This is an intriguing topic that I’m sure few of us have ever thought about. When one species of fish lays her eggs in some shallow pool of water and there are several other species of male fish around — besides the species of the female laying her eggs — how come the only sperm “allowed” to fertilize those eggs, are those from the correct species? For survival of the species, specific “match-making” of the sperm and egg is of course critical. Sperm must come in contact with the egg, and exactly one sperm needs to fertilize exactly one egg. This is because an unfertilized egg cannot develop, and fertilization with too many sperm (polyspermy) is equally fatal.
Fertilization has to provide a barrier between species by creating egg-sperm incompatibilities. Despite these fascinating biological questions, evolutionary implications, and clear applications for human contraception and infertility inherent in understanding the mechanisms by which egg and sperm meet — the mechanisms remain poorly understood. Authors [see attached article & editorial] investigated fertilization in zebrafish and identified Bouncer, as a protein that is important for sperm-egg interaction and which also creates a species-specific barrier.
Fertilization is a highly orchestrated event: in preparation, both egg and sperm must undergo changes. The sperm increases its mobility and the sperm head is reorganized to form a cap-like structure, (acrosome); this prepares the sperm for penetration of the egg coat (a specialized extracellular matrix that surrounds the egg), attachment, and eventual fusion with the egg membrane. Egg activation leads to changes in the egg coat (called the zona pellucida, or chorion). In mammals, the egg coat not only is a structural component that protects the oocyte but also attracts and binds sperm. However, in fish the micropyle (a specific opening in the egg coat) forms the single entry point for sperm. Upon fertilization, the egg coat then becomes less permeable to prevent polyspermy.
Unexpectedly, authors [see attached] found that Bouncer not only is required for sperm-egg interaction but is also sufficient to allow cross-species fertilization between zebrafish and medaka (two species that diverged from one another more than 200 million years ago)..!! This study thus identifies Bouncer as a key determinant of species-specific fertilization in fish. Bouncer’s closest gene homolog in tetrapods (including human) is SPACA4, a gene expressed only in the male germline — which suggests that these findings in fish have relevance to human biology.
Science 7 Sept 2o18; 361: 1029–1033 [article] & pp 974–975 [editorial]