genes G C A T T A C G G C A T Review Whole-Body Regeneration in Sponges: Diversity, Fine Mechanisms, and Future Prospects Alexander Ereskovsky 1,2,3, * , Ilya E. Borisenko 2 , Fyodor V. Bolshakov 4 and Andrey I. Lavrov 4   Citation: Ereskovsky, A.; Borisenko, I.E.; Bolshakov, F.V.; Lavrov, A.I. Whole-Body Regeneration in Sponges: Diversity, Fine Mechanisms, and Future Prospects. Genes 2021, 12, 506. https://doi.org/10.3390/genes12040506 Academic Editor: Joong-Ki Park Received: 8 March 2021 Accepted: 26 March 2021 Published: 29 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institut Méditerranéen de Biodiversité et d’Ecologie Marine et Continentale (IMBE), Aix Marseille University, CNRS, IRD, Station Marine d’Endoume, Rue de la Batterie des Lions, Avignon University, 13007 Marseille, France 2 Department of Embryology, Faculty of Biology, Saint-Petersburg State University, 199034 Saint-Petersburg, Russia; i.borisenko@spbu.ru 3 Evolution of Morphogenesis Laboratory, Koltzov Institute of Developmental Biology of Russian Academy of Sciences, 119334 Moscow, Russia 4 Pertsov White Sea Biological Station, Biological Faculty, Lomonosov Moscow State University, 119192 Moscow, Russia; fedbolsh@mail.ru (F.V.B.); lavrovai.bio@yandex.ru (A.I.L.) * Correspondence: alexander.ereskovsky@imbe.fr; Tel.: +33-662-107-366 Abstract: While virtually all animals show certain abilities for regeneration after an injury, these abilities vary greatly among metazoans. Porifera (Sponges) is basal metazoans characterized by a wide variety of different regenerative processes, including whole-body regeneration (WBR). Consid- ering phylogenetic position and unique body organization, sponges are highly promising models, as they can shed light on the origin and early evolution of regeneration in general and WBR in particu- lar. The present review summarizes available data on the morphogenetic and cellular mechanisms accompanying different types of WBR in sponges. Sponges show a high diversity of WBR, which principally could be divided into (1) WBR from a body fragment and (2) WBR by aggregation of dissociated cells. Sponges belonging to different phylogenetic clades and even to different species and/or differing in the anatomical structure undergo different morphogeneses after similar opera- tions. A common characteristic feature of WBR in sponges is the instability of the main body axis: a change of the organism polarity is described during all types of WBR. The cellular mechanisms of WBR are different across sponge classes, while cell dedifferentiations and transdifferentiations are involved in regeneration processes in all sponges. Data considering molecular regulation of WBR in sponges are extremely scarce. However, the possibility to achieve various types of WBR ensured by common morphogenetic and cellular basis in a single species makes sponges highly accessible for future comprehensive physiological, biochemical, and molecular studies of regeneration processes. Keywords: whole-body regeneration; Porifera; morphogenesis; transdifferentiation; differentiation; body polarity 1. Introduction All organisms tend to maintain their integrity and show abilities to restore after various damages. Such restoration processes occur at different biological levels through dramatically distinct mechanisms and can produce new structures of different similarity to the lost ones. This phenomenon unities as distinct biological processes as reparative regeneration, physiological regeneration, hypertrophy, etc. [1,2]. In this paper, we will consider only post-traumatic restoration processes in multi- cellular animals, i.e., reparative regeneration. While virtually all animals show certain abilities for regeneration after an injury, these abilities vary greatly among metazoans. Rep- resentatives of some phyla have rather limited regenerative abilities, showing imperfect restoration of lost parts even after minor traumas. In contrast, other animals can completely restore their bodies from small body fragments or even suspension of individual cells [3]. Genes 2021, 12, 506. https://doi.org/10.3390/genes12040506 https://www.mdpi.com/journal/genes