Influence of sexual phenotype and genotype, and sex ratio on growth performances in tilapia, Oreochromis niloticus

In tilapia, precocious sexual maturity associated with continuous and asynchronous reproductive activity often results in unwanted reproduction and overcrowding in ponds where the food supply is limited. This constitutes a major limitation in the culture of most tilapia species. Furthermore, divergent growth performances linked to sex are regularly observed. Males present better growth performances than females. The first experiment of the present study analysed the influence of sexual genotype on phenotypic male growth rates. Males with three different sexual genotypes were produced: YY 100 (YY male X YY female), XY 100 (YY male X XX female) and XX 100 (hormonally sex-inverted females). They were compared with classic (XY male X XX female) or all female (XX male X XX female) progenies. The objectives of the second experiment were to study the influence of social interactions through the use of different sex ratios: F100, F75, F50, F25, F0 (F100 = 100% females in the population). These two experiments were carried out in hapas implanted in pond with "Egypt-UCS" strain of Oreochromis niloticus. At the end of the first experiment (140 days postfertilisation), the XY genotype presented the best growth performances and YY genotype the lowest. Final body weights of neomale (XX100) were significantly (P<0.05) higher than those of YY100 males. According to our results, the involvement of a genetic component in sex-related growth difference between males and females is strongly suspected. We stressed upon the involvement of genes bound to the sexual genotype (especially XY in comparison to YY and XX) independently of the sexual phenotype. Furthermore, sexual maturity is not the only phenomenon involved in the sexual dimorphism. From 102 days post-fertilisation until the end of the second experiment, females of the F25 population presented the best growth performances. Their final body weights were significantly higher than those of all the males except F50 population males. In general, female body weight tended to increase inversely to their proportion in the population. Males of the F50 population presented better growth rate than the other males. Thus, tilapias seems to be very sensitive to the effects of social interactions. This social behaviour could strongly influence growth differences between males and females. (Résumé d'auteur