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Tree of Life Reshaped: The Discovery of Solarion arienae, the Phylum Caelestes, and the Rise of the Supergroup Disparia

Updated: 6 days ago

Colorful tree diagram depicting evolution of life domains: TSAR, Archaeplastida, Amorphea, and Excavata, with various microorganisms.

Abstract


The architectural reconstruction of the eukaryotic tree of life (eToL) has long been hindered by the existence of "orphan" lineages—microbial eukaryotes that defy classification within the established supergroups of Amorphea, TSAR (Telonemia, Stramenopiles, Alveolata, Rhizaria), Archaeplastida, and Excavata. These lineages, often termed Protists with Uncertain Phylogenetic Affiliations (PUPAs), represent deep evolutionary branches that hold the keys to understanding the biology of the Last Eukaryotic Common Ancestor (LECA). This report presents a comprehensive analysis of a landmark discovery in this field: the isolation and characterization of Solarion arienae, a novel, deep-branching, free-living heterotrophic protist. Originally identified as a cryptic contaminant in a culture of marine anaerobic ciliates, S. arienae has necessitated the erection of a new phylum, Caelestes, and provided the phylogenetic signal required to unify the orphan lineages Provora, Hemimastigophora, and Meteora into a massive, novel supergroup designated as Disparia.

Beyond its taxonomic significance, S. arienae exhibits ultrastructural and genomic features that challenge fundamental paradigms of eukaryotic evolution. Most notably, its mitochondrial genome encodes a functional SecA gene, a relic of the ancestral alphaproteobacterial protein secretion pathway that has been lost in the vast majority of extant eukaryotes. This retention, shared only with the distantly related Mantamonas, suggests that the mitochondrial import machinery of LECA was a chimera of bacterial export systems and evolving eukaryotic translocases. Furthermore, the organism’s dimorphic life cycle—alternating between a sun-like, axopodial feeder and a flagellated disperser—coupled with a unique predatory apparatus (the stalked extrusome), paints a picture of an ancestral eukaryote that was a sophisticated, metabolically versatile predator. This report integrates data from single-cell transcriptomics, high-pressure freezing transmission electron microscopy (HPF-FS TEM), and multi-gene phylogenomics to define the biology of Solarion and its transformative impact on our understanding of early eukaryotic life.


1. Introduction: The "Dark Matter" of the Eukaryotic World



1.1 The Persistent Challenge of Orphan Lineages


For much of the 20th and early 21st centuries, the classification of eukaryotes was a quest to force a myriad of microbial forms into a handful of "supergroups." This framework, while useful, left a residue of organisms that did not fit. These "orphans" or PUPAs (Protists with Uncertain Phylogenetic Affiliations) constitute the phylogenetic "dark matter" of the eukaryotic tree.1 They are often rare, difficult to culture, and possess morphological traits that are either unique or convergently evolved, confounding traditional taxonomic methods.

The resolution of these lineages is not merely a matter of cataloging biodiversity; it is an archaeological excavation of the cell itself. Because these lineages branch off near the root of the eukaryotic tree, they preserve ancestral characteristics that have been derived or lost in the major, speciose groups like animals, fungi, or plants. The study of PUPAs is, therefore, the study of the intermediate steps between the First Eukaryotic Common Ancestor (FECA) and the Last Eukaryotic Common Ancestor (LECA).


1.2 The Emergence of Disparia


Recent years have seen a piecemeal resolution of some of these orphans. The lineage Hemimastigophora, a group of multi-flagellated soil and sediment dwellers, was recently shown to be a deep-branching distinct lineage.2 Similarly, the Provora—voracious predators dubbed "lions of the microbial world"—were identified as a distinct supergroup-level clade.3 Meteora sporadica, a protist with a bizarre gliding motility and "incredible cell architecture," was linked to Hemimastigophora.2

However, the relationship between these groups remained fragmented until the discovery of Solarion arienae. This organism acted as the missing piece of the puzzle, providing the phylogenetic density required to break up long branches and resolve the topology of this sector of the tree. The result is the proposal of "Disparia," a new eukaryotic supergroup encompassing Provora, Hemimastigophora, Meteora, and the newly erected phylum Caelestes (containing Solarion).3 This consolidation represents one of the most significant reshufflings of high-level eukaryotic taxonomy in a decade.


