Functional division of labor in motility, lignocellulose digestion, and nitrogen metabolism revealed for the Mixotricha paradoxa holobiont.

Mixotricha paradoxa is a large cellulolytic flagellate present in the hindgut of the termite Mastotermes darwiniensis. This parabasalid flagellate is unique in its reliance on ectosymbiotic spirochetes for motility. We analyzed the transcriptome of M. paradoxa and the genomes of the ectosymbiotic spirochete Propulsinema mixotrichae ("Treponematales"), the rod-shaped ectosymbiont Synergitannerella mixotrichae (Bacteroidales), and the endosymbiont Endomicrobiellum mixotrichae (Endomicrobiales), all of which are obligately associated with M. paradoxa and were taxonomically described in this study. Mixotricha paradoxa highly expressed genes for diverse glycoside hydrolases (GHs) and likely ferments sugars to H2, CO2, acetate, ethanol, and glycerol. Similar to the case for parasitic parabasalids such as Trichomonas vaginalis, transcripts for biosynthesis of nucleotides and many amino acids were not detected in our analyses of M. paradoxa. Propulsinema mixotrichae possesses genes encoding proteins for the assembly of flagella and for those in pathways associated with chemotaxis and dinitrogen fixation. Such genes are absent in S. mixotrichae, which instead possesses numerous genes encoding glycoside hydrolase enzymes, which are largely complementary to the glycoside hydrolase repertoire of M. paradoxa. Endomicrobiellum mixotrichae appears to provide nucleotides and nine amino acids to its host, which in turn likely supplies three amino acids, including tryptophan, to E. mixotrichae. Because bacterial cells, in addition to wood particles, were observed in food vacuoles of M. paradoxa, these ecto- and endosymbionts may be digested by the flagellate host. Overall, the distinct roles of each symbiont highlight the efficient functional division of labor that has evolved in this holobiont.