A novel sperm adaptation to evolutionary constraints on reproduction: Pre-ejaculatory sperm activation in the beach spawning capelin (Osmeridae).

Reproduction of external fertilizing vertebrates is typically constrained to either fresh or salt water, not both. For all studied amphibians and fishes, this constraint includes immotile sperm that are activated after ejaculation only by the specific chemistry of the fertilizing medium in which the species evolved (fresh, brackish, or salt water). No amphibians can reproduce in the sea. Although diadromous fishes may migrate between salt and fresh water, they are shackled to their natal environment for spawning in part because of sperm activation. Here, we report for the first time among all documented external fertilizing vertebrates, that in the absence of any external media, sperm are motile at ejaculation in a marine spawning fish (Osmeridae, capelin, Mallotus villosus). To illuminate why, we evaluated sperm behavior at different salinities in M. villosus as well as the related freshwater spawning anadromous rainbow smelt (Osmerus mordax). Surprisingly, sperm performance was superior in fresh water for both species. M. villosus spend their entire life at sea but our results show that their sperm are deactivated by sea water, suggesting a freshwater ancestry. By circumventing constraining water chemistry, we interpret the unique pre-ejaculatory sperm activation in this species as a novel adaptation that enables fertilization in the marine environment. These findings also contribute to understanding the persistence of anadromy, despite great energetic costs to adult fishes.

The effects of rival seminal plasma on sperm velocity in the alternative reproductive tactics of Chinook salmon.

Sperm competition is prevalent and intense in many animal mating systems, and is a major force driving evolution of such mating systems. The objective of this study was to determine the effect of seminal plasma on sperm velocity of male Chinook salmon (Onchorhynchus tshawytscha), which possesses a mating system with male alternative reproductive tactics and intense sperm competition. Male Chinook salmon either adopt a small, precocious sneaking tactic (jack) or a large, dominant tactic (hooknose). To test whether the seminal plasma can effect sperm velocity amongst sperm competitors, two experiments were done whereby males were paired based upon the alternative tactic each male adopted, with the first experiment consisting of jack-hooknose pairs (N = 16) and the second experiment consisting of jack-jack and hooknose-hooknose pairs (N = 12 and 14, respectively). Within each pair, milt of each male was manipulated such that seminal plasma was removed and swapped between the males in each pair and sperm velocity was measured. Jack seminal plasma caused a significant decrease (∼11.9%) in hooknose sperm velocity while causing a significant increase in jack sperm velocity (∼7%), while alternatively, hooknose seminal plasma had no affect on sperm velocity of jack or other hooknose males. This study shows that rival seminal plasma may affect the outcome of sperm competition between males; males adopting a sneaking tactic, that spawn in a disadvantageous mating position, may be able to compensate for this deficit by being more competitive through the effects of their seminal plasma on their competitor's sperm velocity.

Proteomic characterization of seminal plasma from alternative reproductive tactics of Chinook salmon (Oncorhynchus tswatchysha).

Chinook salmon (Oncorhynchus tshawytscha) are external fertilizers that display sneak-guard alternative reproductive tactics. The larger hooknose males dominate mating positions, while the smaller jack males utilize sneak tactics to achieve fertilization. Although poorly understood, previous studies have suggested that differences in spermatozoa quality may play a critical role in sperm competition. Considering that the seminal plasma strongly regulates spermatozoa quality and other processes critical for fertilization success, we employed label free quantitative mass spectrometry utilizing ion mobility separation coupled to cross-species bioinformatics to examine the seminal plasma proteome of Chinook salmon. A total of 345 proteins were identified in all biological replicates analyzed, including many established seminal plasma proteins that may serve as future biomarkers for Chinook salmon fertility and sperm competition. Moreover, we elucidated statistically significant protein abundance differences between hooknose and jack male tactics. Proteins involved in membrane remodeling, proteolysis, hormonal transport, redox regulation, immunomodulation, and ATP metabolism were among the proteins reproducibly identified at different levels and represent putative factors influencing sperm competition between jack and hooknose males. This study represents the largest seminal plasma proteome from teleost fish and the first reported for Chinook salmon. SIGNIFICANCE: Chinook salmon (Oncorhynchus tshawytscha) males represent an example of male alternative reproductive tactics where diverse reproductive strategies are thought to increase sexual selection. While seminal plasma has been shown to play an important regulatory role in sperm competition in many species, little is known about the protein composition of the seminal plasma of salmon. Therefore, seminal plasma isolated from the two alternative reproductive tactics of Chinook salmon (small sneaky jacks and large dominant hooknoses) were analyzed by label free quantitative mass spectrometry employing data independent acquisition and ion mobility separation. This yielded the largest proteome data set of the seminal plasma from salmon and the first to examine protein abundance differences between male alternative reproductive tactics. The quantitative proteomic data provides insight into possible unique mechanistic aspects of Chinook salmon alternative reproductive tactics utilized for sperm competition and fertilization success.