Double Muscled Sheep

History

Double muscling can occur in fish, cattle, dogs, humans, and more as well as sheep.
Presence of gene locus across species suggests it is evolutionarily ancient.
A mutant allele resulting in double muscling naturally occurs at this locus.
Mutant allele is integrated into phenotype of Texel breed sheep, and is also found in a number of others.

Texel Sheep

Originated on Texel Island outside the Netherlands in early 1800s
From crosses of short-tailed and wool varieties including Lincoln
New to the USA, imported in 1985 for herd improvement
Selected for muscular lambs with low fat
Developers of the breed were probably ignorant of mutation or at least never publicized it.
Muscle tissue of Texels contain 1/3 myostatin content compared to wild type sheep.
Texel Sheep Breeders Society of the USA considers myostatin theory controversial.

Genetic Basis

Double muscling results from a naturally mutated allele of growth and differential factor 8 gene (GDF8) locus.
GDF8 codes for myostatin, a protein that regulates and limits muscle growth.
Without myostatin, muscle can hypertrophy over 10-20% without exercise.
Certain mRNA incorrectly binds to mutant DNA with a G to an A.
Functional protein cannot be synthesized from this RNA.
Muscle grows unregulated by genetic limitation.

Description

Myostatin deficient sheep have significantly more muscle mass from birth.
They still have the standard number of muscles, NOT 2 of each.
Larger mass comes primarily from higher myofibril count, but also from fiber cross-sectional area.
Muscularity is especially visible in protruding hindquarters of the sheep.
Muscle mass leaves less subcutaneous and muscular adipose tissue and thinner bones.


Problem

Despite heavy bodies and diminished skeletons, double muscling has no known negative medical consequences for sheep.
Double muscled sires are used often without complication because sheep are good mothers with the ability to bear twins and triplets.
Decreased organ size and reproductive capabilities reported in mutant cattle, but not sheep. Even Texels are known as good breeders.

Genetic Transmission

The mutated GDF8 allele displays autosomal, incomplete dominance
Homozygote dominant sheep are extremely heavily muscled mutants.
Heteroxygotes are heavily muscled.
Homozygous recessive sheep are "wild type" standard sized sheep.


Control of Spreading

Low myostatin is selected for in sheep to increase meat production and for larger, leaner cuts of meat.
Double muscle is selected for especially in Texel sheep herds and they are used to improve other herds.
If eradication was desired, it would be simple because of the visible phenotypes of both homozygotes and heterozygotes.
However, the mutation will rarely appear naturally.

Opinion

Double muscling is useful for lamb and mutton producers and is a natural mutation that is not apparently hazardous or uncomfortable to affected animals or the environment. I think breeding to keep a population of myostatin deficient sheep is beneficial and a good idea so it can be used for quick genotypic and phenotypic changes in populations and the rare mutation will not be lost.

Although the mutation is rare, breeding the allele into a different breed should be simple because the genotype of the GDF8 locus is evident from phenotypes. With double muscled sheep yielding more, leaner meat, why haven't all breeds capitalized on the advantages of this mutation?

References

1. Boman, I.A., Klemetsdal, G., Nafstad, O., Blichfeldt, T., & Vage, D.I. (2010). Selection based on progeny testing induces rapid changes in myostatin allele frequencies - a case study in sheep. Journal of Animal Breeding and Genetics.
2. Clop, A., Marcq, F., Takeda, H., Pirottin, D., Tordoir, X., Bibe, B., Bouix, J., Caiment, F., Elsen, J., Eychenne, F., Larzul, C., Laville, E., Meish, F., Milenkovic, D., Tobin, J., Charlier, C., & Georges, M. (2006). A mutation creating a potential illegitimate microRNA target site in the myostatin gene affects muscularity in sheep [Abstract]. Nature Genetics. Retrieved from:http://www.usatexels.org/pdfs/articles/Mycosin_gene_abstract.pdf
3. Department of Animal Sciences at Oklahoma State University. (2000). Breeds of livestock: Sheep: Texel. Breeds of Livestock resource.
4. Miar, Y. & Salehi, A. Myostatin and its implications on sheep breeding. University of Tehran, Tehran, Iran. Retrieved from: http://dysci.wisc.edu/sglpge/posters/Myostatin%20and%20its%20implications%20on%20sheep%20breeding%20-%20Miar.pdf
5. Tellam, R.L., Cockett, N.E., Vuocolo, T., & Bidwell, C.A. (2012). Genes contributing to variation of muscling is sheep. Front Genet 3: 164. doi:10.3389/fgene.2012.00164. Retrieved from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3429854/
6. Wagner, K.R. (2009) Myostatin-related muscle hypertrophy. GeneReviews [Internet]. Retrieved from: http://www.ncbi.nlm.nih.gov/books/NBK1498/

Picture References