Polymorphism of major ribosomal gene chromosomal sites (NOR-phenotypes) in the hybridogenetic fish Squalius alburnoides complex (Cyprinidae) assessed through crossing experiments

Chromosomal locations of major ribosomal sites, i.e. NOR-phenotypes, were assigned in Squalius alburnoides complex using sequential chromomycin A(3) (CMA(3))- and silver (Ag)-staining. This hybridogenetic Iberian minnow comprises diploid, triploid and tetraploid forms that arose by interspecific hybridisation between S. pyrenaicus and an unknown species. Inheritance of NOR patterns was studied by means of crossing experiments involving most diploid-polyploid forms of the S. alburnoides complex with identified specific genotype constitution. In all the specimens studied, the NORs were localised in the short arms of submetacentric chromosomes. Although S. pyrenaicus presented only one pair of NOR-bearing chromosomes, the data from experimental crosses evidenced that S. alburnoides complex was characterised by a multiple NOR phenotype composed of one chromosome pair with stable NORs and two chromosome pairs with NOR site polymorphism of presence/absence type. These data suggest that the karyotype of the unknown parental species of the S. alburnoides complex should have a multiple NOR pattern and emphasised the role of the all-male diploid linage in the dynamics and evolutionary potential of the S. alburnoides complex allowing the preservation of the missing ancestor genome. Cross-analyses evidenced that in spite of the high polymorphic nature of NORs in this fish complex, we have no reason to reject the hypothesis that their inheritance patterns were in accordance with Mendelian segregation.