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

Discoba（Encavata）是一种单细胞真核生物组合，包括四个亚群：异裂目、裸藻目、Jakobida和Tsukubamonadida。这个分支对于理解线粒体（mt）基因组的早期进化特别重要，因为美洲鳄Reclinomonasamericana保留了迄今为止确定的任何mt基因组中最古老的（即基因丰富的）基因组。其中，Tsukubamonadida是最近建立的组，并包含一个单一的代表性物种（Tsukubamonas globosa;以前被称为双鞭毛虫菌株TKB 055），其在Discoba中的精确系统发育位置尚未解决。我们进行了一个基于焦磷酸测序的转录组调查的T globosa，并根据这些数据，我们组装了一个157个蛋白质的基因组比对。我们对T.裸藻纲和异叶藻纲结合处基部的球状分支。此外，我们还对T. globosa（长度48，463 bp，A+T含量66.2%）。T. globosa基因编码52个蛋白质编码基因和29个RNA基因，表明T. globosa是仅次于R.在编码能力方面，美国。根据157个蛋白质的同源性和mt基因组数据推断出的Discoba进化支主要成员之间的关系有助于阐明该进化支mt基因库的进化。

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.