Physical localization and characterization of the BglI element in the genomes of Atlantic salmon ( Salmo salar L.) and brown trout ( S. trutta L.)

This work describes chromosomal localization, fine physical mapping, and population variation of the BglI element in the genome of Atlantic salmon (Salmo salar L.) and a similar sequence in the genome of brown trout ( S. trutta L.). Results from a variety of complementary approaches, clearly demonstrate that the BglI element does not occur as a satellite-like repetitive DNA in these species but is part of the rDNA cistron as suggested by Goodier and Davidson (1993). Coincident hybridization of BgII clones with rDNA loci in both single and double-probe fluorescence in situ hybridization (FISH) experiments demonstrated physical linkage between the BglI element and rDNA loci. Fine physical mapping by Southern analysis and PCR amplification showed the BgII element to be located approximately 1.6 kb upstream of the 18S gene. The BgII element was used to screen for population-specific markers by Southern analysis. Population-specific banding patterns were only observed in brown trout, allowing identification of individual populations of this species. Sequence comparisons revealed sequences similar to the BgII element present in the rDNA cistron of other salmonids. This result suggests the presence of this sequence in the genome of the salmonid tetraploid ancestor.