ИСТИНА

The organizer region of an amphibian embryo has been described by Mangold and Spemann in 1924. Since this date, the organizers are generally attributed to embryonic development. However, the organizer that keeps its active state in an adult animal has been described even earlier, in 1909. In a cnidarian Hydra, transplantation of a fragment of the hypostome into the body column of a host polyp provokes the formation of a secondary axis of polarity (Browne, 1909). In contrast to Hydra, almost all other hydrozoans have complex life cycle, the majority of them form colonies, and colonies of many species have very elaborated branching pattern and the growing tips functioning like the shoot apical meristem in plants. The obvious questions are: (i) whether the “adult organizer” is a characteristic feature of Hydra polyp? (ii) is it possible to find the organizer regions in colonial cnidarians?, (iii) whether they have the organizer at the larval and embryonic stages? (iv) is there continuity between the embryonic, larval and adult organizers? To answer these questions, we have performed a series of transplantation experiments with the marine colonial hydroids Dynamena pumila, Laomedea flexuosa, Gonothyraea loveni and Clava multicornis. (i) We grafted a fragment of the hypostome of a donor hydrant into the body of a host hydrant. (ii) We implanted a fragment of the colony growing tip into the aggregate of cells pressed out of the colony exoskeleton and into the cell mass obtained from the isolated degenerating hydrant. (iv) We grafted a fragment of the colony growing tip into the proximal zone of the colony branch (v) We transplanted a fragment of the posterior tip of a planula larva into the body of a host planula / embryo of the same or another species. We have found that organizers function throughout the larval development and adulthood of colonial hydroids. Indeed, not only the hypostome, but also the growing tip was able to act as an organizer in our experiments. The posterior tip of the planula preserves organizer capacity even in the species with highly diverged metamorphosis and colony structure. Taking into account the recently published data (Stumpf et al., 2010), this feature can be regarded as conserved among cnidarians. It is likely that organizers have been evolved in cnidarians as an adaptation of adult organisms to continuous growth and self-renewal and were transfered to the larval and embryonic stages later in evolution.