Lack of association between allelic status and myostatin content in lambs with the myostatin g+6723G>A allele.

Lambs with the myostatin (MSTN) g+6723G>A mutation have a greater muscle mass, which is believed to be associated with reduced myostatin protein abundance. This experiment was designed to determine if differences in allelic frequency of the MSTN g+6723G>A mutation affected abundance of myostatin protein from birth to 24 wk of age. A Poll Dorset cross White Suffolk ram (MSTN A/G) was mated to 35 White Suffolk cross Border Leicester cross Merino ewes (MSTN A/G, n=21, and MSTN G/G, n=14). The progeny of these matings delivered 44 lambs with MSTN A/A (n=9), MSTN A/G (n=21), and MSTN G/G (n=14) genotypes. At approximately 1, 4, and 12 wk of age, a biopsy sample was collected and a blood sample was taken to measure the abundance of myostatin protein in muscle and plasma. At approximately 24 wk of age, the wether lambs were slaughtered to determine carcass characteristics and muscle samples were taken from the bicep femoris. The abundance of mature myostatin protein in muscle from 1 wk old lambs was less (P=0.05) in MSTN A/A and MSTN A/G compared with MSTN G/G lambs. However, at 4 and 24 wk the MSTN A/A lambs had a greater (P=0.04) abundance of myostatin protein compared with the MSTN A/G and MSTN G/G lambs. The abundance of mature myostatin did not differ between genotypes in plasma but the myostatin protein did increase as the lambs aged. At slaughter the MSTN A/A wether lambs had greater dressing percentages (P=0.04), shortloin (P=0.01), topside (P<0.001), and round (P=0.01) weights but did not differ in final BW or HCW (P>0.05). The MSTN A/A lambs had more muscle fibers (P=0.02) in the cross-section of LM between the 12th and 13th rib. The MSTN A/A lambs also had greater lean (P=0.002), less fat (P=0.009), and reduced organ (heart, liver, spleen, and kidneys) mass as determined by computed tomography scanning than MSTN G/G lambs. The results of this study demonstrated that lambs homozygous for the MSTN g+6723G>A mutation have changes in carcass characteristics (dressing and total lean), organ weights, and muscle fiber number. This may be due to reduced myostatin protein early in utero, but after 4 wk of age there was no difference in the abundance of mature myostatin protein in muscle or plasma among MSTN A/A, MSTN A/G, and MSTN G/G genotypes.