Molecular mechanism regulating axoneme activation in marine fish: a review

The present review suggests that: (1) there is no single model that can explain the molecular activation and regulation of sperm motility of the marine fish; (2) only in some species (puffer fish, tilapia, gilthead sea bream, and striped sea bream) protein phosphorylation/dephosphorylation has been shown to be involved in flagellar motility regulation; (3) only a few proteins were identified, which show a change in their state of phosphorylation following sperm activation.A model of molecular mechanism controlling the activation of sperm motility in gilthead sea bream is being proposed here, which could be a useful model to clarify the sperm motility activation process in other species.In fish with external fertilization, spermatozoa are usually immotile in the seminal tract (Stoss [1983]). The seminal plasma, whose osmolality and composition depends on the species, protects and immobilizes spermatozoa until they are ejaculated and their motility is initiated (Morisawa [1985]). Many environmental stimuli control sperm activation by triggering the different transduction pathways. In salmonids (Baynes et al. [1981]; Billard [1983]; Morisawa et al. [1983a]; Stoss [1983]; Morisawa [1985]) and sturgeons (Gallis et al. [1991]; Toth et al. [1997]; Alavi et al. [2004]), the reduction of the external K+ concentration, upon dilution of semen, initiates sperm motility. Hypotonic exposure after dilution into freshwater is the trigger signal in non-salmonid freshwater fish (Morisawa and Suzuki [1980]; Morisawa et al. [1983b]; Stoss [1983]; Morita et al. [2003]; Krasznai et al. [2003b]), while hypertonic exposure initiates sperm motility in many marine fishes (Morisawa and Suzuki [1980]; Oda and Morisawa [1993]; Detweiler and Thomas [1998]; Krasznai et al. [2003a]). It has been also reported that an egg-associated molecule triggers sperm activation in herring (Yanagimachi and Kanoh [1953]; Yanagimachi [1957a,1957b]; Yanagimachi et al. [1992]; Oda et al. [1998]).All these factors