The Independent Origins of Blood/Oxygen Transport Mechanisms

This is pretty cool. Science Daily PhysOrg mentions research in PNAS

An extended excerpt from Science Daily PhysOrg:

A team led by Jay Storz (prounounced storts), assistant professor of biological sciences, analyzed the complete genome sequences of multiple vertebrate species and found that jawless fishes (e.g., lampreys and hagfish) and jawed vertebrates (pretty much everything else, including humans) independently invented different mechanisms of blood-oxygen transport to sustain aerobic metabolism.

Specifically, comparative analysis of their gene repertoires revealed that the ancestors of jawed and jawless vertebrates co-opted completely different precursor proteins to serve as oxygen-transport hemoglobins, the respiratory proteins that give blood its red color. The jawed and jawless vertebrates separated about 500 million years ago, late in the Cambrian Period and about 30 million years after the first vertebrate precursors (the chordates) show up in the fossil record.

“What we’ve discovered is that similar physiological problems called forth similar solutions in different branches of the vertebrate family tree,” Storz said. “For example, jawed vertebrates invented one type of hemoglobin as an oxygen transport protein, whereas the other main vertebrate lineage, the jawless fishes, represented today by lampreys and hagfish, came up with their own independent solution for the same physiological problem.”

And here is the abstract of the article from PNAS:

Natural selection often promotes evolutionary innovation by coopting preexisting genes for new functions, and this process may be greatly facilitated by gene duplication. Here we report an example of cooptive convergence where paralogous members of the globin gene superfamily independently evolved a specialized O2 transport function in the two deepest branches of the vertebrate family tree. Specifically, phylogenetic evidence demonstrates that erythroid-specific O2 transport hemoglobins evolved independently from different ancestral precursor proteins in jawed vertebrates (gnathostomes) and jawless fish (cyclostomes, represented by lamprey and hagfish). A comprehensive phylogenetic analysis of the vertebrate globin gene superfamily revealed that the erythroid hemoglobins of cyclostomes are orthologous to the cytoglobin protein of gnathostome vertebrates, a hexacoordinate globin that has no O2 transport function and that is predominantly expressed in fibroblasts and related cell types. The phylogeny reconstruction also revealed that vertebrate-specific globins are grouped into four main clades: (i) cyclostome hemoglobin + cytoglobin, (ii) myoglobin + globin E, (iii) globin Y, and (iv) the α- and β-chain hemoglobins of gnathostomes. In the hemoglobins of gnathostomes and cyclostomes, multisubunit quaternary structures provide the basis for cooperative O2 binding and allosteric regulation by coupling the effects of ligand binding at individual subunits with interactions between subunits. However, differences in numerous structural details belie their independent origins. This example of convergent evolution of protein function provides an impressive demonstration of the ability of natural selection to cobble together complex design solutions by tinkering with different variations of the same basic protein scaffold.

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