Induction of gynogenesis in the turbot (Scophthalmus maximus): effects of UV irradiation on sperm motility, the Hertwig effect and viability during the first 6 months of age

Fish in which gynogenesis has been induced have all their chromosomes inherited from the mother and, if females are the homogametic sex, they usually are all females. Because turbot females grow faster than males, the production of all-female populations is highly desirable. The sperm of turbot is of poor quality and its larvae are small and fragile. These circumstances represent a challenge for the induction of gynogenesis in the turbot. As a first step towards this goal, effective conditions for the induction of gynogenesis through UV irradiation of sperm followed by a cold shock were established. When diluted 1:10 with Ringer-200 saline solution and placed in a thin layer (~0.3 mm), a dose-dependent effect of UV light on sperm motility was found. The dose at which both the amount of motile sperm and the duration of sperm motility was reduced to 50% of the original value (ID50) was ~28,000 erg mm-2. A typical Hertwig effect was elicited with a dose of 30,000 erg mm-2. The resulting embryos exhibited the typical "haploid syndrome" and died shortly after hatching. Application of a cold shock (-1 to 0°C for 25 min starting at 6.5 min after fertilization) to activated eggs with UV-irradiated (30,000 erg mm-2) and diluted (1:10) sperms restored diploidy and resulted in the production of gynogenetic diploids (2n=44 chromosomes). These conditions were used in a pilot-scale experiment and found effective in inducing gynogenesis in ~47,000 eggs. The rate of gynogenesis induction was 100% as verified by an analysis with microsatellite DNA markers. Survival of the gynogenetics was approximately 10% of diploids at 6 months of age, although growth was similar during this period. If this species turns out to have female homogamety, as is the case in most pleuronectiformes examined so far, the method presented here is the first necessary step for the production of all-female populations of this economically important species.