MINIMAL INTRASPECIFIC VARIATION IN THE SEQUENCE OF THE TRANSCRIBED SPACER REGIONS OF THE RIBOSOMAL DNA OF LAKE TROUT (SALVELINUS-NAMAYCUSH)

Intraspecific variation in the sequence of the transcribed spacer regi ons of the ribosomal DNA (rDNA) in lake trout was examined by restrict ion mapping and sequencing of these regions amplified by the polymeras e chain reaction. The length of the first internal transcribed spacer region (ITS-1) was 566 bases and the second internal transcribed space r region (ITS-2) was 368 bases in lake trout. When the 1.4-kb region i ncluding the ITS-1, the 5.8S coding region, and the ITS-2 was amplifie d from 12 individuals from four populations and digested with eight di fferent enzymes only one intraindividual polymorphism was found that o ccurred in each population. When the amplified ITS-1 region was sequen ced from an additional 10 individuals from five populations, no interi ndividual variation was found in the sequence. A 6-kb portion of the r DNA repeat unit including 1.6 kb of the 18S coding region, the 5' exte rnal spacer region (5' ETS), and part of the adjacent intergenic space r was cloned and a restriction map was prepared for these regions in l ake trout. No intraspecific variation was found in the region adjacent to the 18S rDNA, which includes the 5' ETS, although intraspecific an d intraindividual length variation was found in the intergenic spacer region 3-6 kb from the 18S. Sequencing of a 609-b segment of the 5' ET S adjacent to the 18S coding region revealed the presence of two 41-b repeats. The 198-b sequence between the repeats had some similarity to the 18S coding region of other fishes. Primers were designed for ampl ification of 559 b of the 5' ETS using the polymerase chain reaction. No intraspecific variation in this region in lake trout was found when the DNA amplified from this region in 12 individuals from four popula tions was digested with eight restriction enzymes.