The Effects Of The SNP In Promoter Region Of IGF-â…  On Transcriptional And Expressional Level Of The Gene And On Growth And Carcass Traits In Chickens

A QTL affecting body weight of young chickens was mapped to the short arm of chromosome 1 near the centromere using an F2 population crossing of a broiler sire-line and egg-lying line previously. Searching for genes or putative genes in this region, we found that the insulin-like growth factor-I gene is the strongest candidate gene for this effect base on its crucial role in growth and development.We identified a C-A mutation nearing the putative 'TATA' box and the transcriptional start site. Association study demonstrated that the regulatory polymorphism is strongly associated with growth and carcass traits. Birds with A A genotype have more 6 week, 9 week and 12 week body weight than birds with CC and CA genotype, in others traits, the AA birds have also higher traits means than AC and CC birds, such as live weight, carcass weight, eviscerated yield with giblet, eviscerated yield, breast muscle weight, leg muscle weight, heart weight, abdominal fat weight, small intestine length, etc.The mutation occurs in the regulatory region which is highly conserved in several species. The mutation increases a transcription factor of CdxA in vitro and in vivo, which has activity of enhancing transcription level of reporter plasmids in CDXA expressing cells, and expressed exclusively in chicken gut after hatch. We analyzed the transcriptional and expressional level of IGF-I gene in liver and in small intestine between the mutant and the wild-type birds at 3, 6, 9 week by real-time quantitative PCR and radioimmunoassay. In liver, there were no transcriptional and expressional differences between the mutant and wild-type birds at 3, 6, 9 week age; in small intestine, the mutant birds had about one-fold mRNA and 30% protein of IGF-I higher than wild type birds at 3 week age, and no significant genotype difference at 6 and 9 week age. Thus the mutation results in the increase of expressional efficiency in vivo. The mutant birds have higher IGF-I in small intestine, and high IGF-I can promote intestine cell growth faster, then these birds favor the longer small intestine length, with more nutrition digested in and absorbed from intestine the mutant birds finally result in higher growth rate and others carcass traits means.Our results supported the view that selection for high growth rate in birds is linked to an increase in the relative size of the digestive organs and rapid early development of the digestive organs. Our results may be also an example of a complex genetic traits created by multiple susceptibility genes. Identification of QTL gene can provide suggestive information about normal development and disease processes, and now with the development of new genetic technique, statistical tools and better characterized genome, find QTL gene is becoming easier than before. The results are also useful in practice of chicken molecular breeding programme in the near future.