Developmental Evolution of Avian Digit Homology

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Project SummaryEver since the discovery of the dinosaur affinities of birds, a discrepancy has persisted between the developmental and the phylogenetic evidence regarding the identity of digits in the bird wing. The preponderance of phylogenetic evidence demonstrates that the three permanent fingers of birds are digits one, two and three, while the embryological evidence demonstrates that these fingers develop from the anlagen that, in other animals, develop into digits two, three and four. Recent results from the laboratory of Dr Fallon, University of Wisconsin, shows that there is a gene expression signature which can distinguish between digit one and digit two, and which supports the hypothesis that the fingers of birds are in fact digits one, two and (by inference) three. Specifically the expression of Hoxa-13, Hoxd-13, and Hoxd-12 is seen in all digits posterior of digit one, while digit one does not express Hoxd-12. This grant funds a collaborative effort between a developmental biology lab (Dr Fallon, University of Wisconsin) and an evolutionary biology lab (Dr Wagner, Yale University) to follow up on this result. The goal of this project is to validate the inference from gene expression pattern to digit identity by using both experimental as well as comparative data. Objectives: 1) test whether the expression patterns of Hoxa-13, Hoxd-13, and Hoxd-12 are causally relevant for the development of digit morphology. 2) Determine whether the association between Hox gene expression patterns and digit identity, as described by Dr's Vargas and Fallon, is conserved among major groups of tetrapods and in the face of natural variation in digit number. To meet objective 1 Dr Fallon will study the effect of Hoxd-12 misexpression in the digit one anlage as well as knockdown of Hoxa-13 and Hoxd-13. To meet objective 2, Dr Wagner will study the expression of Hoxa-13, Hoxd-13, and Hoxd-12 in alligator, frog and lizard embryos including two species which have lost digit one during evolution, Hemiergis peronii and Chalcides chaldices (skinks). Intellectual merit: while the identity of bird digits can be seen as a subject of limited importance, the persisting disagreement among major scientific disciplines about a matter of scientific fact raises concern. Unresolved high profile controversies like this can have a corrosive effect on the disciplinary integration of evolutionary and developmental biology. For that reason the proposed project seeks to resolve this issue with a combined developmental and evolutionary approach seeking a convergence of evidence between them. Broader impact: The broad impact of this proposal is measured in two ways: (1) by employing a highly interdisciplinary approach for evaluating a complex problem in evolutionary developmental biology; and (2) by providing a wealth of research and educational resources. These training opportunities include postdoctoral training and new summer traineeships for teachers.