Summary: | 碩士 === 國立臺灣大學 === 漁業科學研究所 === 89 === Homeobox genes are important for establishing body plan and determining cell-type or cell-lineage throughout the metazoans. The homeobox genes of the Xlox family are essential for the early morphogenesis of the midgut region and cellular differentiation during embryonic development. Since homeobox sequences and expression patterns of Xlox family homeobox genes are highly conserved in organisms as diverse as leeches to vertebrates, these genes provide an excellent tool to study the evolution of endoderm-derived gut structure among animal phyla. In this study, I identified the homeobox sequences of Xlox family genes from two species of polychaete, Neanthes glandicincta and Hydroides elegans, using the GenomeWalker technique. Phylogenetic analyses of the Xlox homeodomain amino acid sequence show that there are 3 major clades: vertebrates, leeches, and a mixture of marine invertebrates, including amphioxus, starfish, and polychaetes. These results suggest that the Xlox homeodomain sequences of amphioxus, starfish, and polychaetes are more ancestral than those of vertebrates and leeches. Moreover, the Xlox homeodomain sequences of leeches evolved faster than that of polychaetes, implying that the complexity of the leech’s gut structure may be related to the changes of the Xlox genes of the Annelida.
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