1.3 The Metabolic Enigma of LECA


Parallel to these taxonomic upheavals is a shifting understanding of eukaryotic metabolism. The traditional view held that the mitochondrion, once established, rapidly transferred its genes to the nucleus and established the TIM/TOM (Translocase of the Inner/Outer Membrane) protein import complex, discarding its bacterial roots.7 However, the discovery of Solarion challenges this timeline. The presence of a mitochondrial SecA gene in Solarion 3 suggests that the ancestral bacterial export machinery persisted in the mitochondrion for far longer than anticipated, implying that LECA was a "mosaic" entity with a blend of bacterial and eukaryotic cellular systems that is rarely seen in modern lineages.


2. Discovery and Isolation: Serendipity in the Anaerobic Zone



2.1 The "Contaminant" in the Culture


The discovery of Solarion arienae serves as a poignant reminder of the unseen diversity lurking within established laboratory systems. The organism was not the target of a specific environmental survey but was identified as a "cryptic contaminant" in a long-term laboratory culture of a marine anaerobic ciliate.3

The primary subject of the culture was a member of the order Metopida (likely a Metopus or similar anaerobe), which are pivotal players in the microbial food webs of sulfuretums and anoxic sediments.9 These environments are often rich in sulfur-cycling bacteria and archaea.10 In this anaerobic microcosm, Solarion existed as an inconspicuous, free-living heterotroph. Its cells were described as "tiny and only slightly motile," traits that allowed it to evade detection during routine maintenance of the larger ciliate host.3 It was only when researchers, likely investigating the bacterial symbionts or the broader microbiome of the ciliate, applied closer scrutiny that the distinct genetic signature of Solarion emerged.


2.2 Isolation Challenges and Solutions


Culturing deep-branching protists is notoriously difficult. They often have specific, unknown nutritional requirements or rely on complex biotic interactions (syntrophy) that are hard to replicate in axenic (pure) culture.11 Solarion was no exception. It fed on bacteria present in the ciliate culture 12, but isolating it away from the ciliate required precise micromanipulation.

To bypass the hurdles of establishing a monoculture, the research team employed single-cell transcriptomics.13 This technique allows for the amplification and sequencing of the entire expressed gene set (transcriptome) from a handful of manually isolated cells. By picking individual Solarion cells, lysing them, and sequencing the RNA, the team generated a high-quality dataset of protein-coding genes without the need for growing billions of cells. This approach was complemented by the assembly of the mitochondrial genome from the transcriptomic and genomic data, a critical step given the eventual discovery of the SecA gene.13


2.3 The Name: Solarion arienae


The etymology of the name Solarion is likely derived from the Latin sol (sun), referencing the "sun-like" morphology of its primary feeding stage, which possesses radiating axopodia resembling solar rays.12 The specific epithet arienae and the phylum name Caelestes (Latin for "celestial" or "heavenly") reinforce this celestial motif.3 This naming convention aligns with a long tradition in protistology of naming organisms after their visual appearance (e.g., Heliozoa meaning "sun animals").


3. Methodology: Piercing the Veil of the Microscopic


The characterization of S. arienae required a multi-faceted methodological approach, combining classical microscopy with cutting-edge molecular biology. Understanding these methods is essential to appreciating the reliability of the data presented.


3.1 High-Pressure Freezing and Transmission Electron Microscopy (HPF-FS TEM)


To resolve the ultrastructure of a novel organism, particularly one with delicate features like axopodia and extrusomes, standard chemical fixation can be insufficient. Chemical fixatives (like glutaraldehyde) can sometimes cause retraction of cellular extensions or artifacts in membrane structure.

For Solarion, the researchers utilized High-Pressure Freezing followed by Freeze Substitution (HPF-FS).15

  • The Technique: Samples are frozen within milliseconds under high pressure (2000 bar) to prevent ice crystal formation, preserving the cell in a near-native state. The frozen water is then substituted with organic solvents (acetone) containing fixatives (uranyl acetate, osmium tetroxide) at low temperatures (-90°C to -60°C).16

  • Protocol Specifics: The Solarion samples were embedded in EMbed 812 resin and polymerized at 60°C for 48 hours. Ultrathin sections (90 nm) were cut using a diamond knife on a Leica EM UC6 ultramicrotome.16

  • Resulting Imagery: This method allowed for the detailed visualization of the "stalked extrusome," the microtubular cytoskeleton, and the mitochondrial cristae without the distortion often seen in chemically fixed samples. The researchers also performed standard chemical fixation for comparison, noting differences in preservation.12


3.2 Electron Tomography


Beyond standard 2D cross-sections, the study employed electron tomography to generate 3D models of the cell's internal machinery.3 By tilting the sample in the electron microscope and taking a series of images, a 3D volume can be computationally reconstructed. This was crucial for modeling the unique "stalked extrusome," revealing it to be a complex, multipartite organelle rather than a simple vesicle.3


3.3 Phylogenomic Dataset Construction


To determine the evolutionary position of Solarion, a massive phylogenomic dataset was constructed.

