ИСТИНА

Strobilation is a process by which scyphozoan polyps produce ephyrae that develop into medusae. We aimed to study the ephyra formation in three cultures of Aurelia and three cultures of Cassiopea polyps with different geographic origin. To characterize gross-morphology, cell and tissue dynamics in the course of strobilation, we used methods of histology, SEM and CLSM. We found that our polyps demonstrate three forms of strobilation (SF). Two Aurelia cultures (North Sea; Pacific Ocean) possess very typical polydisk strobilation, by which up to 16 stare-shaped ephyrae with 8 long lappets are formed. Aurelia polyps from White Sea demonstrate oligodisk strobilation resulting in the formation of 1-5 bell-shaped ephyrae with 8 short lappets and significantly better developed mesoglea and gastric system. All cultures of Cassiopea polyps exhibit monodisk strobilation and form roundish ephyra with multiple small lappets, well-developed manubrium, mesoglea and gastric system. We subdivided the ephyra formation into the following developmental events: demarcation of the ephyra anlage; enlargement of the ephyra disk; differentiation of the disk margin, development of mesoglea, gastric system and manubrium; ephyra detachment. We created a time-table of ephyra formation for all polyp cultures. We found that the SFs differ in the particular pattern of ephyra formation. We revealed that the set of developmental events as well as their morphogenetic mechanisms are independent of SF. However, the relative timing of these events strongly depends on the SF. Thus, evolutionary transformations of SFs can occur on the basis of heterochronic changes. Financial support: RFBR grant 15-04-06264а.