ONCORHYNCHUS AT THE SOUTHERN EXTENT OF THEIR RANGE - A STUDY OF MTDNACONTROL-REGION SEQUENCE WITH SPECIAL REFERENCE TO AN UNDESCRIBED SUBSPECIES OF O-MYKISS FROM MEXICO

Nucleotide sequences from the right-domain of salmonid mitochondrial D NA (mtDNA) control region flanking the phenylalanine tRNA gene (tRNA(P he)) were determined for 5 species and 14 subspecies of Oncorhynchus a t the southern extent of their range. In all but one population, the r ight domain contained two 72 bp tandem repeats located between the tRN A(Phe) gene and the conserved sequence block CSB-3. At the species lev el we found 46-83% homology between these two repeats. The repeat clos est to tRNA(Phe) contained 43% of the phylogenetically informative sit es. The largest number of transversions (N = 6) were found outside of both repeat sequences. Phylogenetic inference based on mtDNA right-dom ain sequence was congruent with other analyses at the species level, b ut gave variable results in association drawn at the subspecific level . The right-domain contained three highly conserved sequences correspo nding to the mtDNA transcription factor, and the heavy-and light-stran d promoters. In four O. mykiss from Rio Yaqui, Mexico, the repeat cont aining the heavy-strand promoter (HSP) was deleted. The mtDNA HSP is r esponsible for transcription of the 16S and 12S rRNAs, 12 of the 13 pr otein-coding genes, and most of the tRNA genes. Rio Yagui trout showed no heteroplasmy or tissue specificity for this deletion. This HSP del etion suggests the possibility of a primitive transcriptional promoter with bidirectional capacity in this species, similar to that reported in avian and amphibian mtDNA. These data expand our understanding of genetic diversity in Oncorhynchus at the southern extent of their rang e with a knowledge of the evolutionary mechanisms that may have led to that diversity.