ИСТИНА

Many fundamental questions concerning the general patterns and rules of ontogeny evolution remain unanswered. It is unclear to what extent different stages are able to evolve independently of one another. In particular, how can an intermediate stage of development be lost without deleterious effects for further development, which successfully proceeds to an adult stage. We also do not know to what extent the trajectory of development can be affected by the evolutionary changes of reproductive strategy. To address these questions, we use several phylogenetically distant cnidarian species exhibiting very peculiar life history traits and very wired development. Our model species Aglantha digitale, Solmundella bitentaculata, Liriope tetraphylla (Hydrozoa) and Pelagia noctiluca (Scyphozoa) lost the polyp stage and acquired the holopelagic life cycle. Since medusa is normally produced from the polyp tissues, the loss of the polyp stage imposes severe developmental constraints. Comparative analysis of holopelagic medusae versus ‘typical’ ones reveals many dramatic shifts in the timing of the developmental events: some features of the medusa stage such as the bell or tentacles appear very early during embryonic/larval development. These unusual developmental processes help holopelagic medusae to overcome the constraints imposed by the evolution of their life cycles. The staurozoan Lucernaria quadricornis spawns several thousands of tiny eggs (25-30m) into the water column. The most peculiar feature of Lucernaria embryos/larvae is an extremely low number of cells, that may indicate severe developmental constraints imposed on morphogenetic processes. 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 the Lucernaria embryo. We suppose that this peculiar morphogenetic toolkit can be considered as a side effect of the reproductive strategy evolution.