ИСТИНА

A complex life cycle with metamorphosis is widespread among metazoans. Comparing life cycles of evolutionarily distant organisms, we reveal the general patterns and rules of life cycle evolution. Cnidaria, as basal metazoans, are particularly valuable for such a comparison. Recently, the molecular machinery of the polyp-to-medusa transition termed strobilation has been unraveled for the scyphozoan cnidarian Aurelia (Fuchs et al.,2014). However, comparative analysis of metazoan life cycles requires complete set of data on the cnidarian's life-cycle transitions obtained for various representatives of this phylum. Unfortunately, there is an obvious lack of knowledge on the morphogenetic basis of medusa formation. To elucidate the developmental patterns of the polyp-to-medusa transition in scyphozoans, we studied and compared the strobilation of Aurelia(Semaeostomeae) and Cassiopea(Rhizostomeae). We characterized stages and morphogenetic mechanisms of the ephyra(medusa-larva) formation during strobilation at the levels of gross-morphology and histology. We created timetables of the ephyra formation for both species. The polydisc strobilation, typical for Aurelia,leads to formation of multiple star-shaped ephyrae, while the monodisc strobilation, typical for Cassiopea,leads to formation of one ephyra with the circular bell. The main stages and the key morphogenetic processes of the ephyra formation are similar in both species. However, we detected some heterochronic changes in the development of Cassiopea. The newly detached Cassiopea ephyra seems to be more developed than the Aurelia one: it possesses manubrium and gastric system with the complex type of branching. The shape, structure and behavior of the Cassiopea ephyra make it more alike a juvenile jellyfish than a typical ephyra. We suppose that Cassiopea actually skips the stage of ephyra larva: the juvenile jellyfish is formed during strobilation. To summarize, our results demonstrate that strobilation is a promising model system for studying the conserved and evolvable patterns of the life cycle transitions.