Molecular Cytogenetics of the Minute chromosome Syndrome in Alpacas

The Minute Chromosome Syndrome (MCS) in alpacas is associated with infertility and cytogenetically manifested as an abnormally small homolog of chromosome 36 (chr36). The origin of the minute is not known due to the lack of molecular markers for this chromosome. Recently, we developed the first markers for alpaca chr36 and revealed its homology with HSA7p. Here, we used these markers for the analysis of MCS. We mapped 2 genes and 5 genome sequence scaffolds by fluorescence in situ hybridization (FISH) in 7 infertile female alpacas carrying the minute, and confirmed that it is a homolog of chr36. All markers mapped to both the minute and the normal chr36, suggesting that the minute is not a result of a large structural rearrangement. We also showed that the minute is not a natural size polymorphism: karyotyping of 100 normal female alpacas revealed the minute in only one individual, whereas it was present in 15% of reproductively abnormal female alpacas.

However, FISH with rDNA revealed that the size difference between chr36 homologs in infertile alpacas was due to a heterozygous presence of a large nucleolus organizer region (NOR), whereas chr36 in fertile alpacas had no NOR. We theorize that NOR heterozygosity might affect chr36 pairing, recombination and segregation in meiosis, and thus fertility. Whether the NOR in chr36 is a result of a rearrangement or a signature of interspecific hybridization across South American camelids, remains enigmatic. Further studies in llamas, vicuñas and guanacos are needed to decipher the origin of the MCS.