The term callipyge, from the Greek words for ‘beautiful buttocks,’ describes a genetic mutation in sheep that results in pronounced muscular hypertrophy, creating an unusually well-muscled appearance, particularly in the hindquarters. This inherited condition presents a unique opportunity to study the pathways that regulate muscle growth. The initial discovery was in a single Dorset ram, which became the founder for this genetic line.
The Genetic Origin of Callipyge
The callipyge phenotype originates from a single nucleotide polymorphism (SNP), a change in one DNA base pair, located in a specific region on ovine chromosome 18. This mutation is found within a cluster of imprinted genes, meaning that the expression of these genes depends on which parent they are inherited from. The key area is known as the DLK1-GTL2 imprinted domain, which shows high similarity to regions on human and mouse chromosomes.
This genetic trait is notable for its unique and complex inheritance pattern known as polar overdominance. For the muscular hypertrophy to be expressed, the lamb must inherit the mutated allele from its sire (father) and the normal, or wild-type, allele from its dam (mother). If the lamb inherits the mutation from its mother, or if it inherits two copies of the mutation (one from each parent), the muscular phenotype does not develop. This parent-of-origin effect is the only known case of polar overdominance in mammals.
The specific genes implicated in this process are located within the imprinted domain on chromosome 18. The mutation itself lies in what is now considered a novel transcript named CLPG1. Its location suggests it influences the regulation of neighboring imprinted genes, including Delta-like homolog 1 (DLK1) and Maternally Expressed Gene 3 (MEG3, also known as GTL2).
Observable Physical Characteristics
The physical manifestation of the callipyge trait is not present at birth but develops postnatally. The extreme muscle development becomes apparent when lambs are between three and eight weeks of age. This delayed onset indicates that the mutation affects muscle growth and maturation processes rather than initial muscle fiber formation during embryonic development. The hypertrophy is also not uniform across the entire body.
The most dramatic changes occur in the muscles of the hindquarters and loin, such as the longissimus dorsi (loin) and the biceps femoris (a major muscle of the rump). Computerized tomography scans have revealed that these specific muscles can increase in size by approximately 30% compared to those in non-carrier sheep.
This significant shift in muscle mass directly alters the animal’s overall body composition. Sheep expressing the callipyge phenotype have a higher proportion of lean muscle and a reduced percentage of fat. This leads to carcasses that are more muscular and less fatty, which has clear implications for meat production.
Molecular Basis of Muscle Hypertrophy
The muscular hypertrophy seen in callipyge sheep is a result of an increase in the size of existing muscle fibers, not an increase in the number of fibers. This process is driven by complex changes in gene expression. The mutation appears to disrupt the normal regulatory balance, leading to the altered expression of several genes in the muscle tissue of affected animals.
Current research suggests the mutation leads to an upregulation, or increased expression, of paternally expressed genes like DLK1 and PEG11. At the same time, it causes the downregulation, or decreased expression, of maternally expressed non-coding RNAs, such as GTL2/MEG3 and associated microRNAs.
These molecular changes are believed to interfere with the normal pathways that control protein synthesis and degradation within muscle cells. The elevated levels of certain proteins and reduced levels of regulatory RNAs may shift the balance toward muscle protein accretion, causing the individual muscle fibers to grow larger.
Significance in Sheep Farming and Meat Attributes
The most significant advantage of the callipyge mutation is the substantial increase in lean meat yield. Animals with the trait produce carcasses with more muscle and less fat, which can be economically advantageous. The increased muscle mass in high-value areas like the loin and hind limbs means a greater proportion of desirable cuts from each animal.
Despite the higher yield, the mutation has a major drawback that has prevented its widespread use: it negatively affects meat quality, particularly tenderness. Callipyge meat is consistently tougher than meat from non-carrier sheep. This toughness is primarily attributed to changes in the proteolytic systems that naturally tenderize meat after slaughter.
Specifically, the meat from callipyge sheep has higher levels of an inhibitor called calpastatin. Calpastatin blocks the action of calpains, which are enzymes responsible for breaking down muscle proteins during the aging process that makes meat tender. The reduced activity of these tenderizing enzymes means the muscle structure does not break down as effectively post-mortem. This trade-off between muscle quantity and meat quality is a central issue for breeders.