CAA: Evolution of Archosaur Hindlimb Muscles: Use of Muscle Protein Isoforms as Molecular Markers for Homology

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9509088 McKitrick Birds possess more hindlimb muscles than their closest living relatives, the crocodiles and alligators. In addition, some muscles primitively present in birds have been lost numerous times within the avian lineage during the course of history. If historical relationships among crocodilians and birds (archosaurs) are to be understood, hypotheses must be constructed about the sequence of appearance and loss of specific muscles or muscle parts. Prior to such analysis, however, it is crucial to develop hypotheses about which muscles in birds and crocodilians are homologous in the historical sense, i.e., which muscles correspond in the two groups. Identifying these homologies has not been possible with traditional techniques such as gross anatomical study of muscle form. In this proposal, a new approach will be taken to analyze homologies in archosaur hindlimb muscles by the use of developmental studies of muscle proteins using avian and crocodilian embryos. Muscle proteins have different types, termed isoforms, at different stages of organismal development,and these are highly conserved among species. The developmental shifts in isoforms of contractile proteins such as actins, myosins, tropomyosins and troponins, as well as other related proteins follow specific patterns. The earliest forms of these proteins are replaced by embryonic versions and later by neonatal and adult forms. Since individual muscle groups bear "signature" sequential patterns, the PI will examine the protein isoforms and their messages in birds and crocodilians to see whether muscles of known homology in the two groups have retained the ancestral patterns. If so, such patterns can be used to hypothesize historical relationships between muscles of heretofore questionable homology. %%% This approach has the potential to solve important problems in comparative biology about the relationship among traits in related organisms where historical change has in effect obliterated the record. ***