Homeotic transformation and digit evolution in Birds

By | May 7, 2021
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Digit evolution in Birds is a classical problem in the field of evolution owing to the contradictory results observed in morphology and development.Bird wings only have three fingers, which evolved from ancestors (theropod dinosaurs) that, like humans and most reptiles, had five fingers.The digits of Birds wing develop from embryological position that become 2 ,3 and 4 ,meaning they lose digit 1 and 5. But when one look at early birds like Archaeopteryx ,the fossil record clearly suggest that digits four and five were lost and reduced in the dinosaur ancestors of birds.Here lies the classical problem:
How come the two groups with such a significant anatomical difference can be closely related?

Anatomists initially assigned digit numbers I, II, and III to bird limbs on the basis of their morphology, but later had to change that to II, III, and IV on the basis of embryology.

Posterior Hox genes (Hox D complex) are well known for their role in digit formation.In Mice the expression of Hox-D 11 gene is absent only in the region ,which goes on to become digit 1 (thumb) and same is true for the anterior most digit of birds (suggesting it is infact the thumb of birds and develop from position 2 in embryo) .The popular hypothesis to explain this mystery is that ” a homeotic frame shift of digit identity has occurred in evolution of Birds and because of that digits 1,2 and 3 develop from position 2,3 and 4 in the embryo.

If this hypothesis is true one should see the similar expression pattern of Hox-D11 in alligators, closest relatives of birds.On the other hand if there is no frame shift in digit identity had occurred and digits develop directly into digits 2,3 and 4 ,expression of Hox-D11 in alligators will be absent in digit 2.

o investigate this Vargas and colleagues, working at the lab of Gunter Wagner at Yale studied the expression pattern of Hox-D 11 in crocodilian Alligator mississippiensis. The result was ,as in the case of mouse ,in alligator Hox-D11 mRNA was absent only in digit 1 ,which further consolidates the hypothesis of homeotic transformation in digit identity during evolution of Birds,such that fingers one, two and three began to develop from the embryological positions of fingers two, three and four.

Its a remarkable work by Gunter Wagner’s group in order to solve the 120 year old puzzle in evolutionary biology. The results were published in open access journal PLOS ONE.

Reference :
Vargas AO, Kohlsdorf T, Fallon JF, VandenBrooks J, Wagner GP (2008) The Evolution of HoxD-11 Expression in the Bird Wing: Insights from Alligator mississippiensis.
PLoS ONE 3(10): e3325. doi:10.1371/journal.pone.0003325