Genomic analysis of the horse Y chromosome

Stallion fertility is of significant economic importance in the multibillion dollar equine industry. Presently, the underlying genetic causes of infertility in stallions are unknown. Analysis of the human genome has shown that in more than 25% of cases, male infertility is associated with deletions/rearrangements in the Y chromosome. Presently there is no gene map for the Y chromosome in the horse. Therefore, the primary aim of this study is to build a detailed physical map of the chromosome with a long-term aim to identify and analyze Y-specific factors affecting fertility in stallions. To materialize this, we constructed the first radiation hybrid and FISH map of the euchromatic region of the horse Y chromosome. This basic map was used to obtain Y-specific BAC clones that provided new STS markers from the end sequences. Chromosome walking provided 73 BACs comprising 7 contigs that were built across the euchromatic region using 124 markers for content mapping. The results were validated by restriction fingerprinting and Fiber FISH. The map is presently the most informative among the domestic species and second to only human and mouse Y chromosome maps. The construction of this map will pave the way for isolation and functional characterization of genes critical for normal male fertility and reproduction and will in the future lead to the development of a diagnostic test to facilitate early identification of deletions/rearrangements on the Y chromosome of potentially affected foals/stallions. The second part of the study comprised the first extended investigation to assess genetic variation in the horse Y chromosome. Approximately 4.5Mb of the euchromatic region was screened for polymorphic microsatellite markers. Of the 27 markers that were characterized and screened for polymorphism in 14 breeds of the domestic horse and eight extant equids, only one was polymorphic in the domestic horse, suggesting a low level of genetic variation on the chromosome. However, 21 of the markers showed noteworthy variation (on average four alleles/marker) among the eight equids. These markers will be vital in future studies aimed at elucidating the genetic relationships between the various equids through phylogenetic analysis.