Maturases and Group II Introns in the Mitochondrial Genomes of the Deepest Jakobid Branch

Ophirinina is a recently described suborder of jakobid protists (Excavata) with only one described species to date, Ophirina amphinema. Despite the acquisition and analysis of massive transcriptomic and mitogenomic sequence data from O. amphinema, its phylogenetic position among excavates remained inconclusive, branching as sister group either to all Jakobida or to all Discoba. From a morphological perspective, it has not only several typical jakobid features but also unusual traits for this group, including the morphology of mitochondrial cristae (sac-shaped to flattened-curved cristae) and the presence of two flagellar vanes. In this study, we have isolated, morphologically characterized, and sequenced genome and transcriptome data of two new Ophirinina species: Ophirina chinija sp. nov. and Agogonia voluta gen. et sp. nov. Ophirina chinija differs from O. amphinema in having rounded cell ends, subapically emerging flagella and a posterior cell protrusion. The much more distantly related A. voluta has several unique ultrastructural characteristics, including sac-shaped mitochondrial cristae and a complex “B” fiber. Phylogenomic analyses with a large conserved-marker dataset supported the monophyly of Ophirina and Agogonia within the Ophirinina and, more importantly, resolved the conflicting position of ophirinids as the sister clade to all other jakobids. The characterization of the mitochondrial genomes showed that Agogonia differs from all known gene-rich jakobid mitogenomes by the presence of two group II introns and their corresponding maturase protein genes. A phylogenetic analysis of the diversity of known maturases confirmed that the Agogonia proteins are highly divergent from each other and define distant families among the prokaryotic and eukaryotic maturases. This opens the intriguing possibility that, compared to other jakobids, Ophirinina may have retained additional mitochondrial elements that may help to understand the early diversification of eukaryotes and the evolution of mitochondria.