Evolutionary History Of Mitochondrial Genomes In Discoba, Including The Extreme Halophile Pleurostomum Flabellatum (Heterolobosea)

Data from Discoba (Heterolobosea, Euglenozoa, Tsukubamonadida, and Jakobida) are essential to understand the evolution of mitochondrial genomes (mitogenomes), because this clade includes the most primitive-looking mitogenomes known, as well some extremely divergent genome information systems. Heterolobosea encompassesmore than 150 described species, many of themfrom extreme habitats, but only six heteroloboseanmitogenomeshave beenfully sequencedtodate. Herewe complete themitogenome of the heterolobosean Pleurostomum flabellatum, which is extremely halophilic and reportedly also lacks classicalmitochondrial cristae, hinting at reduction or loss of respiratory function. The mitogenome of P. flabellatum maps as a 57,829-bp-long circular molecule, including 40 coding sequences (19 tRNA, two rRNA, and 19 orfs). The gene content and gene arrangement are similar to Naegleria gruberi and Naegleria fowleri, the closest relativeswith sequencedmitogenomes. The P. flabellatummitogenome contains genes that encode components of the electron transport chain similar to those of Naegleria mitogenomes. Homology searches against a draft nuclear genome showed that P. flabellatum has two homologs of the highly conserved Mic60 subunit of the MICOS complex, and likely lostMic19 andMic10. However, electronmicroscopy showednocristae structures. We infer that P. flabellatum, which originates from high salinity (313 parts per thousand) water where the dissolved oxygen concentration is low, possesses a mitochondrion capable of aerobic respiration, but with reduced development of cristae structure reflecting limited use of this aerobic capacity (e.g., microaerophily).