So, a while ago, I went on at length about the confused identities of those stunted little bones in a bird’s wing that were once fingers. (Diagram of wing skeleton by L. Shyamal, Wikimedia Commons.)
For a brief recap: bird wings have (the remnants of) three digits. Based on palaeontological evidence (read: dinosaur hands), these would be digits I, II and III. But in a modern bird embryo, they develop in the places of digits II, III and IV. The II-III-IV hypothesis also seemed more consistent with the way other tetrapods lose digits: if five is too many, DI is typically the first to go.
To solve the apparent conundrum, Wagner and Gauthier (1999) proposed the developmental frame shift hypothesis, meaning that the developing fingers switch identities as they form. The paper I tried to discuss (and probably ended up garbling :-P) in my previous post concluded that the frame shift (if it can be called that) happens very early, before there is anything resembling actual digits in the wing bud.
Well, as always, there is a new twist on the story. (Chicken wings seem to be all the rage this year!) Wang et al. (2011) took a brute force approach towards pinning down those elusive digit identities. They compared all the genes that are active in each developing digit in both the wing and the foot of chicken embryos at two different time points. They used next-generation sequencing to catch mRNAs, the RNA copies of genes that cells make for use as templates for protein synthesis. This means thousands of genes identified from each sample, plus information on how much of each mRNA there is, plenty of material for comparison.
After a considerable amount of statisticking on that giant pile of data, it turns out that avian digit identities are… confused. When they grouped the different digits by how similar their gene expression patterns were, first digits proved starkly and uncontroversially well-behaved. DI from the foot (whose identity no one seriously disputes) and the first digit of the wing obviously belong together. Every other digit, though…
For example, the second finger is most like the third toe, but also kind of like the second toe. The third finger goes from being closest to the second toe to being most similar to the fourth toe as development progresses. And it’s apparently chock full of mRNA belonging to a gene that had never been reported to participate in limb development (never mind determining any digit identity) before.
With all that said, I wouldn’t be too quick to declare that wings are just weird. Well, they certainly are, but these data don’t necessarily tell us that they are weird in this particular respect. I’m sorely missing a comparison to an ordinary, five-fingered forelimb, for example. What if the weirdness is not an issue with birds, but with forelimbs?
I’d say it’s a fair bet that chicken wings will keep evo-devo nerds entertained for years to come…
(I think I’ll be discussing limb development and genetic evidence in evo-devo again in the near future. Well, for certain values of “near”. I’ve had that post in the pipeline for weeks, but the damned thing just doesn’t want to get finished.)
Wagner GP and Gauthier JA (1999) 1,2,3 = 2,3,4: A solution to the problem of the homology of the digits in the avian hand. PNAS 96:5111-5116
Wang Z et al. (2011) Transcriptomic analysis of avian digits reveals conserved and derived digit identities in birds. Nature 477:583-586