Association between polymorphism in the Prolactin I promoter and growth of tilapia in saline-water

Tilapias are a group of species with a variable tolerance to high salinity, which are cultured worldwide in fresh, brackish and seawater. Prolactin I (PRL I) is known as a key hormone in osmoregulatory physiological pathways. A previous study, conducted in a single family, reported on association between polymorphism in a repetitive element within the promoter of the PRL I gene and growth rate of tilapia in saline water. This study was aiming to further validate this association in a larger sample size, and was conducted in nine families over two consecutive breeding seasons. We have confirmed this association in the three F2 families of Oreochromis mossambicus × Oreochromis niloticus hybrids challenged in the first year. The same pattern of improved growth for genotypes with shorter alleles originating from the O. niloticus grand-parental fish, although O. mossambicus is considered to be a more salt tolerant species, was demonstrated. The effects accounted for 13–15% of the phenotypic variance for growth rate (P < 0.05). In the six families from the second spawning season there was no association between the gene polymorphism and the fish growth in saline water. No association was evident between the polymorphism in the PRL I promoter and the expression of the gene.