ИСТИНА

Cnidaria are of crucial importance for our understanding of the evolution of animal embryogenesis. In cnidarians, development is highly diverse, and sometimes closely related species employ different modes of gastrulation (Stern 2004). It seems that some cnidarians are only capable of using a restricted set of morphogenetic movements. For an instance, hydrozoans never use invagination, which is very usual for other cnidarians. How could such a constraint arise in the evolution? We have clarified this question by studying the normal development of the White Sea hydrozoan Rathkea octopunctata. Gastrulation in Rathkea has been described as unipolar ingression of presumptive endoderm cells at the future posterior pole of a larva (Metschnikoff, 1886). We investigated early development of Rathkea using the modern research techniques: immunohistochemistry, confocal microscopy and electron microscopy. We characterized morphology and behaviour of embryonic cells at a set of developmental stages. We have found that unipolar ingression in Rathkea significantly differs from the same morphogenetic process described for other hydrozoans (Byrum, 2001). In Rathkea, presumptive endoderm cells acquire a bottle shape almost synchronously resembling the behavior of archenteron cells in cnidarians, which gastrulate by invagination. Morphologically, the mass ingression of bottle cells looks like invagination. However, Rathkea bottle cells do lose intercellular contacts that never occurs in the course of true invagination. We come to conclusion that hydrozoans have lost the ability to invaginate by losing the ability to synchronize the behavior of presumptive endoderm cells.