Comparative analysis of intra-individual and inter-species DNA sequence variation in salmonid ribosomal DNA cistrons

This study examines sequence divergence in three spacer regions of the ribo somal DNA (rDNA) cistron, to test the hypothesis of unequal mutation rates. Portions of two transcribed spacers (ITS-1 and 5' ETS) and the non-transcr ibed spacer (NTS) or intergenic spacer (IGS) formed the basis of comparativ e analyses. Sequence divergence was measured both within an individual lake trout (Salvelinus namaycush) and among several related salmonid species (l ake trout; brook trout, Salvelinus fontinalis; Arctic char, Salvelinus alpi nus; Atlantic salmon, Salmo salar; and brown trout, Salmo trutta). Despite major differences in the length of the rDNA cistron within individual lake trout, minimal sequence difference was detected among cistrons. Interspecie s comparisons found that molecular variation in the rDNA spacers did not co nform to the predicted pattern of evolution (ITS spacers < ETS spacers < IG S). Specifically, the IGS contains a region that appears to be as highly, o r more conserved than the ITS-1. (C) 2000 Elsevier Science B.V. All rights reserved.