Differential expression of the GTL2 gene within the callipyge region of ovine chromosome 18.

The inheritance pattern of the skeletal muscle hypertrophy phenotype caused by the callipyge gene has been characterized as polar overdominance. We hypothesized that this trait may be caused by a gain or loss of gene expression because of the reversible nature of the phenotype in paternal vs. maternal inheritance. Suppression subtraction cDNA probes were made from skeletal muscle mRNA of normal (NN) and callipyge (C(Pat)N(Mat)) animals and hybridized to Southern blots containing bacterial artificial chromosomes (BACs) that comprise a physical contig of the callipyge region. The CN-NN probes hybridized to two ovine and seven bovine BACs. Sequence analysis of fragments within those BACs indicated short regions of similarity to mouse gene trap locus (gtl2). Northern blots analysis of RNA from hypertrophy-responsive muscles show a population of GTL2 mRNA centred around 2.4 kb that were abundantly expressed in 14-day prenatal NN and C(Pat)N(Mat) lambs but were down-regulated in day 14 and day 56 postnatal NN lambs. The expression of GTL2 remained elevated in 14- and 56-day-old C(Pat)N(Mat) lambs as well as in 56-day-old N(Pat)C(Mat) and CC lambs. Expression of GTL2 in the supraspinatus, which does not undergo hypertrophy, was very low for all genotypes and ages. Isolation of cDNA sequences show extensive alternative splicing and a lack of codon bias suggesting that GTL2 does not encode a protein. The mutation of the callipyge allele has altered postnatal expression of GTL2 in muscles that undergo hypertrophy and will help identify mechanisms involved in growth, genomic imprinting and polar overdominance.

Fine-mapping and construction of a bovine contig spanning the ovine callipyge locus.

The callipyge (CLPG) gene was fine-mapped by linkage analysis to a 4.6-cM chromosome interval on distal ovine OAR18q, flanked by microsatellite markers IDVGA30 and OY3. The OAR18q linkage map and human HSA14q transcript map were aligned by genotyping two bovine-hamster whole-genome radiation hybrid panels with the microsatellite markers, as well as with sequences corresponding to HSA 14q genes. Using Type I loci mapping to the IDVGA30-OY3 interval as anchor points, we have constructed a 1.4-Mb bovine BAC contig containing the IDVGA30-OY3 interval. We demonstrate that the IDVGA30-OY3 interval spans approximately 770 kb and contains at least four genes: YY1, WARS, DLK1, and GTL2.

Analysis of the sheep genome.

The identification of genes controlling several traits of interest in sheep has been accomplished by positional candidate cloning. In these studies, the trait is first mapped to a specific chromosomal region by linkage analysis, which requires families that are segregating for the trait and for polymorphic markers. Microsatellite markers are usually used for these analyses because of their extensive genetic variability. Once the location of a trait is determined by linkage to the markers, possible candidate genes controlling the trait can be inferred because of their proximity to linked markers. It is not necessary to map all possible genes in sheep for this strategy to be effective. Rather, a subset of genes that are mapped in humans and mice have also been mapped in sheep; these genes serve as "anchors" across the comparative maps of the different species. Further study of these positional candidates has revealed naturally occurring mutations that produce phenotypes that are unique to sheep. Thus the genetic analysis of sheep traits advances knowledge not only in this species but provides critical information for understanding biological pathways in mammalian species.

Localization of the locus causing Spider Lamb Syndrome to the distal end of ovine Chromosome 6.

Spider Lamb Syndrome (SLS) is a semi-lethal congenital disorder, causing severe skeletal abnormalities in sheep. The syndrome has now been disseminated into several sheep breeds in the United States, Canada, and Australia. The mode of inheritance for SLS is autosomal recessive, making the identification and culling of carrier animals difficult due to their normal phenotype. Two large pedigrees segregating for the SLS mutation were established, and a genome scan with genetic markers from previously published genome maps of cattle and sheep was used to map the locus causing SLS. Genetic linkage between SLS and several microsatellite markers, OarJMP8, McM214, OarJMP12, and BL1038, was detected, thereby mapping the SLS locus to the telomeric end of ovine Chromosome (Chr) 6. Alignment of ovine Chr 6 with its evolutionary ortholog, human Chr 4, revealed a positional candidate gene, fibroblast growth factor receptor 3 (FGFR3).

The callipyge phenomenon: evidence for unusual genetic inheritance.

In 1983, a male lamb exhibiting a pronounced muscular hypertrophy, particularly noticeable in the hind quarters, was born into a commercial Dorset flock in Oklahoma. The ram was premonitorily called Solid Gold. He subsequently produced offspring expressing the unusual phenotype, which is referred to as callipyge (Greek: calli- beautiful + -pyge buttocks). Animals demonstrating the callipyge phenotype are all descendants of this founder ram. These animals produce leaner, higher yielding carcasses, but there is some concern with decreased tenderness of the loin. Genetic characterization of the locus has demonstrated a unique mode of inheritance termed polar overdominance, in which only heterozygous offspring inheriting the mutation from their sire express the phenotype. The three other genotypes are normal in appearance. Progeny data indicate that reactivation of the maternal callipyge allele occurs after passage through the male germ line, although this reactivation is not absolute. The callipyge gene has been mapped to the distal end of ovine chromosome 18.

Polar overdominance at the ovine callipyge locus.

An inheritable muscular hypertrophy was recently described in sheep and shown to be determined by the callipyge gene mapped to ovine chromosome 18. Here, the callipyge phenotype was found to be characterized by a nonmendelian inheritance pattern, referred to as polar overdominance, where only heterozygous individuals having inherited the callipyge mutation from their sire express the phenotype. The possible role of parental imprinting in the determinism of polar overdominance is envisaged.

Chromosomal localization of the callipyge gene in sheep (Ovis aries) using bovine DNA markers.

A mutation causing muscular hypertrophy, with associated leanness and improved feed efficiency, has been recently identified in domestic sheep (Ovis aries). Preliminary results indicate that an autosomal dominant gene may be responsible for this economically advantageous trait. We have exploited the conservation in sequence and chromosomal location of DNA markers across Bovidae to map the corresponding callipyge locus to ovine chromosome 18 using a battery of bovine chromosome 21 markers. Chromosomal localization of the ovine callipyge locus is the first step toward positional cloning of the corresponding gene.