High-quality, haplotype-resolved reference genomes of the Dutch warmblood horse and Friesian horse using trio binning

In horses, genetic diversity is predominantly observed between breeds, with little variation within breeds. The studbooks of the two largest horse populations in the Netherlands, the Dutch Warmblood horse and Friesian horse population, have ongoing conservation projects including collecting large-scale genotype and sequence data. The current reference genome, derived from a Thoroughbred horse can lead to bias in genetic analyses of other horse breeds. Therefore, the aim of this study was to create high-quality breed-specific reference genomes of Dutch Warmblood and Friesian horses. We performed nanopore long-read sequencing (R10.4, Q20+) of an F1 cross between a Dutch Warmblood horse and a Friesian horse to create two breed-specific reference genomes by trio binning. This resulted in high-quality, haplotype-resolved reference genomes with contig N50 of 37 and 35 Mb and single copy gene completeness of 99.2 and 99.3% for the Friesian and Warmblood, respectively. The majority of the chromosomes contained telomeric and /or centromeric sequences. The Ensembl gene annotation resulted in 19,750 and 19,872 protein coding genes for the Friesian and Warmblood, respectively. No large chromosomal rearrangements were observed between the Friesian and Warmblood genomes. However, a total of 722 large structural variations (> 10 kb) were identified, of which 14 affect the coding sequence of protein-coding genes. The novel breed-specific reference genomes provide a valuable resource for future genetic analysis and breed conservation efforts and will contribute to ongoing equine pangenome efforts.