Sex determination and the feasibility of genetically male tilapia production in the Thai-Chitralada strain of Oreochromis niloticus (L.)

This paper presents the findings from investigations on sex determination a nd attempts to produce monosex male tilapia through the use of novel YY-mal es in the Thai-Chitralada strain of Oreochromis niloticus cultured commerci ally in Thailand. To determine the underlying variability in sex ratio, 95 progeny groups, derived from single pair matings of normal broodstock, were sexed over a period of 2 years. In addition, diallel-type crosses were gen erated from seven females and five males. Of the 7822 progeny sexed, there were 3951 males (50.5%), with the overall sex ratio being not significantly different from 1:1 (P > 0.25). However, progeny sex ratios ranged widely f rom 15.5% to 100% male. The data were highly heterogeneous (P < 0.001) with over 53% of crosses producing sex ratios significantly different from 1:1 (P < 0.05). Analysis of sex ratios from the diallel-type crosses demonstrat ed both maternal and paternal effects on progeny sex ratio. Sex ratios of p rogeny derived from crosses of two novel YY-males previously identified by progeny testing were also highly variable ranging from 36% to 100% male wit h a mean of 80.6%. Progenies derived from repeated spawns of the same pair were also significantly heterogeneous (P < 0.01). The implications for the elucidation of sex determination and for the application of the W-male tech nology for production of genetically male tilapia (GMT) in the strain are d iscussed. We conclude that the consistent production of GMT in the Thai-Chi tralada strain could be made reliable with the application of appropriate s election methods. (C) 1999 Elsevier Science B.V. All rights reserved.