First evidence of sex chromosome mosaicism in the endangered Sorraia Horse breed

The Sorraia Horse is a critically endangered Portuguese breed with an extremely reduced effective size, having reached unusual high levels of inbreeding. Fertility is of crucial importance for the long-term survival of the extant population and it has been shown that chromosomal abnormalities, especially on sex chromosomes, in horses are associated with infertility or subfertility. To date, no cytogenetic studies were performed in the Sorraia breed to assess the extent of chromosome abnormalities in animals with fertility problems. We now report the results of the first studied case — a subfertile mare with a stallion-like behaviour. A mosaic 63,X0/64,XX karyotype (with 10.45% and 89.55% frequency, respectively) was found. Further cytogenetic screenings in Sorraia horses with low breeding performance and ambiguous sexual phenotypes are needed, to determine the extent of chromosomal abnormalities, since early detection of these animals is of paramount importance for the management and conservation of the breed.