Callipyge sheep are distinctive sheep known for their muscular physique. This genetic trait results in a significant increase in muscle mass. Researchers are interested in its unique biological basis, which offers insights into muscle growth and genetic inheritance. They serve as a case study in animal genetics and its impact on livestock production.
Distinctive Characteristics of Callipyge Sheep
Callipyge sheep exhibit hyper-muscularity, especially in their hindquarters and loin. The name “callipyge” comes from Greek, meaning “beautiful buttocks,” describing their prominent muscles. This extreme muscle development becomes evident in lambs around three to eight weeks of age.
The increased muscle mass, up to 30% greater than normal sheep, results from the enlargement of existing muscle fibers, not an increase in their number. There is a larger proportion and greater diameter of fast-twitch muscle fibers, which are associated with powerful, short-duration contractions. Muscles like the adductor, biceps femoris, gluteus group, longissimus, psoas group, quadriceps femoris, semimembranosus, and semitendinosus show increased weight, contributing to their “double-muscled” appearance.
The Unique Genetic Inheritance
The callipyge trait stems from a mutation on chromosome 18. This mutation is a single nucleotide substitution within a region between the DLK1 and GTL2 genes. Its inheritance pattern is unusual, not following typical Mendelian rules; it is known as “polar overdominance.”
Muscular hypertrophy is expressed only when the mutated gene is inherited specifically from the father. If inherited from the mother, or if two copies are inherited (one from each parent), the animal will not display the characteristic muscular phenotype, despite carrying the mutation. This unique parental origin effect, known as genomic imprinting, means that the expression of certain genes is regulated depending on whether they came from the sire or dam.
The CLPG locus, where the mutation is, affects the expression of several genes within an imprinted domain, including DLK1 and GTL2. The mutation causes an enhanced expression of paternally expressed protein-coding genes like DLK1 and PEG11 from the paternal chromosome. Conversely, maternal allele-specific noncoding RNA genes, such as GTL2, antiPEG11, and MEG8, are also affected. Research suggests that micro-RNAs produced from the maternal allele may target and regulate the coding genes from the paternal allele, contributing to the polar overdominance phenomenon.
Impact on Muscle Development and Meat Quality
The callipyge trait influences muscle development, leading to higher lean meat yield. This increased muscle mass, or hypertrophy, results from both enhanced protein synthesis and a reduced rate of protein degradation within the muscle fibers. Callipyge carcasses have a higher dressing percentage, less kidney-pelvic fat, and reduced fat depth at the 12th rib, indicating a leaner composition.
Despite advantages in lean meat production, the callipyge trait presents challenges for meat quality, especially tenderness. Meat from callipyge sheep is tougher than from normal sheep, particularly in muscles like the loin. This toughness links to increased activity of calpastatin, an enzyme inhibiting the tenderization process after slaughter. Reduced juiciness can also be an issue, impacting consumer preference and marketability.
Breeding Considerations and Scientific Interest
The polar overdominance inheritance pattern of the callipyge trait requires specific breeding strategies to express the desired muscular phenotype. Breeders must mate a ram carrying the mutation with a normal ewe to produce callipyge offspring. This ensures the paternally inherited mutated gene leads to hyper-muscularity. A genetic test for the mutation is commercially available, aiding breeders in managing their flocks.
Callipyge sheep are a scientific model for studying muscle growth, genetic imprinting, and unusual inheritance patterns in mammals. Research has provided insights into how a single genetic change can alter muscle phenotype and how parental origin influences gene expression. Understanding callipyge muscle development can advance livestock production, potentially improving muscle growth in other agricultural animals and offering clues for understanding muscle-related conditions in humans.