Extensive NOR site polymorphism in geographically isolated populations of Golden mahseer, Tor putitora

Analyses of nucleolar organizer region (NOR) site polymorphism using silver staining were carried out in 72 individuals of 5 geographically isolated populations of Tor putitora from India. All Ag-NORs analyzed were constituted by rDNA that was confirmed by the presence of positive fluorescence in situ hybridization (FISH) signals. Analyses of the extra NORs among the populations confirmed the hypothesis that multi-chromosomal NOR site polymorphism is not an exception but a rule. We found a negative correlation between the mean number of extra NORs and geographical distance from the Alaknanda River population, and a significant positive correlation between the genetic distances obtained by NOR data and geographical distances among the isolated populations. These findings suggest a single geographical origin of this NOR polymorphism, from which it might have radiated to neighboring populations before their isolation. The distribution of NOR site variation was compared with the sequence variation of the ITS1 region of major rDNA (45S) and the two were found to be positively related. Different mechanisms such as transposable elements adjacent to ribosomal genes, repetitive elements which could serve as a point for chromosome exchange, amplification of minor rDNA loci, and reinsertion of extrachromosomal rDNA amplified during oogenesis have been proposed to explain the apparent NOR site polymorphism. The high dispersion of NOR site variants within and among the populations suggests that whatever transposition phenomenon is responsible for repetitive DNA or NOR jumping, it could still be active in T. putitora.