9707468 Reed The multi-copy ribosomal RNA (rRNA) genes and their associated spacer sequences have been widely applied as molecular tools for both systematic and populational studies. However, mechanisms involved in the evolution of the rRNA genes and the significance of variation in these remain as important genetic and evolutionary questions. Ribosomal DNA sequences evolve through both mutation of individual sequences and homogenization (concerted evolution) of tandem gene arrays. However, origin of new rDNA loci (new chromosomal sites of tandem arrays) may drastically alter the evolutionary dynamics of these multi-copy genes. Salmonid fishes provide an interesting vertebrate model system for investigating rDNA evolution. The ribosomal RNA genes of salmonid fishes appear to have amplified during radiation of this group. Accompanying this amplification is an increase in the number of rDNA loci in a number of species including three of six members of the genus Salvelinus. Mechanisms proposed for the origin of these include chromosomal rearrangements, amplification, reinsertion errors and involvement of mobile genetic elements in either recombination or transposition. The proposed research is intended to characterize the level of intra-individual variation in the rDNA cistron of one species (lake trout, Salvelinus namaycush) by closely examining the molecular structure of the non-transcribed intergenic spacer region (IGS). Molecular analysis of the IGS will focus on screening for sequences responsible for the length variation seen in the rDNA cistron of this species. Cistron-specific probes will be designed to test if length variants are interspersed within individual arrays or occur as locus-specific variants.
