ABSTRACT
Known since the 1970s, intracellular bacterial symbionts of placozoans have recently been genetically classified and their presence confirmed inside the fiber cells of several haplotypes of Trichoplax adhaerens, as well as in Hoilungia hongkongensis. Two symbionts were identified in T. adhaerens haplotype H2, cand.: Grellia incantans (alphaproteobacterial order Rickettsiales), found in the fiber cells, and Ruthmannia eludens (Margulisbacteria), discovered inside ventral epithelial cells. Bacteria have not been observed in these ventral cells in the other haplotypes or species. G. incantans is found in the cytoplasm of the fiber cells, often enclosed in rough endoplasmic reticulum and studded with probable outer membrane vesicles (OMVs), indicative of an intimate interaction. Transcriptomes reveal that G. incantans provides necessary vitamin B2 for the host, and the host provides substrate metabolites for the bacterium's TCA cycle. Although signal molecules have not been identified from either bacterium, a mutualistic symbiosis of G. incantans with its host is thus indicated. In placozoans reproducing by simple fission, each division product can be invested with bacterial symbionts. The situation is less clear for the putative sexually produced embryos seen developing inside the animals. Many questions remain to be answered regarding the relationships between placozoans and their bacterial symbionts.
