Phylogenomic Analysis of 155 Helminth Species Reveals Widespread Absence of Oxygen Metabolic Capacity

The terminal electron acceptor of most aerobic respiratory chains, cytochrome c oxidase (COX), has been highly conserved throughout evolution, from aerobic prokaryotes to complex eukaryotes. Oxygen metabolism in parasitic helminths differs significantly from that of most aerobic eukaryotes, as these organisms can switch between aerobic and anaerobic metabolisms throughout their life cycles. Early studies suggested a lack of COX activity in certain parasitic helminths, and the role of COX in helminth mitochondria remains unclear. To determine whether a functional COX is widely present in helminths, we analyzed the phylogenetic distribution of oxygen metabolism systems across 155 helminth genomes, investigating three distinct sets of protein-coding genes involved in different aspects of oxygen metabolism: COX and its assembly factors, peroxisomes, and the most abundant reactive oxygen species (ROS)-metabolizing proteins. While glycolytic and citric acid cycle enzymes are highly conserved in helminthic species, we observed an apparent widespread absence of essential COX genes across 52% of helminth species investigated. While the most common proteins involved in the defense against ROS are highly maintained across virtually all lineages, we also observed an apparent absence of essential peroxisomal protein-coding genes in 42% of species investigated. Our results suggest that a subset of parasitic helminths utilize oxygen differently from related, nonparasitic species such as Caenorhabditis elegans, with significant differences in their mitochondrial electron transport chains and peroxisomes. The identification of substantive differences between parasite and host metabolism offers a new avenue for the development of anthelmintic agents that could target these divergent pathways.