Fully Phased Telomere-to-Telomere Assemblies for Thoroughbred Horse and Donkey Haplotypes derived from a Mule Illuminate the Peculiar Evolution of Equid Centromeres.

Overview

• This research reports the creation of complete telomere-to-telomere genome assemblies for a Thoroughbred horse and a donkey, generated from their mule offspring.
• The assemblies provide new detailed insights into horse and donkey centromeres and satellite DNA, shedding light on unique features of equid chromosome evolution.

Background

• Equids are a family of mammals including horses, donkeys, and mules.
• Centromeres are essential chromosome regions responsible for proper chromosome segregation during cell division.
• Centromeres often contain repetitive DNA sequences called satellite DNA.
• In many species, centromere function is closely linked to the presence of satellite DNA and specific proteins such as CENP-A and CENP-B.
• Equids exhibit an unusual decoupling of satellite DNA and centromere functionality, meaning centromeres can function with or without satellite DNA, which is uncommon.
• Complete, gapless (telomere-to-telomere) genome assemblies are challenging but crucial for studying structure and evolution of such complex genomic regions.

Methods and Data

• The researchers generated fully phased telomere-to-telomere genome assemblies for a Thoroughbred horse and a donkey by sequencing their mule offspring.
• A mule inherits one haplotype from each parent, enabling separation (phasing) of horse and donkey genomes from a single individual.
• This approach allowed the teams to overcome difficulties in assembling repetitive and complex genome regions.
• The assemblies have been adopted and annotated by the National Center for Biotechnology Information (NCBI) as reference genomes for these species.

Key Findings

• Resolution of previously inaccessible regions: The assemblies include detailed sequences spanning satellite DNA arrays, duplication regions, and telomeres, which were previously unresolved.
• Centromere annotation: Both satellite-based centromeres and satellite-free centromeres were identified and annotated, demonstrating the plasticity of centromeric structures in equids.
• Non-centromeric satellite loci: The study also annotated satellite DNA outside of centromeres, providing a comprehensive catalog of repetitive elements.
• Protein binding variability: Detailed characterization of CENP-A binding domains was performed, revealing variations in the epigenetic markers that define centromeres.
• Absence of CENP-B association: Unlike many other species where CENP-B binds with CENP-A at centromeres, equids lack this association, suggesting a unique centromere protein interaction network.
• Karyotype evolution insights: Comparative analysis of satellite repeats and centromere repositioning indicates rapid chromosomal rearrangements during equid evolution.

Implications

• The newly developed reference genomes serve as foundational resources for future equid genomics research.
• They enable better understanding of chromosome biology, particularly the unusual centromere dynamics in equids.
• The results support large-scale efforts like the Equine Pangenome Project, facilitating broader genomic studies across horse breeds and related species.
• Insights into centromeric plasticity and satellite DNA evolution may inform genetic and evolutionary biology beyond equids.

Conclusion

• The study demonstrates the power of using hybrid offspring such as mules to generate fully phased, complete genome assemblies of parental species.
• This approach revealed unique centromere features in equids that challenge existing paradigms about centromere structure and function.
• The work advances our understanding of chromosome evolution and provides valuable genomic tools for animal genetics and evolutionary research.