Polymorphisms in the 5' flanking region of the insulin-like growth factor I gene are associated with growth traits in largemouth bass Micropterus salmoides

Insulin-like growth factor I (IGF-1) plays an important role in fish growth and development. As the promoter of IGF-I regulates the expression of IGF-I, polymorphisms in the promoter region could be functionally important to differences in IGF-I expression and function. In this study, the 5' flanking region (1,770 bp) of the largemouth bass IGF-I gene was cloned, sequenced and characterized. Several putative liver-enriched transcriptional factor binding sites, including HNF-1alpha, HNF-3beta and C/EBP, were found to have multiple copies in the promoter sequence of the largemouth bass IGF-I gene. It did not contain consensus TATA, CCAAT-like or GAGA sequences. Sequencing of 5' flanking sequences from individual fish revealed the presence of a deletion (GTTT) and two single-nucleotide substitutions, causing two different haplotypes: ATTTTTGTTTTT(A) and ATAATT-TT(B). The haplotypes were tested for their association with growth traits in a population of 91 largemouth bass individuals using polymerase chain reaction. The association analysis was carried out using a general linear model. Results showed the haplotypes had significant effects on body weight and body width (P0.05). The individuals with the AA genotype had significantly higher body weight and body width than those with the AB and BB genotypes (P0.05). Moreover, the effect of the genotypes on IGF-I mRNA expression in the liver was evaluated through the use of a semiquantitative RT-PCR method. The results showed that the IGF-I mRNA expression levels were affected by the promoter polymorphisms (P0.01), and that levels of IGF-I mRNA of the AA genotype in liver were greater than those of both the AB and the BB genotypes (P0.05 and P0.01). Our results implied that the IGF-I gene could be a potential quantitative trait nucleotide that affects growth traits in largemouth bass.