Whole genome duplication: challenges and considerations associated with sequence orthology assignment in Salmoninae

To illustrate some of the challenges and considerations in assigning correct orthology necessary for any comparative genomic investigation among salmonids, sequence data from the non-coding regions of different chromosomes in three members of the subfamily Salmoninae, rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and Arctic charr Salvelinus alpinus, were compared. By analysing c. 55 distinct loci, corresponding to c. 142 kbp sequence information per species, 18 duplicated patterns representative of the two sequential rounds of teleost-specific whole genome duplications (i.e. 3R and 4R WGD) were identified. Sequence similarities between the 4R paralogues were c. 90%, which was slightly lower than those of the 4R orthologues and c. 60% for the 3R products. Through careful examination of the sequence data, however, only 14 loci could reliably be assigned as true orthologues. Locus-specific trees were constructed through maximum parsimony, maximum likelihood and neighbour-joining methods and were rooted using the information from a close relative, lake whitefish Coregonus clupeaformis. All approaches generated congruent trees supporting the {Coregonus [Salmo (Oncorhynchus, Salvelinus)]} topology. The general phenotypic characteristics of sequences, however, were highly suggestive of the basal position of Oncorhynchus, raising the hypothesis of an accelerated rate of nucleotide evolution in this species.