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-1α, HNF-3β 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 (P < 0.05). The individuals with the AA genotype had significantly higher body weight and body width than those with the AB and BB genotypes (P < 0.05). 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 (P < 0.01), 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 (P < 0.05 and P < 0.01). Our results implied that the IGF-I gene could be a potential quantitative trait nucleotide that affects growth traits in largemouth bass.