The Role of Seminal Plasma on Sperm Competition in Alternative Reproductive Tactics of Chinook Salmon

Chinook salmon undergo intense sperm competition due to the existence of male alternative reproductive tactics. The large, dominant hooknoses hold preferential spawning positions near females, while the small, precocious jacks adopt a sneak spawning tactic. Theory suggests that the precocious sneaker males should invest disproportionately more into their gonads rather than in other traits (such as body size) because they always spawn in the presence of at least one rival male, hence high risk of sperm competition. However, investment into gonads doesn’t necessarily mean investment in sperm quality (sperm velocity or number), because seminal plasma is another crucial component of the milt, which is known to negatively impact rival male sperm performance in some taxa. In this study we experimentally examined the effect of rival male seminal plasma on a variety of sperm performance traits that are critical to sperm competition success in salmon. We centrifuged the milt to separate the sperm and seminal plasma and then suspended the sperm from one tactic into the seminal plasma of the other tactic (and vice-versa). Preliminary results suggest jack’s seminal plasma reduces the velocity of hooknose sperm, but hooknose seminal plasma has no effect on jack sperm velocity.

Keywords: Reproductive Success, Spawning