Double Muscled Sheep

Jennifer Bruton

Muscle Hypertrophy

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Background:

  • Muscle hypertrophy
  • Commonly called double muscling
  • Breeders select for this trait in meat producing flocks
  • Many genotypes produce various phenotypes
  • Some phenotypes are desirable, some are not


Double muscling in sheep is viewed as an opportunity for meat producing flocks to become more profitable. In recent years Texel sheep have been used extensively to increase muscling on market lambs. While the genotypes and phenotypes are relatively well understood at this point in time many modes of inheritance are not.

Genotypes and Phenotypes:

Below is a table summarizing the various forms of muscle hypertrophy observed in sheep. The condition involves multiple genes and modes of inheritance. I will be focusing on two specific phenotypes, Calligype and the myostatin mutation of the Texel breed. The Callipyge is the best characterized of all the traits. Double muscling of the Texel breeds is the phenotype most selected for in breeding practices.


*Table adapted from Ref. 1.

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Callipyge Genetics:

Interestingly this type of muscle hypertrophy only affects the pelvic limbs. It is caused by a single mutation that began with the ram Solid Gold. The Callipyge mode of inheritance is polar overdominace. Only heterozygous animals that receive CLPG from their sire will exhibit the Callipyge phenotype. Receiving a CLPG allele from the dam will have no effect, and in the case of a homozygous lamb, the allele from the dam is thought to help suppress expression.

It is not well understood how this mutation enables hypertrophy of only the pelvic limbs. One theory is that the mutation interferes with the slowing down of muscle growth after birth, allowing the hind limbs to continue to grow at an accelerated rate. This is supported by the fact that the extra muscling does not show up until between one and three months after birth.

A punnett square showing the breeding of two heterozygous parents shows that only one in four of their offspring will have the Callipyge phenotype.

Texel breed genetics:

This characteristic muscling of the Texel breed is caused by a mutation on the gene for myostatin, MSTN. The mutation allows for miRNA to bind and interrupt transcription, inhibiting myostatin expression, thus allowing continued muscle growth. This allele has been found to be largely homozygous within the Texel breed, indicating heavy selection for the trait even prior to the identification of the genetics behind it. The mutation appears to be partially recessive, with heterozygotes having a moderate increase in muscling and homozygotes having a larger increase.

The video below gives a good idea just how muscular these sheep are. They have broad backs and look a little bit like the football linebackers of the ovine world.

Texel Sheep judging

Impact on Sheep Production:

The Callipyge phenotype is not selected for in meat production because it causes the meat to be too tough. Texel sheep are the dominant terminal sire breed in Europe and very popular in the US as well. One problem with extensive selection for a single trait within a single breed is the appearance of previously very rare recessive genetic disorders. In the Texel breed microphthalmia is becoming more common. It is an autosomal recessive condition that causes the eyes to be small or not present and always causes blindness. Another issue with selection for muscle hypertrophy is dystocia (difficult birthing). It has been seen in cattle and could show up in sheep. Muscle hypertrophy has also been linked to undersized internal organs, enlarged tongues and fragile bones in cattle. Continued use of these breeding practices will require vigilance on the part of breeders to maintain the health of the Texel breed.

Opinion:

Meat production in general is a very inefficient system. Increasing efficiency by raising the meat yield per animal is a step in the right direction. As long as breeders are paying close attention to the development of negative genetic traits in their flocks and working to prevent them selecting breeding stock to produce muscle hypertrophy can be a vary good thing.

References:

1. Tellam, Ross L., Noelle E. Cockett, Tony Vuocolo, and Christopher A. Bidwell. "Genes contributing to genetic variation of muscling in sheep." Frontiers in Genetics 3.164 (2012): 1-14. http://journal.frontiersin.org/article/10.3389/fgene.2012.00164/full (accessed 29 April 2015)


2. Clop, Alex, et al. "A mutation creating a potential illegitimate microRNA target site in the myostatin gene affects muscularity in sheep." Nature Genetics 38.7 (2006): 813-818. http://www.nature.com/ng/journal/v38/n7/abs/ng1810.html (accessed 29 April 2015)


3. Cockett, Noelle E., et al. “The calligype mutation and other genes that affect muscle hypertrophy in sheep.” Genetics Selection Evolution 37.1 (2005): S65-S81. http://www.gse-journal.org/articles/gse/ref/2005/02/g0405W/g0405W.html (accessed 30 April 2015)


4. Masri, A. Y., et al. "Evaluating the effects of the c. *1232G>A mutation and TM-QTL in Texel x Welsh Mountain lambs using ultrasound and video image analyses." Small Ruminant Research 99 (2011):99-109


5. Becker D, Tetens J, Brunner A, Burstel D, Ganter M, et al. “Microphthalmia in Texel Sheep Is Associated with a Missense Mutation in the Paired-Like Homeodomain 3 (PITX3) Gene.” PLoS ONE 5.1 (2010): e8689. http://www.sheephapmap.org/news/Becker-2010.pdf (accessed 30 April 2015)