Monocercomonoides exilis is considered the first known eukaryote to completely lack mitochondria. This conclusion is based primarily on a genomic and transcriptomic study which failed to identify any mitochondrial hallmark proteins. However, the available genome assembly has limited contiguity and around 1.5 % of the genome sequence is represented by unknown bases. To improve the contiguity, we re-sequenced the genome and transcriptome of M. exilis using Oxford Nanopore Technology (ONT). The resulting draft genome is assembled in 101 contigs with an N50 value of 1.38 Mbp, almost 20 times higher than the previously published assembly. Using a newly generated ONT transcriptome, we further improve the gene prediction and add high quality untranslated region (UTR) annotations, in which we identify two putative polyadenylation signals present in the 3'UTR regions and characterise the Kozak sequence in the 5'UTR regions. All these improvements are reflected by higher BUSCO genome completeness values. Regardless of an overall more complete genome assembly without missing bases and a better gene prediction, we still failed to identify any mitochondrial hallmark genes, thus further supporting the hypothesis on the absence of mitochondrion.

Department of Parasitology Faculty of Science Charles University BIOCEV Průmyslová 595 252 42 Vestec Czech Republic

Institute of Evolutionary Biology Faculty of Biology Biological and Chemical Research Centre University of Warsaw Warsaw Poland

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Genomics of Preaxostyla Flagellates Illuminates the Path Towards the Loss of Mitochondria

A mitochondrion-free eukaryote contains proteins capable of import into an exogenous mitochondrion-related organelle