A phenotype of increased muscle mass (IMM) and reduced fat, comparable to reported effects of deleterious mutations in the myostatin gene (MSTN), has been observed in the Norwegian Spaelsau breed. However, the genotyping of five AI rams producing descendants with this phenotype, failed to reveal any of the known functional MSTN mutations.

In the present study, the coding region of the MSTN gene was sequenced in a Spaelsau ram lamb with this particular phenotype. A one base-pair insertion mutation (c.120insA) producing a premature stop codon in amino acid position 49 was identified. The consequence of this mutation is that the bioactive carboxy-terminal end of the protein is not translated, and a completely non-functional myostatin protein is produced. Among the 98 available AI rams of this breed, all five individuals having descendants with this particular phenotype were found to be heterozygous for the c.120insA mutation. The probability that these five selected AI rams should be heterozygous carriers of the c.120insA mutation purely by chance was calculated to be 3.1 x 10-7. In total, 7 AI rams were found to be heterozygous carriers of c.120insA. The estimated breeding values (EBVs) for EUROP carcass conformation and fat class for these 7 individuals also points towards a strong phenotypic effect of this mutation.

Based upon the completely deleterious effect this novel c.120insA mutation has on myostatin protein function, and the documented phenotypic effect of comparable MSTN mutations in Norwegian White Sheep and other species, we conclude that this mutation is the functional explanation underlying the IMM phenotype in Norwegian Spaelsau. The allele distribution among the 98 genotyped AI rams support this conclusion, and also suggests that c.120insA is the most common reason for this phenotype in the Norwegian Spaelsau breed.