Temperature dependent sex ratios in selected lines and crosses with a YY-male in Nile tilapia (Oreochromis niloticus)

Sex determination in Nile tilapia is very labile. It was shown that the sex ratio in temperature treated groups (36 degrees C.10 days) of Oreochromis niloticus can be selected for as a quantitative trait. The standardized test procedure involved the artificial reproduction of breeders with nine days of egg incubation at 28 degrees C and subsequent testing of fry at an elevated temperature of 36 degrees C for 10 days. Family selection was carried out upon the male percentages in treatment groups (high-line>80%, low-line<60%). Selected candidates were taken from corresponding full sib control groups, which were kept at 28 degrees C. After two generations of divergent selection for the male proportion in temperature treated groups, male percentages of 90% and 54% were obtained in the high- and low-line, respectively. Herein, the data for a third generation of selection are presented. Twelve families in the high-line and 20 in the low-line were tested. Average male percentages in the treatment groups of 92.7% and 50.4% were obtained. After three generations of selection the cumulated realized heritabilities for the temperature dependent sex ratio were 0.63 in the high- and 0.84 in the low-line, still indicating good potential for selection. Two families in the high-line exhibited only males, whereas in the low-line two families showed a feminising effect in the treatment groups. Generally elevated temperatures (>34 degrees C) are considered to be masculinising in Nile tilapia, feminising effects so far have only been reported in genetic all-male progenies (YY, XY). In the third generation of selection, it could be shown that elevated temperatures might take over a feminising role through selection in genetically mixed sex (XX/XY) progenies. Consequently both, temperature-masculinising and -feminising genes, exist in the same population and can be accumulated through selection. However, the load of temperature-masculinising genes of dams from the high- or low-line had no effect on mean sex ratios in temperature treated groups when a YY-male was used as a sire. From an evolutionary point of view, the prevalence of both temperature-masculinising and -feminising effects might allow the sex ratio to equilibrate at the population level if random mating occurs. This in turn might counteract the evolution of sex chromosomes. (C) 2011 Elsevier B.V. All rights reserved.