Ghrelin gene single nucleotide polymorphisms and their effects on Nile tilapia (Oreochromis niloticus) growth

In teleosts, ghrelin plays an important role in body weight gain. This hormone has been used in functional studies on food intake and foraging-related behaviors. However, the regulatory role of its 5' flanking and coding regions is unknown. To examine the effects of single nucleotide polymorphisms (SNPs) in the ghrelin gene of Nile tilapia, we cloned sequences of its 5' flanking and coding regions to analyze the regulatory elements binding sites and nonsynonymous mutation sites, and screened the SNPs in these two regions to find their association with mRNA expression levels and superior growth traits. No SNPs loci were found in the coding region. However, a SNPs locus (C-226 T) was identified in the amplified 5' flanking region, for which we analyzed the distribution of three genotypes (CC, CT and TT) at frequencies of 17.80 %, 50.00 % and 32.14 %, respectively. TT is associated with an increased number of putative octamer transcription factor 1 binding sites (from 2 to 3); the TT genotype also exhibited more superior growth traits (body weight, total length, standard length, body depth) than either the CC and CT genotypes. Expression of ghrelin mRNA in Nile tilapia with the TT genotype was significantly upregulated (P < 0.05) compared with the CC genotype. Thus, the polymorphism at the ghrelin locus C-226T is associated with superior growth in Nile tilapia, rendering the C-226T locus a potential marker for future selection of breeding stock to improve the quality and growth rate of farmed fish.