Setting up embryonic polarity in the hydrozoan Clytia hemisphaerica

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• Evelyn Houliston, Developmental Biology Unit, CNRS/Université Pierre et Marie Curie, Villefranche-sur-mer, France
• Wednesday 09 November 2011, 13:00-14:00
• Part II Lecture Theatre, Department of Zoology.

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Setting up embryonic polarity in the hydrozoan Clytia hemisphaerica.

Evelyn Houliston. Developmental Biology Unit, CNRS /Université Pierre et Marie Curie, Villefranche-sur-mer, France.

The characteristic oral-aboral polarity of cnidarian larvae was long thought to emerge progressively during embryonic development, rather than to be directed by localised maternal determinants as is commonly the case for axis specification in bilaterian animals. In recent years, the results of experimental manipulations in variety of cnidarian species have changed this perception, indicating that cnidarian axis specification, as well as germ layer formation though gastrulation, is directed by maternal localised determinants. Furthermore, accumulating molecular evidence has shown that Wnt signalling plays a key role in regulating axial properties and gastrulation in cnidarians, as it commonly does in bilaterians. The common eumetazoan ancestor may thus have exhibited embryonic patterning based on maternal mRNA localisation acting upstream of regionlised Wnt signalling. Using an emerging experimental model from the Hydrozoa, Clytia hemisphaerica1, we have identified embryonic axis determinants at the molecular level as localised maternal mRNAs, coding for a Wnt ligand and two Frizzled-family receptors with antagonistic activities2,3. These mRNAs become translated following fertilisation, initiating a sequence of interactions that restricts Wnt-beta catenin signalling to the future oral territory of the blastula. Wnt-beta catenin activity is required for transcription of many target genes in the oral territory, including two Brachyury orthologues necessary for gastrulation. In parallel, the same maternally provided Wnt ligand is required to coordinate Strabismus–mediated Planar Cell Polarity along the oral-aboral axis. I will describe experiments that have led us to this current picture, and some ongoing efforts to identify novel players in defining oral identity downstream of Wnt signalling.

1) Houliston, E., Momose, T. & Manuel, M. (2010) “Clytia hemisphaerica: A jellyfish cousin joins the laboratory. Trends in Genetics  26, 159-167

2 ) Momose, T. & Houliston, E. (2007) Two Oppositely Localized Frizzled RNAs as Axis Determinants in a Cnidarian embryo PLoS Biology 5 (4) E70

3) Momose, T., Derelle, R. & Houliston, E. (2008) A maternally localised Wnt ligand required for axial patterning in the cnidarian Clytia hemisphærica. Development 135, 2105-2113

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