Genetic improvement of farmed tilapias: growth performance in a complete diallel cross experiment with eight strains of Oreochromis niloticus

A complete diallel cross experiment was carried out with eight strains of Nile tilapia (Oreochromis niloticus). The strains represented four wild populations collected from various locations in Africa and four populations that had been reproduced over a large number of generations for tilapia farming in Asia. Growth performance was recorded in a total of 23739 individually tagged progeny of the 64 different strain combinations after a grow-out period of about 90 days in seven different test environments representing applied farming systems in the Philippines. Least square means of body weight at harvest were computed for each strain combination within and across test environments, and additive genetic effects, maternal (reciprocal) effects and non-additive genetic effects (heterosis) were estimated. The least square mean heterosis of both reciprocals of all strain crosses across all test environments was 4.3%, and the cross that expressed the largest non-additive genetic effect showed a least square mean heterosis of 14%. However, only seven out of the 22 crosses that expressed a significant heterosis were better performing than the best pure strain, and the largest gain was then about 11%. The least square mean heterosis within test environments ranged from 0 to 9.6%, and the largest heterosis was observed in some of the test environments with poor growth performance. Significant reciprocal effects were observed, showing that some strains performed better as sire strains and others as dam strains. Significant genotype (strain combination) by test environment interactions were also detected. The interactions were mainly associated to an expected variation in the magnitude of the differences between genotypes that was proportional to the mean performance in the test environments. However, some examples of re-ranking of the genotypes were observed, mainly associated to the non-additive component of performance. It was concluded that specific crossing schemes, possibly involving specialised sire and dam lines, may improve growth performance in Nile tilapia, but that this improvement will be quite marginal and should be accompanied by selection within the parent strains. Furthermore, the growth performance of such hybrids may be more sensitive to genotype by environment interactions affecting the non-additive component of performance. A regular selection program based on additive genetic performance will normally result in a comparable genetic improvement after a short period of repeated selection, and may then continue beyond the results obtained in the present cross experiment without the complications caused by the laborious dissemination procedures required by a crossbreeding program and the problems caused by a possibly increased environmental sensitivity in the hybrids.