Fertility and 63,X Mosaicism in a Haflinger Sibship.
Authors: Neuhauser Stefanie, Handler Johannes, Schelling Claude, Pieńkowska-Schelling Aldona
Journal: Journal of equine veterinary science
Summary
Chromosomal abnormalities represent a recognised cause of infertility in horses, typically manifesting as poor oestrous cyclicity and underdeveloped reproductive tracts; however, this 2019 investigation challenged assumptions about the clinical significance of X-chromosome mosaicism by documenting a Haflinger mare carrying 63,X/64,XX/65,XXX mosaicism who remained fully fertile with at least three healthy offspring. Using Giemsa and DAPI banding alongside fluorescence in situ hybridisation and PCR analysis, Neuhauser and colleagues identified similar mosaicisms in two closely related mares and extended chromosomal screening to seven further relatives without detecting subfertility, developmental abnormalities, or apparent fitness deficits in either sex. The findings highlight a critical gap in current knowledge: whilst X0 mosaicism is theoretically associated with reproductive dysfunction, this family pedigree demonstrates that such conditions do not inevitably compromise fertility or general health, suggesting that phenotypic expression of chromosomal mosaicism may be more variable than conventional veterinary literature suggests. For equine practitioners, the practical implication is clear—a chromosomal abnormality identified during diagnostic investigation should not automatically predict infertility or justify culling decisions without further clinical correlation, particularly in mares demonstrating normal oestrous cycles and structural development. Further longitudinal studies characterising X0 mosaic populations are essential to establish accurate prognostic thresholds and refine breeding recommendations in affected families.
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Practical Takeaways
- •Chromosomal abnormalities detected via karyotyping alone should not be used as absolute predictors of infertility in mares—clinical assessment and reproductive history remain essential for fertility evaluation
- •X0 mosaicism in mares may be clinically inconsequential; consider additional investigation beyond cytogenetics before making breeding or management decisions
- •Incidental chromosomal findings in fertile individuals warrant cautious interpretation and should prompt multi-generational family studies rather than immediate exclusion from breeding
Key Findings
- •A fertile Haflinger mare with three healthy offspring was found to have 63,X/64,XX/65,XXX mosaicism, demonstrating that X0 mosaicism does not necessarily impair fertility
- •Two related mares were 63,X/64,XX mosaics, with one carrying a marker chromosome, yet neither showed evidence of subfertility or developmental abnormalities
- •Eight family members (4 mares and 3 stallions) with various chromosomal mosaicisms displayed normal fertility and reproductive development
- •The study highlights the need for further research to improve the predictive value of chromosomal X0 mosaics on actual fertility outcomes in horses