Posted by:
CKing
at Mon Nov 3 22:10:20 2003 [ Email Message ] [ Show All Posts by CKing ]
BGF wrote:
"The remarkable conservation of pax6 expression and pax6-regulatory circuit suggests that all bilaterian eyes share a common developmental genetic circuit and that this circuit was present in the bilaterian ancestor."
I was right. You are using dated information. Dr. Fernald has already told us that eyes evolved independently in different animal phyla and that the Pax-6 gene is neither eye specific nor is the development of eyes impossible without the Pax-6 gene. Like crystallin and opsin, the Pax-6 gene has apparently been co-opted into regulating the development of the eye independently in different animal groups. This is the case because different eyes are derived from different embryonic tissue in different animal groups and because eyes differ in anatomical detail. As Dr. Fernald points out, "the fish retina is inverted, meaning the light-sensing cells are at the very back of the eye (inverse) while those in squid are at the front of the retina (everse).
"Ah yes but we are not relying on the simple presense of a particular toxin type but rather examined through rigorous phylogenetic analyeses and the results robustly supported a single origin of venom."
A phylogenetic hypothesis is only as good as the characters that one uses. Chemical similarity, as scientists have learned from the study of eye evolution, is not a good character. It is subject to homoplasy. The fact that the lampropeltines and Old World Elaphe sensu lato are non-venomous poses a serious problem for the venom-evolved once hypothesis. The proponents of this hypothesis have so far been unable to explain why venom should be lost completely without a trace in the Old World ratsnakes, the New World ratsnakes, and the lampropeltine genera such as Pituophis, Arizona, and Lampropeltis, even though these genera could definitely benefit from the retention of venom since they are hunters of active prey.
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RE: The Pax-6 gene and the evolution of venom - CKing, Mon Nov 3 22:10:20 2003