Lateral Acquisitions Repeatedly Remodel the Oxygen Detoxification Pathway in Diplomonads and Relatives

Abstract

Oxygen and reactive oxygen species (ROS) are important stress factors for cells because they can oxidize many large molecules. Fornicata, a group of flagellated protists that includes diplomonads, have anaerobic metabolism but are still able to tolerate fluctuating levels of oxygen. We identified 25 protein families putatively involved in detoxification of oxygen and ROS in this group using a bioinformatics approach and propose how these interact in an oxygen detoxification pathway. These protein families were divided into a central oxygen detoxification pathway and accessory pathways for the synthesis of nonprotein thiols. We then used a phylogenetic approach to investigate the evolutionary origin of the components of this putative pathway in Diplomonadida and other Fornicata species. Our analyses suggested that the diplomonad ancestor was adapted to low-oxygen levels, was able to reduce O2 to H2O in a manner similar to extant diplomonads, and was able to synthesize glutathione and l-cysteine. Several genes involved in the pathway have complex evolutionary histories and have apparently been repeatedly acquired through lateral gene transfer and subsequently lost. At least seven genes were acquired independently in different Fornicata lineages, leading to evolutionary convergences. It is likely that acquiring these oxygen detoxification proteins helped anaerobic organisms (like the parasitic Giardia intestinalis) adapt to low-oxygen environments (such as the digestive tract of aerobic hosts).

Metabolic Reconstruction Elucidates the Lifestyle of the Last Diplomonadida Common Ancestor

“…Only two reactions were classified as LGT candidates in the G. intestinalis ancestor since the Giardiinae ancestor, MsrA and flavohemoprotein, both of which have been reported before ( 28 ) ( Fig. 2 and Table S2 ).…”

“…Most likely, both species acquired this capacity via LGT independently. Cases of convergence via LGT have been reported before in diplomonads and other eukaryotes ( 28 , 37 ). This activity is absent in Giardia intestinalis and Giardia muris , indicating either that no successful LGT of genes needed to degrade melibiose has occurred yet in these lineages or that this compound is less frequent in the intestinal tract of their hosts.…”

“…We have recently shown that the last Diplomonadida common ancestor was well adapted to low-oxygen environments ( 28 ). Even though this characteristic evolved in a free-living lifestyle, being able to survive in low-oxygen environments is a requirement to colonize the intestinal tract of the host.…”

“…Any reaction with a majority of hits from prokaryotes was considered an LGT candidate. Reactions whose evolutionary history has been described previously were manually curated to be consistent ( 14 , 28 , 30 ). Classifications of all reactions are listed in Table S1 in the supplemental material.…”

“…We performed a phylogenetic analysis by following the approach previously described ( 28 ). One sequence from Trepomonas sp.…”