Abstract
BackgroundDespite extensive study on hemoglobins and hemocyanins, little is known about hemerythrin (Hr) evolutionary history. Four subgroups of Hrs have been documented, including: circulating Hr (cHr), myohemerythrin (myoHr), ovohemerythrin (ovoHr), and neurohemerythrin (nHr). Annelids have the greatest diversity of oxygen carrying proteins among animals and are the only phylum in which all Hr subgroups have been documented. To examine Hr diversity in annelids and to further understand evolution of Hrs, we employed approaches to survey annelid transcriptomes in silico.ResultsSequences of 214 putative Hr genes were identified from 44 annelid species in 40 different families and Bayesian inference revealed two major clades with strong statistical support. Notably, the topology of the Hr gene tree did not mirror the phylogeny of Annelida as presently understood, and we found evidence of extensive Hr gene duplication and loss in annelids. Gene tree topology supported monophyly of cHrs and a myoHr clade that included nHrs sequences, indicating these designations are functional rather than evolutionary.ConclusionsThe presence of several cHrs in early branching taxa suggests that a variety of Hrs were present in the common ancestor of extant annelids. Although our analysis was limited to expressed-coding regions, our findings demonstrate a greater diversity of Hrs among annelids than previously reported.
Highlights
Despite extensive study on hemoglobins and hemocyanins, little is known about hemerythrin (Hr) evolutionary history
Following translation Pfam domain evaluation and manual removal of sequences with less than 100 amino acid residues, 214 putative novel Hr genes were retained from all taxa examined in this study, representing 44 annelid species in 40 different families (Table 1)
Incongruence between our gene genealogy relative to current knowledge of annelid evolutionary history indicates that Hrs have a complex history which possibly involved events of gene losses and duplications, paralogs replacements and lateral gene transfer [26]
Summary
Despite extensive study on hemoglobins and hemocyanins, little is known about hemerythrin (Hr) evolutionary history. Natural selection has presumably favored proteins that can reversibly bind and transport oxygen [1]. Such oxygen-binding proteins likely originated from enzymes whose primary function was to protect the body from oxygen toxicity, and, secondarily, these enzymes acquired the ability to carry oxygen molecules [2]. Several different classes of oxygen-carrying proteins, or respiratory pigments, are found across animal life These molecules can reversibly bind oxygen, their binding affinities and evolutionary origins differ. Oxygen-binding proteins are usually divided into two main groups: proteins that use iron to bind oxygen, including hemoglobins and hemerythrins, Hemerythrins (Hrs) are a non-heme oligomeric protein family within the ‘four-helical up-and-down bundle’ fold and ‘all alpha proteins’ class according to the Structural Classification of Proteins database (SCOP) [14]. Residues involved in iron binding include histidines (His) in positions 26 56, 75, 79, and 108, glutamic acid residue (Glu) in position 60 and aspartic acid residue (Asp) in position 113
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