Can sexual dimorphism and body shape be altered in Nile tilapia (Oreochromis niloticus) by genetic means?

The objective of this study was to estimate genetic parameters in GIFT (Genetically Improved Farmed Tilapia), especially focusing on the genetic correlation between trait expressions in both sexes and among measurements of body size. Body weight, length, depth and width data at harvest from 12,308 individuals, progeny of 232 sires and 340 dams, were analyzed by restricted maximum likelihood methods fitting a multi-trait animal model. To explore the genetic variation in sexual dimorphism the trait expressions in the two sexes were treated as if they were different traits. Heritabilities and maternal and common environment effects for all the traits were very similar in females and males. The genetic correlations between sexes for all traits were close to unity (0.91 to 0.96), indicating that there was no sex by genotype interaction. When treated as a single trait the heritabilities (±SE) for body weight, length, depth and width were moderate to high, ranging from 0.20 to 0.35 (±0.04 to 0.05). The maternal and common environment effects accounted for 16 to 24% of the variance. Genetic correlations among the four body measurements were highly positive (0.94 to 0.99), suggesting the existence of little or no genetic variation independent of each other.We concluded that there was no need to treat trait expressions in the two sexes as different traits in genetic improvement programs. Furthermore, that the relative dimensions of the body were essentially controlled by the same genes, but that continued selection for live weight would result in relatively longer and thinner fish because of the greater correlated response in length relative to width and depth.