Induction of triploidy in the turbot (Scophthalmus maximus) - I. Ploidy determination and the effects of cold shocks

The basis for induction of triploidy in the turbot by applying cold shooks shortly after fertilization (AF) was studied. Since this species exhibits a polymorphism in a number of nucleolar organizing regions (NOR), determinat ion of the ploidy level through NOR analysis was first validated. Results s howed that NOR analysis could discriminate well between diploid and triploi d individuals whose ploidy level was verified karyotypically (n = 44 chromo somes in diploids; n = 66 in triploids). In diploids, the mean number of NO R per cell ranged from 1.10 to 1.85, whereas in triploids, it ranged from 1 .50 to 2.35. However, histo,sram distribution of data on mean number of NOR per cell showed that the number of fish in the overlapping region (NOR num ber between 1.50 and 1.85) was very low. Cold-shocked fish with a NOR value > 1.735 were considered triploids. The error in ploidy assessment using NO R analysis in the turbot was found to be consistently around 3% and always < 5%. In this way, NOR analysis could be safely applied to monitor the effe cts of cold shocks on triploidy induction. Cold shocks were applied 5 min A F for 5, 10, 20 or 40 min at either 0 degrees C, 2 degrees C, or 4 degrees C. Results showed that the number of triploids increased with lower shock t emperatures and longer shock duration in the range from 5 to 20 min. In par ticular, cold shocks of 0 degrees C applied during 20 min consistently resu lted in similar to 90% triploid turbot (P < 0.001). Shocks longer than 20 m in (40 min) did not increase the number of triploid turbot in contrast to w hat has been found in other flatfish species. This is probably related to t he higher pre-shock temperature at which turbot eggs are incubated. Surviva l, 1 day after hatching, was similar to 80% of the untreated controls and n ot different (P > 0.05) from appropriate sham controls, indicating that low er survival is due to the effects of mechanical handling and stress during triploidy induction rather than the triploid condition per se. The highest triploid yield obtained in this study, similar to 70%, is higher than the t riploid yield obtained in other flatfishes. (C) 2000 Elsevier Science B.V. All lights reserved.