Polarised expression of FoxB and FoxQ2 genes during development of the hydrozoan Clytia hemisphaerica
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We have characterised the expression of four genes coding for Forkhead box-containing (‘Fox’) transcription factors identified from the hydrozoan (Leptomedusa) Clytia hemisphaerica. Phylogenetic analyses including all available non-bilaterian Fox sequences placed these genes in subfamilies B, Q2 (two genes) and O, and indicated that at least 17 Fox subfamilies were present in the common cnidarian/bilaterian ancestor, with multiple subsequent losses in cnidarian lineages. Chordate FoxB and FoxQ2A subfamily genes show polarised expression in early embryos. Correspondingly, Clytia CheFoxB expression was localised around the gastrulation site (future oral pole) at blastula and gastrula stages, with CheFoxQ2a expressed in a complementary aboral domain, maintained through larval development. Distinct later expression domains were observed for CheFoxB in the larval endoderm region, and in the statocyst, gonad and tentacle bulb of the medusa. A second Clytia FoxQ2 gene, CheFoxQ2b, not expressed in the embryo, larva or polyp, was detected uniquely in the gonads of the medusa. In contrast, CheFoxO, whose sequence indicates regulation by the PI3-Kinase/PKB signalling pathway consistent with known roles in bilaterian developmental regulation, was detected throughout the Clytia life cycle. CheFoxO expression was enhanced in regions associated with growth control including larval poles, gonad and the margin of the medusa bell. These results support the idea that an early embryonic patterning system involving FoxB and FoxQ2 family genes has been evolutionary conserved and indicate that Fox family genes have also acquired distinct roles during other phases of the hydrozoan life cycle.
KeywordsForkhead Cnidarian Embryonic polarity Statocyst Hydromedusa
We thank Dany and Claude Carré for encouraging us to develop Clytia as a laboratory model and for giving us advice on its culture. We also thank all the members of our two research groups, in particular, Tsuyoshi Momose, Michaël Manuel, Elsa Denker and Muriel Jager, for useful discussions and for participation in developing tools and techniques. We also thank Mark Martindale for providing a detailed Nematostella in situ hybridisation protocol, and one of the reviewers for alerting us to the Drosophila FoxQ2 expression data.
This research was funded by the CNRS, by joint contracts to our two groups from the GIS “Marine genomics”/ANR, and by ACI, ANR and ARC contracts to EH.
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