With the rapid increase of the quantity of molecular data, many animals joined the ranks of the so-called 'emerging models' of Evo-Devo. One of the necessary steps in converting an emerging model into an established one is gaining comprehensive knowledge of its normal embryonic development. The marine colonial hydrozoan Hydractinia echinata - an excellent model for research on stem cells, metamorphosis, and allorecognition - has been studied for decades. Yet knowledge of its embryonic development remains fragmentary and incomplete. Here we provide a detailed account of H. echinata embryonic development using in vivo observations, histology, immunohistochemistry, and electron microscopy. Furthermore, we propose a model describing the cellular basis of the morphogenetic movements occurring during development and also reveal a functional link between canonical Wnt signaling and regional differences in the morphology of the embryo. Hydractinia embryogenesis is an example of the diversity and plasticity of hydrozoan development where multiple routes lead to the same result - the formation of a normal planula larva.

We have studied the evolution of Wnt genes in cnidarians and the expression pattern of all Wnt ligands in the hydrozoan Hydractinia echinata. Current views favor a scenario in which 12 Wnt sub-families were jointly inherited by cnidarians and bilaterians from their last common ancestor. Our phylogenetic analyses clustered all medusozoan genes in distinct, well-supported clades, but many orthologous relationships between medusozoan Wnts and anthozoan and bilaterian Wnt genes were poorly supported. Only seven anthozoan genes, Wnt2, Wnt4, Wnt5, Wnt6, Wnt 10, Wnt11, and Wnt16 were recovered with strong support with bilaterian genes and of those, only the Wnt2, Wnt5, Wnt11, and Wnt16 clades also included medusozoan genes. Although medusozoan Wnt8 genes clustered with anthozoan and bilaterian genes, this was not well supported. In situ hybridization studies revealed poor conservation of expression patterns of putative Wnt orthologs within Cnidaria. In polyps, only Wnt1, Wnt3, and Wnt7 were expressed at the same position in the studied cnidarian models Hydra, Hydractinia, and Nematostella. Different expression patterns are consistent with divergent functions. Our data do not fully support previous assertions regarding Wnt gene homology, and suggest a more complex history of Wnt family genes than previously suggested. This includes high rates of sequence divergence and lineage-specific duplications of Wnt genes within medusozoans, followed by functional divergence over evolutionary time scales.

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