Developmental expression of homeobox genes in the ctenophore Mnemiopsis leidyi
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Homeobox genes are a large family of genes that encode helix–turn–helix transcription factors that play fundamental roles in such developmental processes including body axis formation and cell specification. They have been found in a wide variety of organisms, from fungi to plants and animals, with some classes being specific to the Metazoa. While it was once thought that organismal complexity was tied to gene complexity, sequencing of genomes from a cnidarian, poriferan, and placozoan have shown no clear correlation. However, little attention has been paid to ctenophores, another early branching taxon. Ctenophores are mostly pelagic marine animals, with complex morphological features, so understanding the gene content and expression of this nonbilaterian phylum is of key interest to evolutionary biology. Expression information from developmental genes in ctenophores is sparse. In this study, we isolated seven homeobox genes from the ctenophore Mnemiopsis leidyi and examined their expression through development. Phylogenetic analyses of these genes placed four in the ANTP class and three in the PRD class. These are the first reported full-length PRD class genes, although our analyses could not place them into specific families. We have found that most of these homeobox genes begin expression at gastrulation, and their expression patterns suggest a possible role in patterning of the tentacle apparati and pharynx.
KeywordsHomeobox Ctenophore Metazoan evolution
The authors would like to thank Andreas Hejnol, Eric Röttinger, and two anonymous reviewers for comments that greatly improved this manuscript. We also thank David Q. Matus for assistance in phylogenetic analyses and Atsuko Yamada and students from the Embryology course for helping to collect animals. Without the support of the Marine Biological Laboratory (Woods Hole, MA, USA), this work could not have been done. This work was supported by the NSF Graduate Research Fellowship Program to K.P. and grants from NASA and NSF to M.Q.M.
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