Previous studies in goldfish (Carassius auratus) showed that the oocyte maturation-inducing steroid 4-pregnen-17,20ss-diol-3-one (17,20ssP) functions after release as a pheromone that increases male serum gonadotropin II (GtH II) concentration, milt (sperm and seminal fluid) volume and sexual activity, effects hypothesized to increase male reproductive success in the sperm competition of multi-male spawnings. The present study tested this hypothesis by determining whether overnight exposure to 17,20ssP increases fertility. In pair spawnings, 17,20ssP-exposed males fertilized a greater percentage of eggs than did control males, apparently because 17,20ssP-exposed males had more releasable sperm at the onset of spawning. Microsatellite DNA paternity analysis showed that 17,20ssP-exposed males also fertilized more eggs in competitive spawnings involving one control male and one 17,20ssP-exposed male. This effect of 17,20ssP on competitive fertility could be due to demonstrated increases in spawning activity, milt volume, duration of sperm motility and proportion of motile sperm. However, it appears that a change in sperm quality is a major component of the pheromonal effect because, in competitive in vitro fertilizations, sperm from 17,20ssP-exposed males fertilized more eggs than did sperm from control males. The results indicate that the response to pheromonal 17,20ssP is a major determinant of reproductive success in male goldfish.

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