08 June 2009

Deep homology and design: a new series

Recently I was reading a superb review article [doi] on the subject of a famous and important cellular signaling pathway called the Notch pathway. The author, Mark Fortini of Thomas Jefferson University, quoted James Puckle (an 18th-century English inventor and writer) on the "wonderful frame of the human body" in which "so many strings and springs" which all must "be in their right frame and order" for life and concluding that "it is next to a miracle we survived the day we were born." (If you must know, it's maxim #914 in The Club, in a section called "Death.")

This reminded me of some personal tragedy in our own family, after which Puckle's conclusion was repeated almost verbatim. It also reminded me of my need to write about the amazing homology of developmental signaling mechanisms in animals. For many months, I've listed an article on "deep homology" as the subject of my next Journal Club. But this topic won't fit into one article review, so I've decided to turn it into a little series.

Here's what Fortini writes in his introduction, after quoting Mr. Puckle:

Surprisingly, research over the past few decades has revealed that the orderly differentiation and arrangement of these many physiological ‘‘strings and springs’’ are controlled by a relatively small number of developmental signaling pathways. These pathways, including the Notch, Ras/MAPK, Hedgehog, Wnt, TGFβ, and JAK/STAT pathways, among others, are widely conserved throughout the animal kingdom and they cooperate throughout development to pattern a diverse array of tissues in different animal species.
The lingo might seem strange, but I hope the point is clear. The vast diversity of animal life, with "endless forms most beautiful," is assembled through the action of a small set of signaling systems. And, remarkably, the systems are used in the same ways in animals that couldn't be more different in behavior or structure.

This fact raises interesting questions about design and evolution. Why so few systems? Why are they used again and again, for the very same purpose? Are these choices forced by design constraints of some kind, or is there another explanation? Could it have been otherwise? Can it be otherwise? I'll tackle those questions while discussing some recent experiments in evolutionary developmental biology, or evo-devo.

And what of this phrase "deep homology"? It was coined by some of the founding minds of evo-devo – Neil Shubin, Cliff Tabin and Sean Carroll – as they considered the fact that animal limbs of every kind are "organized by a similar genetic regulatory system that may have been established in a common ancestor." And we mean limbs of every kind: whale flippers, fish fins, bat wings, human arms, and, amazingly, insect limbs. Such disparate structures may not be evolutionarily homologous (meaning that they were modified from a common ancestor) but the signaling systems that create them are homologous.

This, then, is deep homology: the sharing of signaling mechanisms that are used to create diverse (though often functionally similar) animal structures. So please join me, and maybe we'll lure interesting commenters into the discussion.

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