Description of Imasa heleensis, gen. nov., sp. nov. (Imasidae, fam. nov.), a Deep-Branching Marine Malawimonad and Possible Key Taxon in Understanding Early Eukaryotic Evolution.

Malawimonadida is a deep-level (arguably "kingdom-scale") lineage of eukaryotes whose phylogenetic affinities are uncertain but of great evolutionary interest, as the group is suspected to branch close to the root of the tree of eukaryotes. Part of the difficulty in placing Malawimonadida phylogenetically is its tiny circumscription: at present, it comprises only two described and one cultured but undescribed species, all of them are freshwater suspension-feeding nanoflagellates. In this study, we cultivated and characterised Imasa heleensis gen. nov., sp. nov. (Imasidae fam. nov.), the first marine malawimonad to be described. Light and electron microscopy observations show that Imasa is largely similar to other malawimonads, but more frequently adheres to the substrate, often by means of a pliable posterior extension. Phylogenetic analyses based on two ribosomal RNA genes and four translated protein-coding genes using three different taxon sets place Imasa as sister to the three freshwater malawimonad strains with strong support. Imasa's mitochondrial genome is circular-mapping and shows a similar gene complement to other known malawimonads. We conclude that Imasa represents an important expansion of the range of taxa available for future evolutionary study.

Rickettsial endosymbiont in the "early-diverging" streptophyte green alga Mesostigma viride.

A bacterial endosymbiont was unexpectedly found in the "axenic" culture strain of the streptophyte green alga Mesostigma viride (NIES-995). Phylogenetic analyses based on 16S rRNA gene sequences showed that the symbiont belongs to the order Rickettsiales, specifically to the recently designated clade "Candidatus Megaira," which is closely related to the well-known Rickettsia clade. Rickettsiales bacteria of the "Ca. Megaira" clade are found in a taxonomically diverse array of eukaryotic hosts, including chlorophycean green algae, several ciliate species, and invertebrates such as Hydra. Transmission electron microscopy, fluorescence in situ hybridi-zation, and SYBR Green I staining experiments revealed that the endosymbiont of M. viride NIES-995 is rod shaped, typically occurs in clusters, and is surrounded by a halo-like structure, presumably formed by secretory substances from the bacterium. Two additional M. viride strains (NIES-296 and NIES-475), but not SAG50-1, were found to house the rickettsial endosymbiont. Analyses of strain NIES-995 transcriptome data indicated the presence of at least 91 transcriptionally active genes of symbiont origins. These include genes for surface proteins (e.g., rOmpB) that are known to play key roles in bacterial attachment onto host eukaryotes in related Rickettsia species. The assembled M. viride transcriptome includes transcripts that code for a suite of predicted algal-derived proteins, such as Ku70, WASH, SCAR, and CDC42, which may be important in the formation of the algal-rickettsial association.

One high quality genome and two transcriptome datasets for new species of Mantamonas, a deep-branching eukaryote clade.

Mantamonads were long considered to represent an "orphan" lineage in the tree of eukaryotes, likely branching near the most frequently assumed position for the root of eukaryotes. Recent phylogenomic analyses have placed them as part of the "CRuMs" supergroup, along with collodictyonids and rigifilids. This supergroup appears to branch at the base of Amorphea, making it of special importance for understanding the deep evolutionary history of eukaryotes. However, the lack of representative species and complete genomic data associated with them has hampered the investigation of their biology and evolution. Here, we isolated and described two new species of mantamonads, Mantamonas vickermani sp. nov. and Mantamonas sphyraenae sp. nov., for each of which we generated transcriptomic sequence data, as well as a high-quality genome for the latter. The estimated size of the M. sphyraenae genome is 25 Mb; our de novo assembly appears to be highly contiguous and complete with 9,416 predicted protein-coding genes. This near-chromosome-scale genome assembly is the first described for the CRuMs supergroup.