ИСТИНА

The class Staurozoa comprises enigmatic and understudied cnidarian species. While the anatomy of adult staurozoans has been recently characterized (Miranda et al., 2016), their embryonic and larval development has never been studied using the modern research techniques. It is known that staurozoan embryos and larvae are very small and have a very low number of cells (Wietrzykowski, 1912), that may indicate severe developmental constraints imposed on morphogenetic processes. Our model species is the White Sea staurozoan Lucernaria quadricornis (O. F. Müller, 1776). To characterize Lucernaria embryonic development, we used light microscopy, histology, immunocytochemistry, CLSM and TEM. We created a time-table of Lucernaria embryogenesis and described morphogenetic processes during gastrulation and elongation of the primary body axis. We have shown that different kinds of cell rearrangement rather than cell-shape-change driven epithelial sheet morphogenesis play major roles in the primary germ layers formation 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. Financial support: RFBR grant 15-04-08214.