Population analysis of an unusual NOR-site polymorphism in brown trout (Salmo trutta L.)

A population analysis of an unusual NOR-site polymorphism previously detected in brown trout (Salmo trutta L.) from North-western Spain was carried out in 225 individuals from 20 native populations from this area. The analysis performed has permitted us to reveal: (i) the ubiquity of this phenomenon in most river basins from NW Spain, 13 extra-NORs not observed in standard trout being detected; (ii) the rDNA constitution of all extra-NORs, as confirmed by CMA3-staining and rDNA-FISH, and their capability to constitute their own nucleolus; (iii) the constant location of extra-NORs within individuals and their stable transmission across generations; and (iv) the telomeric location of Ag-NORs, which were randomly distributed in the karyotype of S. trutta, mostly in the heterozygous condition. Repetitive sequences in the IGS or scattered along rDNA units, rather than chromosome rearrangements, could play an important role in the dispersion of NORs. The polymorphism described seems to have a single geographical origin, since a positive correlation was demonstrated between the degree of polymorphism and geographical distance to a central point of the river basin where the phenomenon showed a higher intensity. The distribution of NOR-site variation as compared with allozyme variation in the populations studied, showed significant statistical differences. Selection against high copy number, or mutation due to changes in NOR location, could be acting on this polymorphism to explain the observed distribution. The high dispersion of NOR-site variants within and among populations suggests the transposition phenomenon responsible for NOR jumping could be still active.