Evidence for an Independent Hydrogenosome-to-Mitosome Transition in the CL3 Lineage of Fornicates

• Published in: Frontiers in microbiology 2022-05, Vol.13, p.866459-866459
• Main Authors: Vargová, Romana, Hanousková, Pavla, Salamonová, Jana, Žihala, David, Silberman, Jeffrey D, Eliáš, Marek, Čepička, Ivan
• Format: Article
• Language: English
• Subjects: Caviomonadidae; caviomonads; codon reassignment; Fornicata; hydrogenosome; Microbiology; mitochondrial evolution
• ISSN: 1664-302X; 1664-302X
• DOI: 10.3389/fmicb.2022.866459

Fornicata, a lineage of a broader and ancient anaerobic eukaryotic clade Metamonada, contains diverse taxa that are ideally suited for evolutionary studies addressing various fundamental biological questions, such as the evolutionary trajectory of mitochondrion-related organelles (MROs), the transition between free-living and endobiotic lifestyles, and the derivation of alternative genetic codes. To this end, we conducted detailed microscopic and transcriptome analyses in a poorly documented strain of an anaerobic free-living marine flagellate, PCS, in the so-called CL3 fornicate lineage. Fortuitously, we discovered that the original culture contained two morphologically similar and closely related CL3 representatives, which doubles the taxon representation within this lineage. We obtained a monoeukaryotic culture of one of them and formally describe it as a new member of the family Caviomonadidae, gen. et sp. nov. In contrast to previously studied caviomonads, the endobiotic and , possesses an ultrastructurally discernible MRO. We sequenced and assembled the transcriptome of , and by sequence subtraction, obtained transcriptome data from the other CL3 clade representative present in the original PCS culture, denoted PCS-ghost. Transcriptome analyses showed that the reassignment of only one of the UAR stop codons to encode Gln previously reported from does not extend to its free-living relatives and is likely due to a unique amino acid substitution in 's eRF1 protein domain responsible for termination codon recognition. The backbone fornicate phylogeny was robustly resolved in a phylogenomic analysis, with the CL3 clade amongst the earliest branching lineages. Metabolic and MRO functional reconstructions of CL3 clade members revealed that all three, including , encode homologs of key components of the mitochondrial protein import apparatus and the ISC pathway, indicating the presence of a MRO in all of them. evidence indicates that the organelles of and PCS-ghost host ATP and H production, unlike the cryptic MRO of . These data suggest that the CL3 clade has experienced a hydrogenosome-to-mitosome transition independent from that previously documented for the lineage leading to .