio n has been well documented in several taxa (9, 10), this is the first recor d of sperm competition between species, or interspecific sperm competition.