RT Journal Article SR Electronic T1 The Trichoplax microbiome: the simplest animal lives in an intimate symbiosis with two intracellular bacteria JF bioRxiv FD Cold Spring Harbor Laboratory SP 568287 DO 10.1101/568287 A1 Harald R. Gruber-Vodicka A1 Nikolaus Leisch A1 Manuel Kleiner A1 Tjorven Hinzke A1 Manuel Liebeke A1 Margaret McFall-Ngai A1 Michael G. Hadfield A1 Nicole Dubilier YR 2019 UL http://biorxiv.org/content/early/2019/03/05/568287.abstract AB Placozoa is an enigmatic phylum of simple, microscopic, marine metazoans. Although intracellular bacteria have been found in all members of this phylum, almost nothing is known about their identity, location and interactions with their host. We used metagenomic and metatranscriptomic sequencing of single host individuals, plus metaproteomic and imaging analyses, to show that the placozoan Trichoplax H2 lives in symbiosis with two intracellular bacteria. One symbiont forms a new genus in the Midichloriaceae (Rickettsiales) and has a genomic repertoire similar to that of rickettsial parasites, but does not appear to express key genes for energy parasitism. Correlative microscopy and 3-D electron tomography revealed that this symbiont resides in an unusual location, the rough endoplasmic reticulum of its host’s internal fiber cells. The second symbiont belongs to the Margulisbacteria, a phylum without cultured representatives and not known to form intracellular associations. This symbiont lives in the ventral epithelial cells of Trichoplax, likely metabolizes algal lipids digested by its host, and has the capacity to supplement the placozoan’s nutrition. Our study shows that even the simplest animals known have evolved highly specific and intimate associations with symbiotic, intracellular bacteria, and highlights that symbioses with microorganisms are a basal trait of animal life.