Our work provides unprecedented insight into the architecture of the centriole proximal region, which is key for a thorough understanding of the mechanisms governing centriole assembly.

Background: Centrioles are cylindrical microtubule-based structures whose assembly is critical for the formation of cilia, flagella, and centrosomes. The centriole proximal region harbors a cartwheel that dictates the 9-fold symmetry of centrioles. Although the cartwheel architecture has been recently analyzed, how it connects to the peripheral microtubules is not understood. More generally, a high-resolution view of the proximal region of the centriole is lacking, thus limiting understanding of the underlying assembly mechanisms. Results: We report the complete architecture of the Trichonympha centriole proximal region using cryotomography. The resulting 3D map reveals several features, including additional densities in the cartwheel that exhibit a 9-fold symmetrical arrangement, as well as the structure of the Pinhead and the A-C linker that connect to microtubules. Moreover, we uncover striking chiral features that might impart directionality to the entire centriole. Furthermore, we identify Trichonympha SAS-6 and demonstrate that it localizes to the cartwheel in vivo. Conclusions: Our work provides unprecedented insight into the architecture of the centriole proximal region, which is key for a thorough understanding of the mechanisms governing centriole assembly.

Termites consume an estimated 3–7 billion tonnes of lignocellulose annually, a role in nature which is unique for a single order of invertebrates. Their food is digested with the help of microbial symbionts, a relationship that has been recognized for 200 years and actively researched for at least a century. Although DNA- and RNA-based approaches have greatly refined the details of the process and the identities of the participants, the allocation of roles in space and time remains unclear. To resolve this issue, a pioneer study is reported using metabolomics to chart the in situ catabolism of 13C-cellulose fed to the dampwood species Hodotermopsis sjostedti. The results confirm that the secretion of endogenous cellulases by the host may be significant to the digestive process and indicate that a major contribution by hindgut bacteria is phosphorolysis of cellodextrins or cellobiose. This study provides evidence that essential amino acid acquisition by termites occurs following the lysis of microbial tissue obtained via proctodaeal trophallaxis.

In 1944, Harold Kirby described microorganisms living within nuclei of the protists Trichonympha in guts of termites; however, their taxonomic assignment remains to be accomplished. Here, we identified intranuclear symbionts of Trichonympha agilis in the gut of the termite Reticulitermes speratus. We isolated single nuclei of T. agilis, performed whole-genome amplification, and obtained bacterial 16S rRNA genes by PCR. Unexpectedly, however, all of the analyzed clones were from pseudogenes of 16S rRNA with large deletions and numerous sequence variations even within a single-nucleus sample. Authentic 16S rRNA gene sequences were finally recovered by digesting the nuclear DNA; these pseudogenes were present on the host Trichonympha genome. The authentic sequences represented two distinct bacterial species belonging to the phylum Verrucomicrobia, and the pseudogenes have originated from each of the two species. Fluorescence in situ hybridization confirmed that both species are specifically localized, and occasionally co-localized, within nuclei of T. agilis. Transmission electron microscopy revealed that they are distorted cocci with characteristic electron-dense and lucent regions, which resemble the intranuclear symbionts illustrated by Kirby. For these symbionts, we propose a novel genus and species, 'Candidatus Nucleococcus trichonymphae' and 'Candidatus Nucleococcus kirbyi'. These formed a termite-specific cluster with database sequences, other members of which were also detected within nuclei of various gut protists, including both parabasalids and oxymonads. We suggest that this group is widely distributed as intranuclear symbionts of diverse protists in termite guts and that they might have affected the evolution of the host genome through lateral gene transfer.

Termite guts harbor diverse yet-uncultured bacteria, including a non-photosynthetic cyanobacterial group, the class “Melainabacteria”. We herein reported the phylogenetic diversity of “Melainabacteria” in the guts of diverse termites and conducted a single-cell genome analysis of a melainabacterium obtained from the gut of the termite Termes propinquus. We performed amplicon sequencing of 16S rRNA genes from the guts of 60 termite and eight cockroach species, and detected melainabacterial sequences in 48 out of the 68 insect species, albeit with low abundances (0.02–1.90%). Most of the melainabacterial sequences obtained were assigned to the order “Gastranaerophilales” and appeared to form clusters unique to termites and cockroaches. A single-cell genome of a melainabacterium, designated phylotype Tpq-Mel-01, was obtained using a fluorescence-activated cell sorter and whole genome amplification. The genome shared basic features with other melainabacterial genomes previously reconstructed from the metagenomes of human and koala feces. The bacterium had a small genome (~1.6 Mb) and possessed fermentative pathways possibly using sugars and chitobiose as carbon and energy sources, while the pathways for photosynthesis and carbon fixation were not found. The genome contained genes for flagellar components and chemotaxis; therefore, the bacterium is likely motile. A fluorescence in situ hybridization analysis showed that the cells of Tpq-Mel-01 and/or its close relatives are short rods with the dimensions of 1.1±0.2 μm by 0.5±0.1 μm; for these bacteria, we propose the novel species, “Candidatus Gastranaerophilus termiticola”. Our results provide fundamental information on “Melainabacteria” in the termite gut and expand our knowledge on this underrepresented, non-photosynthetic cyanobacterial group.