ИСТИНА

The majority of metazoans possess complex life cycles (CLCs). However, many fundamental questions concerning the evolution of CLC remain unanswered. First of all, if later developmental stages depend on earlier ones, how can an intermediate stage of CLC be lost without deleterious effects? To address this question, we established a model system, which include cnidarian species exhibiting similar evolutionary modifications of CLCs: Aglantha digitale, Solmundella bitentaculata, Liriope tetraphylla (Hydrozoa) and Pelagia noctiluca (Scyphozoa). Many cnidarians have CLC consisting of a juvenile stage (polyp), an adult stage (medusa) and a planula larva. CLC of our model species underwent evolutionary changes: they lost the polyp stage and acquired the holopelagic LC. Since medusa is normally produced from polyp tissues, the loss of the polyp imposes severe developmental constraints. Using histology, immunocytochemistry, CLSM, SEM and TEM we described normal development of our model species. We investigated very peculiar traits of their development, which can be considered as side effects of LC evolution. Comparative analysis of holopelagic medusae versus ‘typical’ ones reveals many dramatic shifts in the timing of the developmental events (heterochronic changes): some features of the medusa stage such as the bell or tentacles appear very early during embryonic/larval development. We characterized morphogenetic basis of these unusual developmental processes, which help holopelagic medusae to overcome the constraints imposed by the loss of the polyp. We consider holopelagic medusae as an exciting novel system, which will help us to clarify the general rules of CLC evolution. Financial support: RFBR-15-04-08214, EMBRC-France OOV-2016-658, AAP2017–952.