Evo-Devo in NYR Books!

Is anyone else having trouble connecting to Pharyngula just recently?

Yes, me too! It's just a blank page. Eek.

Yep, not to mention the damn typekey registration cookie monster madness.

NY Review of Books isn't part of the NY Times, as far as I know. However they're classy enough to own Granta. Our little consultancy group did the website for both...

I had a question prompted by that review that perhaps someone can help me answer. It mentioned in the discussion of hox genes that some of them "produce eyes", whether in a fly or a mammal. Does this call into doubt the usual claim about eyes evolving independently ~40 times?

Does this call into doubt the usual claim about eyes evolving independently ~40 times?

So, is the HOX gene sort of a generic sensory system regulator, such that it tends to get co-opted into visual systems when they arise?

Henry

No, Hox genes are transcription factors that regulate the expression of other genes. Here's an overview.

Ah, so they're more of an outline as to where to put various major body parts. Interesting.

Henry

It's probably particularly pinheaded to try to follow PZ on this thread, but let me throw a couple of things out there, hoping that PZ or others will correct me if I go astray.

While the HOX genes do serve as "an outline as to where to put various major body parts," these--and other regulatory--genes can function at multiple times and places during the cascade of events that make up development. It's almost fractal: first the HOX genes divide the developing embryo into major body zones, and then they--and other regulatory genes (the "eye gene" is not, technically, one of the HOX genes, I don't think, though it's also shared across major animal phyla)--are redeployed later on within those zones to lay out "subdivisions," specify areas in which particular types of cells and tissues will localize, etc. And so on and so on. Thus, some of the same genes which lay out the major body-segment modules may later encourage the budding of limbs or sense organs from those segments, and then be used again to mark off the even-finer division of the limb into still-more-specialized sub-segments (rays, fingers, etc.). All the way down to which scales on a butterfly's wing will help paint in the colors that make up the "eye-spot."

With regard to eyes, the shared gene--to the best of my recollection--appears to have been initially associated with the light-receptor pigments and the associated nerve pathways at a very rudimentary stage of affairs. Perhaps well before anything like complex optical systems--eyes!--had developed, when these primitive light-sensing organs (or pits or whatever) may have functioned only as a simple day-night "circadian rhythm" detector: Yo, worms, it's dark! It's safe to emerge from the burrow and browse, 'cause all the predators are catching Zzzzs!

As the various phyla evolved apart from each other, different kinds of much more-sophisticated eyes may have indeed "independently" evolved from this common, rudimentary base. But these different kinds of independently-evolved eyes were all erected upon the common foundation, and the (different, from there on down) developmental cascades that now build those different eyes, in critturs as different as flys and humans, are still intially triggered by that "same" (homologous) gene on that same ancestral substrate.

Likewise, vertebrate limbs and arthropod limbs are not themselves homologous--our common ancestor was probably something like a little legless wormlike thing. But it may have had some sort of--let's call it a primitive body-segment extension--perhaps a sensing-tasting "feeler"? And a regulatory gene which specified where that segment-sprout would go. Which has been redeployed for that same "build extension (of whatever later-evolved kind) here" task ever since. Thus, while our fingers and toes are not homologous to the distal subdivisions of an insect's leg, the homologous regulatory genes (and I think they may be true HOX genes, in this case) that the arthropods and the vertebrates use to "build" their highly-differing body-segment-extensions upon still retain their ancestral similarities, even though the downstream limb-building cascades differ considerably in detail.

still retain their ancestral similarities

So there is no single gene for homosexuality, as the article declares!

That is not politically correct!

And only homophobes and the scientifically ignorant insist that a single gene for homosexuality is politically correct. The actual politically correct position is that homoexuality is genetically determined. (The nuance is that homosexuality is also partially determined by non-genetic factors)