Alligator Genetics and the Development of Bird "Thumbs"

Here is a new article from open-access journal PLoS One: "The Evolution of HoxD-11 Expression in the Bird Wing: Insights from Alligator mississippiensis," (link) which uses color illustrations to explain the genetics and development of the finger digits in birds and alligators. Read the summary on Palaeoblog for a good explanation on the study. Basically, it's a study on Hox genes and how although developmentally birds seem to have digits 2, 3, and 4 from the ancestral 5, this is a result of a "hometic frameshift," causing HoxD-11 expression to move, so ancestrally, their digits are 1, 2, and 3, which is consistent with the fossil evidence such as Archaeopteryx as well as genetic evidence seen in their closest living relatives, the crocodilians.

It's satisfying when evidence adds up in such a neat way.

Anyway, on to the drawings!

Figure 1: Three levels to the avian digit homology problem: embryology, gene expression, and morphology
First of all, the illustrations in this article are just fantastic. Full of information, but designed in a clear way that is in no way overwhelming. There is a didactic use of color, which allows the viewer to follow each of the 5 digits across both development and evolution. The next example includes a more complex clade as well as specific fossil and extant examples.

Figure 2: The evolution of digit morphology
This drawing makes it so easy to compare across the cladogram. You can watch as the digits disappear one by one. I have seen similar illustrations used to show the gradual loss of toes in the evolution of the horse. Janel? You out there? This illustration on Wikipedia is awful! I know there are better ones out there.

And here are the more technical illustrations...

Figure 3: Identification of the alligator HoxD-11 exon 1 sequence.
Figure 4:The expression of HoxD-11 in alligator and chicken limbs.
I have to admit I often I roll my eyes at the use of photos in lieu of illustrations, but in this case they work amazingly well in tandem. Development is shown in similar stages in alligators and birds, both limbs, and you can trace the development to see where they diverge, and where and which fingers are lost in the bird.