Two Egg-Derived Molecules in Sperm Motility Initiation and Fertilization in the Pacific Herring (Clupea pallasi)
Sperm of the Pacific herring are immotile at spawning. Two egg-derived molecules are capable of initiating sperm motility. One is herring sperm activating protein(s) (HSAPs) and the other is sperm motility initiation factor (SMIF). These two motility initiators differ in their location and association with the chorion, and in their isoelectric points and molecular weights. In this study we have investigated the roles of these two inducers with respect to motility and fertilization. Using computer analysis of sperm motility, we found that HSAPs, as well as the Cterminal HSAPs peptide, elicit a linear motility pattern, while SMIF induced a highly circular and asymmetric pattern. HSAPs induced a two-fold increase in intracellular calcium, whereas SMIF induced a four-fold increase of motility initiation. SMIF-exposed sperm, preloaded with BAPTAAM, showed a more linear motility and this motility trajectory decreased with their fertilizing capability. The difference in intracellular calcium levels between HSAPs and SMIF is consistent with the observed linear and circular motility. In the absence of SMIF, HSAPs do not support fertilization. Fertilization is rescued in these experiments if SMIF is reintroduced. We propose that diffusible HSAPs are not essential for fertilization, but enhance sperm-egg collisions via linear motility. SMIF, which is bound to the micropylar region of the chorion, is required for fertilization and induces circular motility that is a prerequisite for sperm to enter the micropylar canal and fertilize the egg.