The Genome of the Mitochondrion-Related Organelle in , a Large Endosymbiotic Opalinid Inhabiting the Recta of Frogs.

Mitochondrion-related organelles (MROs) are loosely defined as degenerated mitochondria in anaerobic and microaerophilic lineages. Opalinids are commonly regarded as commensals in the guts of cold-blooded amphibians. It may represent an intermediate adaptation stage between the conventional aerobic mitochondria and derived anaerobic MROs. In the present study, we sequenced and analyzed the MRO genome of . It has a linear MRO genome with large inverted repeat gene regions at both ends. Compared to and , the MRO genome of has a higher G + C content and repeat sequences near the central region. Although three Opalinata species have different morphological characteristics, phylogenetic analyses based on eight concatenated genes indicate that they are close relatives. The phylogenetic analysis showed that clustered with with strong support. The 18S rRNA gene-based phylogeny resolved the Opalinea clade as a sister clade to , which then further grouped with . The paraphyly of Proteromonadea needs to be verified due to the lack of MRO genomes for key species, such as , and . Besides, our dataset and analyses offered slight support for the paraphyly of Bigyra.