Novel biflagellate TKB055 as a possible early-branch in the global eukaryotic phylogeny : studies on its morphology, transcriptome, and mitochondrial genome

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Discoba (Excavata) is a unicellular eukaryotic assemblage that comprises four subgroups: Heterolobosea, Euglenozoa, Jakobida and Tsukubamonadida. This clade is especially important for understanding the early evolution of mitochondrial (mt) genomes, as the jakobid Reclinomonas americana retains the most ancestral (i.e. gene-rich) genome of any mt genomes determined to date. Amongst discobids, the Tsukubamonadida is the most recently established group and contains a single representative species (Tsukubamonas globosa; formerly known as the biflagellate strain TKB055) whose precise phylogenetic position within the Discoba has not yet been resolved. We conducted a pyrosequencing-based transcriptomic survey of T globosa and, based on these data, we assembled a phylogenomic alignment of 157 proteins. Our phylogenomic analyses show that T. globosa branches at the base of the union of Euglenozoa and Heterolobosea. In addition, we completely sequenced the mt genome of T. globosa (48,463 bp in length and A+T content of 66.2%). The mt genome of T. globosa can be mapped as circular, and encodes 52 putative protein-coding and 29 RNA genes, indicating that T. globosa is second only to R. americana in terms of its coding capacity. The relationships amongst the majormembers of the Discoba clade inferred from the 157-protein phylogeny and their mt genome data enalbe us to elucidate the evolution of mt gene repertoires in this protist clade.