Egg cell gatekeeper keeps zebrafish sperm in line

What happens when a sperm and egg meet? Scientists know surprisingly little about the molecular mechanics of fertilization; only three proteins central to the process have ever been identified, all in mammals. A new study in Science reports that a tiny protein on the surface of zebrafish eggs acts as a species-specific gatekeeper for sperm, giving fertilization the green light only if the sperm is also from a zebrafish (DOI: 10.1126/science.aat7113).

Researchers in Andrea Pauli’s lab at the Research Institute of Molecular Pathology in Vienna stumbled upon the gene for this 80-amino-acid protein in a poorly-understood region of the zebrafish genome. Zebrafish engineered to lack the gene produced eggs, or oocytes, that couldn’t be fertilized by zebrafish sperm. “We discovered this was a new fertilization factor,” Pauli says. “So far, our data suggest that it helps the sperm bind to the oocyte surface.”

The protein has homologs in other species, but their amino acid sequences vary significantly. “That made us very curious,” Pauli says, “so we decided to do a crazy experiment.” The team used the gene editing tool CRISPR to engineer zebrafish so that their eggs expressed a homolog from a different fish called a medaka. Now, zebrafish sperm couldn’t fertilize the egg, but medaka sperm could. The researchers called the protein Bouncer, a reference to a security guard at a bar.

“This little protein was the only thing we changed on the surface of the oocyte, and that was sufficient to allow sperm from the wrong species to get in,” Pauli says. “That was completely unexpected.” Such species specificity is crucial for animals like fish and frogs, who release their sperm and eggs into the water for fertilization.

“This represents an important contribution to the field of reproduction,” says Enrica Bianchi, a molecular biologist at the Wellcome Sanger Institute who was not involved in the work. The study “is shedding light on an essential step for life that is still full of mysteries.”

Pauli’s team is now looking more closely at how Bouncer works and searching for its binding partner in zebrafish sperm. They are also studying whether Bouncer’s mammalian homolog—a protein called SPACA4, which is expressed in sperm rather than eggs—is also involved in fertilization.