Phylogenetic tree of teleost and Xenopus tropicalis β hemoglobins. The β hemoglobin cDNAs (exclusive of untranslated regions) annotated within the Ensembl 54 database for medaka, zebrafish, tetraodon, stickleback and X. tropicalis, as well as those identified in Atlantic salmon here and the hemoglobin genes identified as embryonic within rainbow trout  were independently aligned using EBioX . Phylogenetic trees were constructed using the a Bayesian approach with (5 runs, 100,000 generations, 40% burn-in period) within the TOPALi V.2 software package  running the MrBayes program  under the best selected model (SYM). For simplicity, as well as to clearly indicate the source chromosome of the gene, the teleostean hemoglobin genes were named using the same system used to name those of Atlantic salmon. That is, an abbreviated three letter (g enus sp ecies) name followed by chromosome/linkage group name followed by α or β followed by a number indicating the sequential order of the genes from 5' to 3' as defined by Ensembl (Additional file 4, Table S3). Hemoglobin genes that were previously identified via expression analysis as being expressed exclusively during embryogenesis, and that are identified as embryonic within the Ensembl 54 database are denoted with "emb" following the assigned gene name. Branch numbers indicate posterior probabilities.