ИСТИНА

Complex life cycles that present in a wide range of metazoans possess high level of evolutionary plasticity. There are multiple examples of evolutionary changes in the set of the phylum-specific life cycle stages, and the most impressive of them are known from the phylum Cnidaria. Among all cnidarians, the class Hydrozoa is the most diverse group, and some of the most interesting and enigmatic cnidarians belong to the hydrozoan subclass Trachylina. In the 'typical' hydrozoan life cycle, a juvenile stage (polyp) asexually produces an adult stage (medusa); the me¬dusa produces gametes, which fuse and the resulting embryo develops into a planula larva; the planula metamorphoses into a polyp (solitary or colonial). The main evolutionary trend of trachylinians is a reduction/loss of the polyp stage and acquiring the holopelagic life cycle. What do they 'pay' for such drastic evolutionary changes in their life cycles? Our data on the development of the trachymedusa Aglantha digitale, as well as analysis of old papers (e.g. Metschnikoff, 1886) demonstrate that trachy¬linians possess developmental traits, which are not only very unusual to hydrozoans, but also very peculiar to metazoans. We identified several traits of Trachylina development, which can be considered as side effects of life cycle evolution: the trachylinian larval development drastically dif¬fers from the 'typical' hydrozoan one, as their planula directly transforms into a medusa. Some features of the medusa stage appear very early during embryonic development; in several species, the embryo/larva has a very low number of cells resembling early embryos of ascidians and nematodes, which indicate very specific changes of developmental trajectories coupled with very severe developmental constraints. We consider the Trachylina as a crucial group for our ability to correctly interpret data regarding the early development and life cycle evolution. We are working on introduction of several Trachylina species as exciting novel models for comparative studies in evolutionary developmental biology.