The embryonic development of the cnidarianHydractinia echinata

Abstract

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.

Embryonic development of thecate hydrozoan Gonothyraea loveni (Allman, 1859)

“…The mode of cleavage observed in our essay corresponds in general to that described in details by Krauss and Rodimov (1999) and that can be considered as "typical" for hydrozoans. As it was described previously for certain hydrozoan and antozoan speices (Fritzenwanker, Genikhovich, Kraus, & Technau, 2007;Kraus et al, 2014;Wulfert, 1902), the cytokinesis in G. loveni embryo often delays during the first two to three rounds of cleavage leading to incomplete separation of blastomeres. Perhaps variability of cleavage pattern, blastomeres active movements and incomplete cytokinesis during early cleavage stages can be a characteristic feature for the cnidarians species brooding embryos.…”

“…In present essay, the cleavage in G. loveni starts about 3 hpf that is later than in most other known hydrozoans species (Kraus et al., ; Martin & Thomas, ). Even for the G. loveni from the Black Sea the cleavage stats in 0.5–1.5 hpf (Dementjev & Minichev, ).…”

“…Different environmental conditions (e.g., temperature, salinity) can affect certain parameters of cleavage but cannot change the general sequence of early development passing through morula, parenchymula and planula stages. So the example of G. loveni supports the idea that during embryonic development multiple developmental trajectories pass through nodal stages (Dorfman & Cherdantsev, ) leading finally to the same final stage (Kraus et al., ).…”

“…In studied athecate species, the planula is more differentiated than in thecate species: besides ectoderm, the endoderm is epithelized to surrounding inner (gastric) cavity (Kraus et al., ; Pennati et al., ; Piraino et al., ; Weis & Buss, ). Supportive cells constituting the prevailing volume of both epithelia do not need replacement (substitution); only the ectodermal supportive cells lose their flagella during metamorphosis.…”

“…One of the intriguing points is the development of the pseudostratified columnar epithelium of parenchymula (Polteva & Aizenstadt, 1980). With the onset of gastrulation, the nuclei of the fu- In the middle and oral parts of preplanula, the endodermal cells with centrally located nuclei form a dense mass that gradu- (Dorfman & Cherdantsev, 1977) leading finally to the same final stage (Kraus et al, 2014).…”