Yale researchers solve mystery of disappearing bird digit

Yale scientists now have a good handle on how these developmental changes are orchestrated in the embryo, but there is still one outstanding debate on birds: Which digits are they: a thumb with index and middle fingers, or the index, middle and ring fingers?

In five-digit vertebrates, the thumb comes from the precursor stem cells labeled pa. While birds have a digit that looks like a thumb, pa precursor cells die off during development and never produce a digit in adults. As a result, scientists have wondered whether precursor cells in pb can make a thumb.

Yale scientists have completed a genomic analysis of birds that reveals the answer (shown in orange on illustration). It is a hands down “yes” — even though the first bird digit develops where the index finger on a five-finger vertebrae should be.

The results are published online Sept. 4 in the journal Nature. Authors are Zhe Wang, Rebecca L. Young, Huiling Xue, and Gunter P. Wagner from the Department of Ecology and Evolutionary Biology.