Sperm activation for fertilization requires robust activity of the TAT-5 lipid flippase.

During fertilization, sperm and egg membranes signal and fuse to form a zygote and begin embryonic development. As lipids participate in signaling and membrane fusion, we investigated the role of lipid asymmetry in gametogenesis, fertilization, and embryogenesis. We show that the lipid flippase TAT-5, an essential P4-ATPase that maintains phosphatidylethanolamine asymmetry, is required for both oocyte formation and sperm activation, albeit at different levels of flippase activity. Loss of TAT-5 significantly decreases fertility in both males and hermaphrodites and decreases sperm activation. TAT-5 localizes to the plasma membrane of primary spermatocytes but is sorted away from maturing spermatids after meiosis. We also find that phosphatidylethanolamine asymmetry is lost in residual bodies prior to phosphatidylserine exposure. Our findings demonstrate that phosphatidylethanolamine asymmetry plays key roles during gametogenesis and sperm activation, expanding the roles of lipid dynamics in developmental cell fusion.