A unique case of mosaicism for two Robertsonian translocations, rob(17;27) and rob(17;29), in a subfertile mare (Equus caballus)

Introduction Robertsonian translocations (ROBs) or centric fusions of acrocentric chromosomes, are the most common structural chromosomal rearrangements in mammals. ROBs are of medical and veterinary concern because of associated subfertility and congenital disorders but are also of interest as a mechanism of chromosome and karyotype evolution. While ROBs are well documented in humans, mouse, and cattle/bovids, they are extremely rare in horses, this despite of the 18 acrocentric chromosomes in the horse karyotype. Methods We characterize the case using conventional and molecular cytogenetic approaches and DNA analysis. Results We report about the first case of ROB between nonhomologous chromosomes in the horse, whereas the carrier was 50/50 mosaic for two different ROB cell lines – 63,XX,rob(17;27) and 63,XX,rob(17;29), and had no cells with normal karyotype. Both derivative ROB chromosomes had retained two structural centromeres which is also a typical feature of human and bovine ROBs. Clinical phenotype of the mare included small ovaries, irregular estrus and two pregnancy losses – all consistent with ROB. Discussion We discuss the role of centromeric satellite sequences in the formation of ROBs but also differences in the prevalence of ROBs in different species, regardless of the number of acrocentric chromosomes. In this context, further molecular studies of the presented case may provide additional clues about the features of centromeric satellite repeats that facilitate or prevent ROBs in general.