Molecular genetic analysis of IGF1 in Romney sheep and its role in growth

Abstract

In an effort to improve livestock breeding, recent focus has been on using marker-assisted selection (MAS) to increase the accuracy of the choices made in selecting breeding stock. MAS may increase the annual rate of genetic gain in livestock by as much as 15-30%. Traits that determine the economic value of livestock are of primary concern in livestock breeding. Insulin-like Growth Factor 1 (IGF-1: also known as somatomedin C) affects young animal growth and a range of other anabolic processes in adults from a number of species, but little is known about its role in sheep. Variation in the IGF-1 gene (IGF1) has also been reported in other animal species, but once again little is known about ovine IGF1 variation. Using a polymerase chain reaction – single strand conformational polymorphism (PCR-SSCP) approach, 50 New Zealand (NZ) Romney rams were investigated to ascertain whether variation existed in two regions (an exon 2 fragment and an exon 3 fragment) of ovine IGF1. Two PCR-SSCP banding-patterns were discovered for each region, with one or a combination of two banding patterns detected for each sheep. For each region, these patterns were named A and B, and upon sequencing a unique DNA sequence was identified. 150 lambs (obtained from the NZ Romney Progeny Test; 2007-present), that were the progeny of a single ram that produced lambs over two seasons (2007 and 2009) were investigated for the association analysis. Phenotypic data for growth and carcass traits were available for these lambs and statistical analyses (Minitab v17) were performed using stepwise regression to assess the effect of the presence or absence of the IGF1 variants on the various lamb phenotypes. In these analyses the presence of the exon 2 IGF1 A variant was associated (P = 0.049) with increased birth weight in the 2007 lambs, although this effect did not persist in the 2009 lambs, or when the data from both years was combined. For the 2009 lambs, the presence of exon 2 A was associated (P = 0.017) with an increased growth rate from birth to tailing and this effect persisted (P = 0.035) when the 2007 and 2009 lamb data was combined. Trends (0.05 < P < 0.2) were observed between the presence of exon 2 A and increased tailing weight and growth from birth to weaning in the 2009 lambs and these trends persisted when the data from the 2007 and 2009 lambs was combined. No effects were observed for the exon 3 genotypes in 2007, but in 2009, there was a trend for AA to be associated with increased tailing weight and weaning weight, with the tailing weight effect persisting when both years’ data was combined. Overall these results suggest that variation in IGF1 could be of value as a genetic marker to complement genetic evaluations for early growth in sheep.