Fig. 4.

Endodermal polyp tentacle marker in the developing bell endoderm of Clytia supports a homology of radial tubes and the bell endoderm. (a) Ch-six3/6 expression in the tentacle endoderm of polyps, and additional weak staining of the mouth tip ectoderm. (b–d) Ch-six3/6 expression in the radial tubes and in later medusa buds also in the ectoderm of the mouth anlage (arrowhead). In the young medusa, Ch-six3/6 is found in cell clusters on the tentacles and the bell rim. (e) Ch-tbx4/5 expression in polyp tentacle endoderm. (f–h) In the medusa bud, Ch-tbx4/5 expression is restricted to the basal ‘bell part’ of the radial tubes (dotted line). In young medusa Ch-tbx4/5 remains expressed in the endoderm of the future canals of the gastro-vascular system. (i) Ch-bmp5/8 expression at the base of the polyp tentacles. (k–m) During medusa formation, Ch-bmp5/8 is expressed in the radial tubes at early stages. in young ephyrae Ch-bmp5/8 is expressed in sensory structures at the bell rim. (n) In polyps, Ch-twist is expressed in the polyp tentacle endoderm, strongest at the base. (o–q) During medusa formation, the expression of Ch-twist is specifically expressed in the laterally outgrowing cells from each side of the bell part in the radial tubes, initially within the Ch-tbx4/5 domain (see f), subsequently in the whole umbrellar plate endoderm. In the medusa stage, Ch-twist is expressed at a low level in the plate endoderm but more strongly at the bell rim. r–u Schematic summary of the expression patterns. r Polyp. s–u Early, intermediate and late medusa bud. Only endodermal tissues shown for clarity