Genetic linkage of loci on the ovine X chromosome

One of the main objectives in gene mapping today is to construct genetic maps for commercially important animals. These maps provide a useful resource for directing matings and improving livestock. In New Zealand, sheep (Ovis aries) are one of our most important residential and export market livestock and hence a map of the sheep genome would aid in the study, control and manipulation of economically important traits. The sheep genome contains approximately 100, 000 genes distributed over 27 pairs of chromosomes (26 pairs of autosomes and two sex chromosomes). The ovine X chromosome is the fourth largest chromosome and is estimated to contain about 5% of the total DNA content of the cell (Ohno, 1973). To date, very little is known about the location of genes on the ovine X chromosome and as part of a longer term programme to identify markers to a fecundity gene (FecX) on the ovine X chromosome (Davis et ai., 1991), a linkage map was constructed. Based on the observation by Ohno that X chromosome genes are conserved in eutherian mammals (Ohno, 1973), comparative mapping techniques were used to identify DNA markers on the ovine X chromosome. A total of one hundred and four cDNA/genomic probes and thirty-one microsatellite markers were collected from human, cattle, sheep, mouse, rabbit and rat sources to test for homology and polymorphism in sheep. Sixteen markers were identified which could be used for X chromosome mapping in sheep: seven RFLP (Type I) markers (ATP7A, DMD, PDHA1, PHKA1, PLP, TBG, TIMP1), and nine microsatellite (Type II) markers (ILSTS017, MAF45, OarAE25, OarAE133, OarMP4, TGLA15, TGLA68, TGLA72, TGLA89). This set included one microsatellite marker (MAF45) previously shown to be located in the pseudo autosomal region (PAR) on the ovine X chromosome (Swarbrick et at., 1992). The relative chromosomal location of these markers on the ovine X chromosome was determined from the segregation of alleles in a three-generation sheep pedigree containing; 480 individuals. The maximum number of informative meioses was 703, and the average was 614 for each positioned marker. The total genetic length of the ovine X chromosome is 151.3 cM, based on female recombination frequencies. This includes the positioning of MAF45 within the PAR at a distance of 6.2 cM from the PAR boundary. Comparison of homologous gene markers on the ovine X chromosome with other eutherian mammals is in agreement with Ohno's prediction of mammalian X chromosome conservation.