Thursday, August 12, 2010

Independently-derived globins?

There's an interesting report in this week's PNAS about lamprey hemoglobin, and I can hear the mistakes about this report already. The authors Hoffmann et al. argue that the hemoglobin of gnathostomes (vertebrates with jaws) is derived independently from the hemoglobin of cyclostomes (lampreys and hagfish). When I first saw it, I was surprised, since I'm fairly familiar with globin evolution. I didn't think the authors could be arguing for the independent origin of globins, and that was not their argument. It's a lot more subtle than that.

The basic idea goes like this: There was an ancestral globin gene that was independently co-opted to function in oxygen transport in the ancestor of the gnathostomes and in the ancestor of the cyclostomes. It's the function of oxygen transport that's independently-derived. The proteins that do that function just happen to be homologous. It sounds weird to say that homologous proteins (globins) are also homoplastic (functionally), but it makes sense. They are definitely not saying that globins originated independently, which really would be wacky. We can rule out independent origins using simple statistical tests of randomness. The evidence they use is a phylogeny of globins that shows a sister relationship between lamprey hemoglobins (shown above) and cyclostome cytoglobins, which are not oxygen transport molecules. If the oxygen-transport function was ancestral, you'd expect gnathostome and cyclostome hemoglobins to be sisters.

What does it mean for design? I'm not sure, but it's very interesting.

Hoffmann et al. 2010. Gene cooption and convergent evolution of oxygen transport hemoglobins in jawed and jawless vertebrates. PNAS 107:14274-14279.

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