Opening the "Black Box": The Genetic and Biochemical Basis of Eye Evolution by Todd H. Oakley and M. Sabrina Pankey (PDF) published in Evolution: Education and Outreach.
Warning: Long Post. If you only read one thing, read this:
One might argue that we are just pushing back the origins, changing the question of "phototransduction" origin to the question of "transduction" origins. In a way this is true, but it is also a fundamental insight about how evolution works. New features are not breathed into organisms by some unknown force, they evolve by duplication/divergence or recombination of existing features. Trace a feature like phototransduction back far enough in evolutionary time, component by component, and it grades into something else altogether.
Here is the story:
A week or two ago, I was contacted by an undergraduate from a university in the UK (I see no reason to reveal her name). She had questions about the article, writing:
Based on this email, it seemed the student had a pretty good grasp of the issues, and I replied:I've just been reading your 2008 paper 'Opening the Black Box: the genetic and biochemical basis of eye evolution' and found it really interesting! I'm just writing an essay on eye evolution atm and am trying to get to the crux of the issue, and find out how the phototransduction cascade itself evolved. After explaining that opsin probably arose by a mutation in a serpentine gene/protein, you mention in your paper that:
"In yeast...these receptors [GPCR's - the serpentine proteins] are sensitive to pheremones, and they even direct a signal through proteins homologous to non-opsin phototransduction proteins."
What I'm wondering is, is it this whole yeast pathway that has been modified for the metazoan phototransduction cascade? Or is it only the opsin which has been derived from it? (With the subsequent molecules involved in the phototransduction cascade being co-opted from other proteins not involved in the yeast signalling pathway).
Thanks for you questions. I found out after writing the paper that
As for other components of the yeast pheromone cascade, these are different than phototransduction. Yeast pheromones activate a MAP kinase cascade. So, I think all that is similar is the GPCR and G-protein. So, it really is an open question as to what the ancestral function was of some of the genes of phototransduction, although some of these genes do function in other sensory transduction pathways...the yeast pheromone proteins are not the "rhodopsin type" GPCR, so they are distantly related at best to opsins. So they should not be considered anything like direct ancestors of opsin.
Unfortunately, she misinterpreted this email, understanding it to mean (written over at PT, using the pseudonym "Green"):
First of all, this is an incorrect interpretation of what I wrote. In fact, I wrote "So, I think all that is similar is the GPCR and G-protein." This is far from "not homologous in any way", as claimed by the student. The G-protein is undeniably homologous, and the yeast pheromone receptor is a 7-transmembrane protein, at least conformationally like opsin (whether or not the opsin and pheromone receptor sequences are homologous is a trickier issue).
But the larger issue is I think an issue of "linear thinking", which I address quite often on this blog. The student seems to think that if we cannot identify in yeast (taken as a linear ancestor of animals) a cascade identical to phototransduction except for opsin, then the origin of phototransduction requires numerous simultaneous mutations. This is not the case. First of all, yeast is a more distant relative of animals with phototransduction than is sponges. I just mentioned the yeast pheromone photoreceptor in the paper as a well studied example of a pathway outside of animals with partial homology (some components homologous, some not) to photoreception. There are closer "relatives" of phototransduction in sponges (poorly studied functionally, but the genes are known) and in other animals (better studied).
Another issue is a difficulty that people have with thinking about partial homology - that some components can be homologous and some not, depending on the time scale of the comparison (see my post The Red Herring of Eye Evolution).
Partial homology is a pattern that indicates a mechanism of co-option in the evolution of features. Co-option is the combination of existing things in a new way (analogy: dijonaisse = dijon mustard plus mayonaisse). All of the components of phototransduction pre-date animals, except opsin. And if we consider opsin to be a GPCR, which it is, then all of the components of phototransduction pre-date animals. This may be considered a pattern of co-option, or exaptation. Signaling pathways were already present before phototransduction. Some of the phototransduction components function together as far back as the yeast + human common ancestor (GPCR + G-protein). Other components of phototransdcution function together in non-phototransduction cascades of other animals. This indicates that phototransduction did not assemble all at once, but built incrementally upon an existing scaffold.
One might argue that we are just pushing back the origins, changing the question of "phototransduction" origin to the question "transduction" origins. In a way this is true, but it is also a fundamental insight about how evolution works. New features are not breathed into organisms by some unknown force, they evolve by duplication/divergence or recombination of existing features. Trace a feature like phototransduction back far enough in evolutionary time, and it grades into something else, component by component.
There were other comments, too. Again at PT, she also commented:
The difficulty with this comment is that the origin of opsin defines the origin of phototransduction. The other components of the cascade were already there, they all predate opsin, as described above.
This is also a "God in the Gaps" argument, or maybe, a "God under the surface" argument, stating that describing the origin of the keystone molecule of phototransduction (opsin) "only scratches the surface".
Also, I don't understand what the difference is between "comparisons of genes" and "biochemical explanation". What would a biochemical explanation be for the evolutionary origins of things that doesn't involve "comparisons of genes". The genes of the phototransduction pathway have biochemical interactions with each other, many mediated by interactions between specific amino acids.