Distribution and origin of rDNA loci in triploid A . × cornutum . (a) Chromosomes carrying 5S and 35S rDNA in A. × cornutum and three putative parental taxa; (b) idiogram of A. × cornutum (modified from Puizina et. al. 1999); (c) origin and localisation of the 5S and 35S rRNA genes in triploid onion. Tri-colour circles, squares and pentagons were used to label the chromosomes that carry 5S and 35S rDNA in the progenitor species and the corresponding chromosomes in the triploid hybrid A. × cornutum.