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

9509088只麦基特里克鸟的后肢肌肉比它们现存的近亲鳄鱼和短吻鳄还多。此外，一些原始存在于鸟类中的肌肉在历史进程中已经在鸟类谱系中丢失了无数次。如果要了解鳄鱼和鸟类(始祖龙)之间的历史关系，就必须建立关于特定肌肉或肌肉部分出现和丧失的顺序的假说。然而，在这样的分析之前，关键是要提出假设，即鸟类和鳄鱼的哪些肌肉在历史意义上是同源的，即这两个群体中的哪些肌肉是对应的。用传统的技术，如肌肉形态的大体解剖学研究，不可能识别这些同源性。在这项建议中，将采用一种新的方法来分析始祖龙后肢肌肉的同源性，方法是使用鸟类和鳄鱼胚胎对肌肉蛋白质进行发育研究。肌肉蛋白在生物发育的不同阶段有不同的类型，称为异构体，并且在物种之间高度保守。肌动蛋白、肌球蛋白、原肌球蛋白和肌钙蛋白等收缩蛋白亚型以及其他相关蛋白的发育变化遵循特定的模式。这些蛋白质的最早形式被胚胎形式取代，后来被新生儿和成年形式取代。由于个别肌肉群具有“签名”序列模式，PI将检查鸟类和鳄鱼的蛋白质异构体及其信息，以查看两组中已知同源的肌肉是否保留了祖先模式。如果是这样的话，这样的模式可以用来假设在此之前同源性有问题的肌肉之间的历史关系。这种方法有可能解决比较生物学中有关相关生物性状之间关系的重要问题，而历史变化实际上已经抹去了这一记录。***

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. ***