DIVERGENT TRAJECTORIES FOR ANAEROBIC MITOCHONDRIAL EVOLUTION IN BREVIATE PROTISTS
Mitochondrion-related organelles (MROs) have evolved as adaptations to low oxygen conditions multiple times in the eukaryote tree of life. However, the evolutionary steps by which aerobic mitochondrial functions were replaced by anaerobic pathways are still poorly understood. The breviate Pygsuia biforma is particularly interesting because it is the only protist known to have replaced the canonical mitochondrial iron-sulfur cluster (ISC) system with a horizontally acquired SUF-like minimal system (SMS) protein. This functions within an MRO possessing a uniquely configured electron transport chain (ETC). To investigate the evolutionary path by which the P. biforma MRO evolved these features, we conducted a comparative transcriptomic study of eight diverse marine breviate species and predicted their MRO proteomes. We found three distinct patterns of iron-sulfur cluster biosynthesis machinery across the breviates where organisms would encode: the canonical ISC system alone, the ISC system and cytoplasmic SMS system, and a cytoplasmic and MRO-localized SMS system. Phylogenetic analyses suggests that the SMS system was acquired via lateral gene transfer in an ancestor of all breviates and later duplicated, with one copy gaining mitochondrial targeting and replacing the ISC system in a subset of breviates. We observed similarly divergent evolutionary trajectories for quinone-utilizing proteins. Two species have completely lost the ETC while the remaining lineages retain a partial ETC and mitochondrial contact site and cristae organizing system (MICOS), previously thought to be absent in breviates. These patterns reflect divergent biochemical configurations of MROs shaped by gene transfer, loss, and duplication within a single eukaryotic lineage and underscores dynamic remodeling of organellar metabolism in response to marine hypoxic environments.