The case
The genome of the oxymonad Monocercomonoides (LIT-1567) lacks every hallmark mitochondrial protein. The mitochondrial iron–sulfur cluster assembly pathway, thought conserved in virtually all eukaryotes, is replaced by a cytosolic SUF system acquired by lateral gene transfer from bacteria. Phylogeny places the organism among mitochondriate relatives, so the loss was secondary. It is the first eukaryote known to lack any form of mitochondrion.
The full analysis of M. exilis (LIT-1574) finds its genome as complex as other eukaryotes’, less reduced than some related metamonads’, with a canonical cytoskeleton, endomembrane system and biosynthesis. The only apparent preadaptations for the loss were gaining SUF and losing the glycine cleavage system.
What it can show
- The same process, the opposite outcome. Loss of function deepens a symbiont’s dependence in the other cases. Here, once a third party’s genes replaced the compartment’s essential job, the compartment itself was lost. As A2 put it, “losing genetic information is not equivalent to losing function, losing autonomy, or increasing integration” (QUESTION-277).
- An integration that can be undone. A partnership as old as the eukaryotic cell was dissolved, so integration is not a ratchet.
Documented in works the record has not read. What it shows rests on their abstracts until they are read.
Note of 2026-10-27 (batch 2): LIT-1574 read
Karnkowska et al. 2019 (LIT-1574, read in NOTE-1243; THEORY-1098) correct the previous title’s “one indispensable job”:
- SUF alone was not enough. The SUF system replaced the mitochondrial ISC pathway “in all examined Preaxostyla”, including relatives that kept a mitochondrion-related organelle.
- A second step. The glycine cleavage system and SHMT sit in the organelle of the close relative Paratrimastix pyriformis. Their loss “took place after the split from the P. pyriformis lineage” and “removed another essential function”. The authors count both as preadaptations, so the title is revised.
- What went with it. Mitophagy is the one system “likely lost together with mitochondria”. Peroxisomes are also gone, which is common in anaerobes. No compensating gene was found.
- The origin of SUF. Its lateral-transfer origin is LIT-1567’s claim and is not argued here.
- “First eukaryote known to lack any form of mitochondrion” stands, with two caveats from the paper. FIB-SEM “could still allow for undetected highly reduced mitosomes”, and relatives may share the status.
- A substituted dependence. The organism is a gut commensal that salvages its nucleotides. It lost an internal partner and depends on a host gut instead. This inference is the record’s, not the paper’s.