1, 2, 3: Counting the fingers on a chicken wing

The forelimbs and hindlimbs of vertebrates are serially homologous structures. Bird limbs differ from many other vertebrates in that they have only three digits in the forelimb (wing), but four digits in the hindlimb (leg). This pattern has emerged through a process of digit loss during evolution. While the morphological digit identities of the hindlimb are widely accepted as digits I to IV, corresponding to digits I to IV of the ancestral pentadactyl (five digit) limb (Figure 1), such definitive identities have yet to be designated to the digits of the wing. The precise identities of these digits have been the focus of a long-standing and ardent debate amongst paleontologists and embryologists, since they underpin the mechanism by which the bird wing has evolved from the theropod dinosaur lineage; however, a recent article by Wagner et al. [1] has shed light on this conundrum.Paleontological evidence demonstrates the progressive loss of the two most posterior digits, V and IV, in theropods, implying digits I to III from the ancestral hand remain in the avian wing. However, embryological evidence identifies the digits as II to IV, since the most posterior digit of the wing is the first to condense (the first visible digit) and forms in alignment with the primary axis of cartilage condensation [2]; this digit is consistent with digit IV in the ancestral hand, which is a pattern retained in the mouse.In order to settle this debate, Wagner et al. [1] utilized contemporary RNA-seq techniques to uncover a gene expression 'signature' of digit identity taking the rationale that morphological digit identity is ultimately determined by gene expression profiles, which are unique to each digit.Specifically, the group sequenced the transcriptomes of the digit primordia of the forming wing and leg dissected from chicks at two different stages (HH28/29 and HH31). To establish a neutral digit annotation, the anterior-most digit was designated digit A and, in sequence, the more pos