Bouncer's receptor on sperm: Investigating sperm-egg compatibility in fish

Fertilization requires the successful binding and fusion of sperm and egg. In zebrafish, sperm-egg binding is mediated by the Spaca6-Izumo1-Tmem81 complex on sperm interacting with Bouncer on the egg. We previously found that expressing medaka Bouncer on zebrafish eggs and vice versa allows hybridization between medaka and zebrafish, two species that can normally not hybridize. Here, we tested whether providing zebrafish Spaca6 and Izumo1 on medaka sperm and vice versa enables cross-species compatibility from the side of the sperm. To this end, we generated spaca6 and izumo1 knock-out (KO) lines in medaka, which are male sterile, and introduced zebrafish spaca6 and izumo1 transgenes. Transgenic medaka males did not fertilize zebrafish or medaka eggs with zebrafish Bouncer. Similarly, zebrafish males expressing medaka Spaca6 and Izumo1 failed to fertilize zebrafish eggs presenting medaka Bouncer. Unexpectedly, providing either full-length medaka Spaca6 or the Bouncer binding site of medaka Izumo1 in zebrafish sperm rescued the sterility of spaca6 and izumo1 KO, respectively. Therefore, medaka Spaca6 and Izumo1 can interact with zebrafish Bouncer when paired with their zebrafish sperm complex members underscoring the nuanced interplay between molecular restrictions and compatibilities during sperm-egg interaction across teleosts.