Clinical and molecular cytogenetics of camelids: A comprehensive review

Abstract Despite distinct anatomical and physiological differences and very specialized adaptations, all camelid species share almost identical karyotypes except for the Y chromosome which is different between dromedary and Bactrian camel and South American camelids (SAC). However, the high diploid number (2n = 74) and morphological and banding similarities between autosomes, make conventional cytogenetic analysis difficult. Due to this, camelid chromosome nomenclature for definite chromosome identification and exchange between researchers, requires molecular markers assigned by fluorescence in situ hybridization (FISH). Consequently, camelid chromosomes have been defined by approximately 300 gene-specific and DNA markers assigned to all autosomes and the sex chromosomes, as well as by conserved synteny with human chromosomes. Molecular cytogenetic markers have been instrumental for chromosomal assignment of sequence scaffolds in camelid genome assemblies and to a lesser extent, for clinical cytogenetics. Compared to other domestic species, camelid clinical cytogenetics falls behind in quantity and quality, with only a handful of autosomal and X chromosome abnormalities found in alpacas and llamas. No chromosome abnormalities have been reported in camels. The signature chromosome abnormality in alpacas and llamas is the Minute Chromosome Syndrome (MCS), which makes over 50% of all known cytogenetic abnormalities. Other cytogenetic findings include an autosomal translocation, a few cases of X-monosomy and X-trisomy, several freemartin females with XX/XY blood chimerism, and animals with normal karyotypes but discrepancy between the genetic sex (XX or XY) and gonadal and/or phenotypic sex. At the same time, camelids have multiple male and female reproductive and developmental disorders that, like in other species, may be caused by chromosome abnormalities. Therefore, veterinarians should be encouraged to seek cytogenetic evaluation in animals with reproductive or congenital abnormalities. This will advance clinical diagnostics and genetic research on these conditions but will also help in the future breeding programs of these species. As evidenced from human medicine and veterinary practice in other domestic animals, cytogenetic evaluation remains the fastest and cheapest first level genetic evaluation of individuals with developmental or reproductive disorders and should also be a part of assisted reproductive technologies.