Progress towards the identification of the sex-determining mechanism of the sole, Solea solea (L.), by the induction of diploid gynogenesis

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Abstract
Methods were determined for the induction of diploid gynogenesis in the sole, Solea solea (L.), in order to investigate the sex‐determining mechanism. The experiments utilized gametes obtained by dissection of fish caught at sea. Activation of eggs with UV‐irradiated sole sperm was not feasible but development was initiated by exposure to halibut, Hippoglossus hippoglossus (L.), sperm. These embryos displayed the typical characteristics of haploids and few survived beyond hatching. High levels (> 90%) of diploidy were induced in such eggs by subjecting them to a cold shock (2°C or 4°C) for 1‐2 h about 10‐15 min after activation at 12°C. The resultant embryos were indistinguishable from those developing from normally fertilized eggs and were considered to be diploid gynogenomes. Fifteen of these were reared to a length of 5‐10 cm by which time gonad differentiation had been initiated. Both sexes were represented, indicating that sex in this species is not determined by a simple XX‐XY system with female homogamy.