Cytogenetic and genome research2026; 1; doi: 10.1159/cgr/achag004

Comparative analysis of the karyotypes of Equus caballus and Equus asinus in the context of contemporary genomic data.

Kowal-Mierzwa, W
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Bugno-Poniewierska, M

Abstract: Karyotypes of species within the genus Equus display considerable variation in chromosome number and are characterised by numerous structural rearrangements that have played a pivotal role in their evolutionary divergence and speciation. Equus caballus (horse), 2n = 64 and Equus asinus (donkey), 2n = 62, constitute particularly interesting models for comparative studies due to the diversified morphology of their chromosomes, including acrocentric and submetacentric pairs, documented Robertsonian translocations, and centric fusions. In recent decades, widely applied molecular cytogenetic techniques, especially fluorescence in situ hybridisation (FISH), have enabled the identification of homologous regions, the determination of breakpoint locations, and the precise mapping of chromosomal fragments. Simultaneously, the rapid development of sequencing technologies has allowed for the generation of complete genomic assemblies and the detection of numerous rearrangements that were not identified by earlier methods. Despite these advances, comprehensive physical validation of the most variable chromosomal regions in both species is still lacking, which limits a full understanding of karyotype dynamics and its evolutionary significance. For this reason, further studies involving systematic BAC probe mapping and the integration of FISH data with advanced genomic techniques are warranted. Such an interdisciplinary framework would facilitate the construction of a high-resolution physical map of the genomes of Equus caballus and Equus asinus, thereby advancing our understanding of the mechanisms underlying speciation within the genus Equus.

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Publication Date: 2026-08-20 PubMed ID: 42623319DOI: 10.1159/cgr/achag004Google Scholar: Lookup

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Overview

This study compares the chromosome structures (karyotypes) of the horse (Equus caballus) and donkey (Equus asinus) using both classical and modern genomic techniques to better understand their evolutionary differences and speciation processes.

Background and Significance

The genus Equus, which includes horses and donkeys, exhibits significant variation in chromosome number and structure.
Chromosome number differences: Horses have 64 chromosomes (2n=64) while donkeys have 62 chromosomes (2n=62).
The chromosomes show variation in morphology, including:
- Acrocentric chromosomes (centromere near one end)
- Submetacentric chromosomes (centromere slightly off-center)
Known chromosomal rearrangements include Robertsonian translocations and centric fusions, which are structural changes important for the evolutionary divergence between these species.

Technological Advances Enhancing Comparison

Fluorescence in situ hybridisation (FISH) allows for:
- Identification of homologous chromosome regions between species
- Pinpointing breakpoint locations where rearrangements occur
- Precise mapping of chromosomal fragments
Advancements in DNA sequencing technologies have enabled:
- Complete genome assemblies for both species
- Detection of many chromosomal rearrangements that older techniques missed

Current Limitations and Research Gap

Despite the available technology, certain highly variable and complex regions of chromosomes have not been physically validated in detail.
This lack of detailed mapping limits comprehensive understanding of karyotype evolution and its implications for speciation.

Future Directions and Recommendations

The study suggests further research using:
- Systematic mapping of bacterial artificial chromosome (BAC) probes—a method that can target specific large DNA regions for detailed study
- Integration of FISH data with cutting-edge genomic sequencing to produce a high-resolution physical map of chromosomes
This integrated approach will:
- Provide a more complete picture of chromosome structure differences between horses and donkeys
- Enhance understanding of genome evolution within the genus Equus
- Clarify mechanisms underlying speciation driven by chromosomal rearrangements

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APA

Kowal-Mierzwa W, Bugno-Poniewierska M. (2026). Comparative analysis of the karyotypes of Equus caballus and Equus asinus in the context of contemporary genomic data. Cytogenet Genome Res, 1. https://doi.org/10.1159/cgr/achag004