Adult Stem Cells Host Intracellular Symbionts: The Poriferan Archetype

Adult stem-cells (ASC) in many aquatic invertebrates not only are very common, but are also morphologically highly diverse, and exhibit a wide range of differentiation states as well as somatic and germline characteristics. One unexpected phenomenon is the presence of intracellular symbionts in the ASCs of some invertebrates. We review the literature on ASCs intracellular symbionts in sponges (phylum Porifera) and others invertebrates. We showed that the intracellular symbionts (either prokaryotic or eukaryotic) of sponges are found only in representative species of only one sponge class Demospongiae. Eukaryotic symbionts are exclusively unicellular photosynthetic algae in sponges, and are found only in pluripotent stem cells, the archaeocytes; they are documented in five orders of Demospongiae. Bacteriocyte-like cells have been reported in four other phyla, indicative of their independent evolutionary origins. Our results would add considerable insight into the establishment and maintenance of intracellular symbioses in ASCs, and provide new insight into the diversity of symbiotic associations seen across the Tree of Life.