Genetic studies in Tilapias

Abstract

Tilapia as a group have suffered from a lack of detailed genetic research in a number of important applied and basic areas. Sex determination in tilapia is of great importance because of the need to produce monosex populations for aquaculture. In this study sex ratio data produced from a total of 41 intra- and interspecific crosses utilizing pure species have been analysed. The results so far obtained do not support the existing hypothesis of chromosomal sex determination. On the basis of the existing data it seems that sex in these fishes is determined by a polygenic system.

Cytogenetic studies on chromosome number, chromosome morphology and DNA content show many similarities between the 7 species belonging to three genera. The DNA value varies between 0.84pg for 0. macrochir and S. galilaeus and up to 1.21pg for O. aureus. The chromosome number (2n = 44, n = 22) is the same in all the species. No heteromorphic sex specific chromosome pair has been found in any species. Arm number (NF) difference in the species indicates the involvement of pericentric inversions in the karyotypic evolution of these species. C-banding of the metaphase chromosomes shows that the heterochromatin is localised around the centromere in all the species of Oreochromis and Sarotherodon, but T. zillii has more heterochromatin with six chromosomes having their short arm completely C-positive. mossambicus and spilurus, two closely related species, can also be distinguished by their C-banding pattern.

Comparative growth trials on different species, their hybrids and the effect of hormone treatment on growth performance have all been analysed. Different species showed large differences in growth performance based on SGR and FCR values, when compared under standardized conditions. Comparison between hybrids and their parental species showed no significant heterosis for SGR and FCR except in the hybrid between O. spilurus female and niloticus male. Hormone treatment of fry improved growth performance but did not alter the relative growth found in the untreated group.