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Animals depend on the sequential oxidation of organic molecules to survive; thus, oxygen-carrying/transporting proteins play a fundamental role in aerobic metabolism. Globins are the most common and widespread group of respiratory proteins. They can be divided into three types: circulating intracellular, noncirculating intracellular, and extracellular, all of which have been reported in annelids. The diversity of oxygen transport proteins has been underestimated across metazoans. We probed 250 annelid transcriptomes in search of globin diversity in order to elucidate the evolutionary history of this gene family within this phylum. We report two new globin types in annelids, namely androglobins and cytoglobins. Although cytoglobins and myoglobins from vertebrates and from invertebrates are referred to by the same name, our data show they are not genuine orthologs. Our phylogenetic analyses show that extracellular globins from annelids are more closely related to extracellular globins from other metazoans than to the intracellular globins of annelids. Broadly, our findings indicate that multiple gene duplication and neo-functionalization events shaped the evolutionary history of the globin family. [ABSTRACT FROM AUTHOR]

Multicellular organisms depend on oxygen-carrying proteins to transport oxygen throughout the body; therefore, proteins such as hemoglobins (Hbs), hemocyanins, and hemerythrins are essential for maintenance of tissues and cellular respiration. Vertebrate Hbs are among the most extensively studied proteins; however, much less is known about invertebrate Hbs. Recent studies of hemocyanins and hemerythrins have demonstrated that they have much wider distributions than previously thought, suggesting that oxygen-binding protein diversity is underestimated across metazoans. Hexagonal bilayer hemoglobin (HBL-Hb), a blood pigment found exclusively in annelids, is a polymer comprised up to 144 extracellular globins and 36 linker chains. To further understand the evolutionary history of this protein complex, we explored the diversity of linkers and extracellular globins from HBL-Hbs using in silico approaches on 319 metazoan and one choanoflagellate transcriptomes. We found 559 extracellular globin and 414 linker genes transcribed in 171 species from ten animal phyla with new records in Echinodermata, Hemichordata, Brachiopoda, Mollusca, Nemertea, Bryozoa, Phoronida, Platyhelminthes, and Priapulida. Contrary to previous suggestions that linkers and extracellular globins emerged in the annelid ancestor, our findings indicate that they have putatively emerged before the protostome–deuterostome split. For the first time, we unveiled the comprehensive evolutionary history of metazoan HBL-Hb components, which consists of multiple episodes of gene gains and losses. Moreover, because our study design surveyed linkers and extracellular globins independently, we were able to cross-validate our results, significantly reducing the rate of false positives. We confirmed that the distribution of HBL-Hb components has until now been underestimated among animals. [ABSTRACT FROM AUTHOR]

Animal tissues need to be properly oxygenated for carrying out catabolic respiration and, as such, natural selection has presumably favored special molecules that can reversibly bind and transport oxygen. Hemoglobins, hemocyanins, and hemerythrins (Hrs) fulfill this role,with Hrs being the least studied. Knowledge of oxygen-binding proteins is crucial for understanding animal physiology. Hr genes are present in the three domains of life, Archaea, Bacteria, and Eukaryota; however, within Animalia, Hrs has been reported only in marine species in six phyla (Annelida, Brachiopoda, Priapulida, Bryozoa, Cnidaria, and Arthropoda). Given this observed Hr distribution, whether all metazoan Hrs share a common origin is circumspect. We investigated Hr diversity and evolution in metazoans, by employing in silico approaches to survey for Hrs from of 120 metazoan transcriptomes and genomes. We found 58 candidate Hr genes actively transcribed in 36 species distributed in 11 animal phyla, with new records in Echinodermata, Hemichordata, Mollusca, Nemertea, Phoronida, and Platyhelminthes. Moreover, we found that "Hrs" reported from Cnidaria and Arthropodawere not consistentwith that of othermetazoan Hrs. Contrary to previous suggestions that Hr geneswere absent in deuterostomes, we find Hr genes present in deuterostomes and were likely present in early bilaterians, but not in nonbilaterian animal lineages. As expected, the Hr gene tree did not mirror metazoan phylogeny, suggesting that Hrs evolutionary history was complex and besides the oxygen carrying capacity, the drivers of Hr evolution may also consist of secondary functional specializations of the proteins, like immunological functions. [ABSTRACT FROM AUTHOR]