ИСТИНА

The basal metasoans Cnidaria have evolved a variety of morphologies, developmental trajectories and complex life cycles. The cnidarian class Staurozoa (stalked jellyfish) comprises enigmatic and understudied species: their embryonic and larval development has never been studied using the modern research techniques. Our model species is the White Sea staurozoan Lucernaria quadricornis. To characterize Lucernaria development, we used light microscopy, histology, immunocytochemistry, CLSM and TEM. Embryonic and larval development of Lucernaria is strikingly different from the development of other cnidarians. It seems that severe developmental constraints are imposed on all morphogenetic processes by a low number of cells in the embryo (< 100). We have shown that different kinds of cell rearrangement rather than cell-shape-change driven epithelial sheet morphogenesis play major roles in the gastrulation and in the shaping of a Lucernaria embryo. Comparative analysis based on our new data demonstrates that phylogenetically unrelated animals, namely nematodes, cnidarians and ascidians, characterized by a low number of cells developed convergently similar morphogenetic trajectories to overcome the developmental constraints.