Cell-cycle dynamics in cnidarian-dinoflagellate symbioses

The eukaryotic cell cycle is an evolutionarily conserved process that underpins cell division and growth in multicellular and unicellular organisms alike. The regulation of cell cycles of the partners in an endosymbiosis is complex, wherein coordinated growth is central to the maintenance of biomass homeostasis. Imbalanced growth can result in the starvation of one or both partners, the loss of immune tolerance, and the subsequent collapse of the symbiosis. The cnidarian-dinoflagellate symbiosis is an intracellular partnership where unicellular dinoflagellate symbionts reside within cnidarian host cells, provide their hosts with photosynthetically-derived carbon in return for inorganic carbon and nitrogen, and participate in a broader metabolic exchange. This nutritional symbiosis is the foundation of critically important coral reef ecosystems. Cell-cycle arrest is the primary mechanism of symbiont biomass regulation, yet despite this fundamental role in symbiosis homeostasis, many knowledge gaps remain. Here, we summarize our current understanding of host and symbiont cell-cycle regulation during the development and breakdown of cnidarian-dinoflagellate symbiosis.