Molecular Mechanisms of Pharyngeal Endoderm Evolution in Vertebrates

University Of Georgia Research Foundation Inc, Athens GA

Investigators

Abstract

0131314 Manley During embryonic development the pharyngeal endoderm forms a series of pharyngeal pouches in many, if not all chordates. The pharyngeal pouches develop into a diverse array of specialized structures in different organisms. For example, the pharyngeal pouches form gills in many invertebrate and vertebrate species, while in terrestrial animals, the pouches form pharyngeal organs, such as the thymus, but do not form gills. Recent work from the P.I.'s laboratory suggests that in mice, Hoxa3 expression in the 3rd pharyngeal pouch endoderm is required for both initiation of thymus organogenesis and suppression of gill formation. Hoxa3 is also required to up regulate the Pax1/9 genes in the 3rd pouch endoderm. The proposed experiments will test the hypothesis that the expression of Hoxa3 in the pharyngeal pouch endoderm in higher vertebrates modifies Pax1/9 gene expression, resulting in the development of different pharyngeal pouch-derived structures in different vertebrate species. The proposed studies will use 3 different vertebrate species, the lamprey, zebrafish, and mouse, representing a range of vertebrate body plans. Specific aim 1 will determine whether Hox3 and Pax1/9 gene expression patterns in the pharyngeal region in these three species can be correlated with differences in pharyngeal morphogenesis and organogenesis. The second aim will test whether zfHoxa3a is required for thymus formation in the zebrafish, as it is in mouse. In aim 3, misexpression of the lamprey, zebrafish, and mouse Hox3 genes will test whether Hox3 proteins from all of these species can induce an ectopic thymus and/or block gill formation. The fourth aim will use gene targeting to "knock-in" the lamprey Hox3 and zebrafish zfHoxa3a gene sequences to the mouse Hoxa3 locus, thereby determining to what extent these genes can substitute functionally for mouse Hoxa3. The experiments in Aims 3 and 4 will test whether there has been an acquisition of new functions by the Hoxa3 protein during vertebrate evolution. The results from the proposed studies will provide evidence for possible mechanisms by which vertebrates have evolved "newer" functions, such as adaptive immunity, from evolutionarily ancient embryonic structures. In addition, these studies may provide molecular insights into the loss of gills in air-breathing vertebrates.