REPORT OF MEETING 2nd general meeting and working group meetings of the COST Action 16203: STEM CELLS OF MARINE/AQUATIC INVERTEBRATES: FROM BASIC RESEARCH TO INNOVATIVE APPLICATIONS (MARISTEM), November 28-30, 2018, Marine Biology Station -Laboratoire Arago, Banyuls-sur-Mer, France Organizer: A-M Genevière

International audienceSponges (Porifera) are thought to be the sister group of all other animals and the earliest branching multicellular lineage of extant animals and as such a key group for understanding of the evolutionary history of animal stem cells and their regulation. Sponges are known to possess remarkable reconstitutive and regenerative abilities and high cell dynamic. There is a widespread opinion that all sponges cells are capable of transdifferentiation and under certain conditions exhibit properties of pluripotency. However, the experiments on the regeneration and reaggregation of dissociated cells, have shown that not all cells exhibit the properties of stem cells. Sponges do not have well-established stem cell lineages. Furthermore, presumable stem cells differ between four sponge classes. The most consistent model of the stem cell system is elaborated for fresh-water Demospongiae. According to this model demosponges have two stem cell lineages: archaeocytes and choanocytes. Both express the ortholog of the stem cell marker Piwi and show the proliferation activity in the intact sponges (Funayama, 2018). During regeneration in demosponges these cell types play an important role: they give rise to the new exopinacoderm and participate in the restoration of the choanosome structures. Additionally, the archaeocytes and choanocytes have ability for (trans)differentiation to various cell types during the restoration processes after sponge tissue dissociation (Borisenko et al. 2015; Lavrov, Kosevich, 2016). Despite the importance of archaeocytes as stem cells of demosponges, there is still no ultrastructural characterization of this type of cell; moreover, there are contradictory and unclear interpretations of the morphology of this cell type. However, both Calcarea and many Homoscleromorpha do not have mesohyl cells similar to demosponges archaeocytes. In these sponge clades choanocytes and pinacocytes exhibit properties of polypotentiality, as follow from gametogenesis, experiments on regeneration, and cell dissociation. These cells can directly transdifferentiate into other cell types without archaeocytes-like stage (Ereskovsky et al. 2015; Lavrov et al. 2018). Finally, it is necessary to emphasize the importance of models diversification: the comparison between different sponge taxa may help to shed light on the diversity of stem cells in Porifera and their properties. Financial support by the Russian Science Foundation n° 17-14-01089 is gratefully acknowledged