Sixty-seven new equine microsatellite loci assigned to the equine radiation hybrid map

The horse genome derby: racing from map to whole genome sequence

The map of the horse genome has undergone unprecedented expansion during the past six years. Beginning from a modest collection of approximately 300 mapped markers scattered on the 31 pairs of autosomes and the X chromosome in 2001, today the horse genome is among the best-mapped in domestic animals. Presently, high-resolution linearly ordered gene maps are available for all autosomes as well as the X and the Y chromosome. The approximately 4350 mapped markers distributed over the approximately 2.68 Gbp long equine genome provide on average 1 marker every 620 kb. Among the most remarkable developments in equine genome analysis is the availability of the assembled sequence (EquCab2) of the female horse genome and the generation approximately 1.5 million single nucleotide polymorphisms (SNPs) from diverse breeds. This has triggered the creation of new tools and resources like the 60K SNP-chip and whole genome expression microarrays that hold promise to study the equine genome and transcriptome in ways not previously envisaged. As a result of these developments it is anticipated that, during coming years, the genetics underlying important monogenic traits will be analyzed with improved accuracy and speed. Of larger interest will be the prospects of dissecting the genetic component of various complex/multigenic traits that are of vital significance for equine health and welfare. The number of investigations recently initiated to study a multitude of such traits hold promise for improved diagnostics, prevention and therapeutic approaches for horses.

High-resolution gene maps of horse chromosomes 14 and 21: additional insights into evolution and rearrangements of HSA5 homologs in mammals

High-resolution physically ordered gene maps for equine homologs of human chromosome 5 (HSA5), viz., horse chromosomes 14 and 21 (ECA14 and ECA21), were generated by adding 179 new loci (131 gene-specific and 48 microsatellites) to the existing maps of the two chromosomes. The loci were mapped primarily by genotyping on a 5000-rad horse x hamster radiation hybrid panel, of which 28 were mapped by fluorescence in situ hybridization. The approximately fivefold increase in the number of mapped markers on the two chromosomes improves the average resolution of the map to 1 marker/0.9 Mb. The improved resolution is vital for rapid chromosomal localization of traits of interest on these chromosomes and for facilitating candidate gene searches. The comparative gene mapping data on ECA14 and ECA21 finely align the chromosomes to sequence/gene maps of a range of evolutionarily distantly related species. It also demonstrates that compared to ECA14, the ECA21 segment corresponding to HSA5 is a more conserved region because of preserved gene order in a larger number of and more diverse species. Further, comparison of ECA14 and the distal three-quarters region of ECA21 with corresponding chromosomal segments in 50 species belonging to 11 mammalian orders provides a broad overview of the evolution of these segments in individual orders from the putative ancestral chromosomal configuration. Of particular interest is the identification and precise demarcation of equid/Perissodactyl-specific features that for the first time clearly distinguish the origins of ECA14 and ECA21 from similar-looking status in the Cetartiodactyls.

A 1.3-Mb interval map of equine homologs of HSA2

A comparative approach that utilizes information from more densely mapped or sequenced genomes is a proven and efficient means to increase our knowledge of the structure of the horse genome. Human chromosome 2 (HSA2), the second largest human chromosome, comprising 243 Mb, and containing 1246 known genes, corresponds to all or parts of three equine chromosomes. This report describes the assignment of 140 new markers (78 genes and 62 microsatellites) to the equine radiation hybrid (RH) map, and the anchoring of 24 of these markers to horse chromosomes by FISH. The updated equine RH maps for ECA6p, ECA15, and ECA18 resulting from this work have one, two, and three RH linkage groups, respectively, per chromosome/chromosome-arm. These maps have a three-fold increase in the number of mapped markers compared to previous maps of these chromosomes, and an increase in the average marker density to one marker per 1.3 Mb. Comparative maps of ECA6p, ECA15, and ECA18 with human, chimpanzee, dog, mouse, rat, and chicken genomes reveal blocks of conserved synteny across mammals and vertebrates.

A high-resolution physical map of equine homologs of HSA19 shows divergent evolution compared with other mammals

A high-resolution (1 marker/700 kb) physically ordered radiation hybrid (RH) and comparative map of 122 loci on equine homologs of human Chromosome 19 (HSA19) shows a variant evolution of these segments in equids/Perissodactyls compared with other mammals. The segments include parts of both the long and the short arm of horse Chromosome 7 (ECA7), the proximal part of ECA21, and the entire short arm of ECA10. The map includes 93 new markers, of which 89 (64 gene-specific and 25 microsatellite) were genotyped on a 5000-rad horse x hamster RH panel, and 4 were mapped exclusively by FISH. The orientation and alignment of the map was strengthened by 21 new FISH localizations, of which 15 represent genes. The approximately sevenfold-improved map resolution attained in this study will prove extremely useful for candidate gene discovery in the targeted equine chromosomal regions. The highlight of the comparative map is the fine definition of homology between the four equine chromosomal segments and corresponding HSA19 regions specified by physical coordinates (bp) in the human genome sequence. Of particular interest are the regions on ECA7 and ECA21 that correspond to the short arm of HSA19-a genomic rearrangement discovered to date only in equids/Perissodactyls as evidenced through comparative Zoo-FISH analysis of the evolution of ancestral HSA19 segments in eight mammalian orders involving about 50 species.