DIVERGENT SELECTION FOR GROWTH IN THE DEVELOPMENT OF A FEMALE LINE FOR THE PRODUCTION OF IMPROVED GENETICALLY MALE TILAPIA (GMT) IN

The culture of genetically male tilapia (GMT) is becoming established as an alternative to hormonal sex reversal in some locations such as in the Philippines. However, concerns exist over the growth performance of the strain of Oreochromis niloticus in which GMT have been developed and the consistency of sex ratios. As part of a project to develop improved GMT, a selection programme was initiated in a female line to be crossed with YY males. Three generations of divergent within family selection for 16-week weight were applied to a base population consisting of five strains of pure O. niloticus from diverse origins, with each base strain forming a family. Rotational mating was carried out between the families from the base strains. A high intensity of selection was applied with standardized selection differentials ranging from 1.6 to 2.6 per generation. Selection for combining ability for sex ratio was carried out in the selected females (ten heaviest females per family) in generation one and three. After three generations of selection, high line and low line selected males and females were used to produce progeny for growth performance trials in four culture environments to assess the response to selection. Growth differences between high and low lines in the four environments ranged from 37 % (extensive ponds) to 102 % (intensive tanks) indicating highly significant response to selection (P < 0.01). Estimates for realised heritability (h2) ranged from-0.08 (high line males) to +0.40 (low line females). GMT was produced by crossing YY males from an Egyptian strain with females from the high line. The mean sex ratio of GMT using this selected female line was 97.4% male and preliminary data from growth trials indicate significant increases in growth rate of this improved GMT compared to the original inter-strain GMT