HYPERVARIABILITY OF RIBOSOMAL DNA AT MULTIPLE CHROMOSOMAL SITES IN LAKE TROUT (SALVELINUS-NAMAYCUSH)

Variation in the intergenic spacer (IGS) of the ribosomal DNA (rDNA) o f lake trout (Salvelinus namaycush) was examined. Digestion of genomic DNA with restriction enzymes showed that almost every individual had a unique combination of length variants with most of this variation oc curring within rather than between populations. Sequence analysis of a 2.3 kilobase (kb) EcoRI-DraI Fragment spanning the 3' end of the 28S coding region and approximately 1.8 kb of the IGS revealed two blocks of repetitive DNA. Putative transcriptional termination sites were fou nd approximately 220 bases (b) downstream from the end of the 28S codi ng region. Comparison of the 2.3-kb fragments with two longer (3.1 kb) fragments showed that the major difference in length resulted from va riation in the number of short (89 b) repeats located 3' to the putati ve terminator. Repeat units within a single nucleolus organizer region (NOR) appeared relatively homogeneous and genetic analysis found vari ants to be stably inherited. A comparison of the number of spacer-leng th variants with the number of NORs Found that the number of length va riants per individual was always less than the number of NORs. Examina tion of spacer variants in five populations showed that populations wi th more NORs had more spacer variants, indicating that variants are pr esent at different rDNA sites on nonhomologous chromosomes.