  • Scale: The primary dataset consisted of 240 protein-coding genes across 87 taxa, totaling 77,133 amino acid sites.3

  • Curation: Genes were selected from the PhyloFisher database 17, a standard tool for eukaryotic phylogenomics. This ensures that the genes used are "single-copy orthologs" (genes that track the species history, not gene duplication history).

  • Analysis: The data were analyzed using complex evolutionary models (e.g., LG+C60 mixture models) that account for site-specific heterogeneity.18 This is critical for avoiding Long Branch Attraction (LBA), a common artifact where fast-evolving lineages (like many parasites or rare protists) are erroneously grouped together.


4. Systematic Description: Phylum Caelestes and Supergroup Disparia


The discovery of Solarion triggered a taxonomic cascade, necessitating new ranks to accommodate its distinctness.


4.1 Phylum Caelestes


  • Definition: The phylum Caelestes is defined by the presence of the genus Solarion and its unique combination of traits: a dimorphic life cycle (sun-like and flagellate), stalked extrusomes, and the retention of the mitochondrial SecA gene.3

  • Diagnosis: Free-living, bacterivorous heterotrophs. Mitochondria with tubular or discoidal cristae (inferred from similar lineages, though specific shape in Solarion is described generally as "cristae" in 12). The presence of a unique transmembrane domain in the mitochondrial SecA protein is a molecular synapomorphy for the group.19


4.2 Supergroup Disparia


The establishment of Disparia is the broader implication. It unifies Caelestes with three other orphan groups.


Taxonomic Group

Key Morphological Features

Ecological Niche

Previous Status

Caelestes (Solarion)

Sun-like/Flagellate dimorphism, Stalked Extrusomes.

Marine anaerobic/microaerophilic

New Discovery

Provora (Nibbleromonas, Nebulomonas)

"Nibbling" predation, distinct cortical alveoli, ventral groove.

Marine/Freshwater Predators

Described 2022 (Nature) 3

Hemimastigophora (Hemimastix, Spironema)

Multi-flagellated (two rows), complex pellicle, bottle-shaped extrusomes.

Soil/Sediment

Deep-branching Orphan 2

Meteora (Meteora sporadica)

Gliding motility via lateral arms, no visible flagella (cryptic?), intricate cytoskeleton.

Marine sediments

Orphan PUPA 2

Phylogenetic Topology of Disparia:

Current analyses suggest a specific internal topology for Disparia:

  1. Hemimastigophora + Meteora: These two form a robust clade. Despite their morphological differences (many flagella vs. gliding arms), they share deep phylogenomic signal.2

  2. Provora: This group branches as sister to the Hemimastigophora+Meteora clade.3

  3. Caelestes: Solarion branches deeply, likely sister to the Provora/Hemimastigophora/Meteora cluster, or nested in a way that anchors the entire group.3

This supergroup is defined not by a single morphological trait (like the flagellar hairs of Stramenopiles) but by their shared evolutionary history and, increasingly, their shared retention of "ancestral" predatory features like complex extrusomes.


5. Morphology and Ultrastructure: The Anatomy of a Predator


Solarion arienae is a shapeshifter. Its life cycle involves at least two distinct forms, a strategy known as "polymorphism" which is common in protists that must balance feeding efficiency with dispersal.


5.1 The Sun-Like Form (The Trophont)


The "sun-like" form is the vegetative, feeding stage of the organism.

  • Appearance: In light microscopy, it appears as a spherical cell with numerous radiating projections.12 These are not simple pseudopodia; they are axopodia-like, supported by microtubule bundles that radiate from a central point.

  • The MTOC: Ultrastructural analysis reveals a central Microtubule Organizing Center (MTOC) from which the microtubules radiate.12 This centrosomal structure organizes the cytoskeleton and likely coordinates the movement of the axopodia during prey capture.

  • Feeding Behavior: Time-lapse microscopy shows this form is sedentary but active. It uses its axopodia to ensnare bacteria. Once captured, the prey is drawn toward the cell body and engulfed into a food vacuole (phagosome).12 The presence of food vacuoles containing bacteria in the TEM sections confirms this bacterivorous lifestyle.


5.2 The Flagellated Form (The Dispersal Stage)


When conditions change—perhaps when local bacterial prey is exhausted—Solarion transforms.

  • Morphology: The cell elongates and develops a flagellum. This flagellum is described as "posterolaterally directed," meaning it emerges from the side/rear of the cell and trails behind.12

  • The Posterior Tail: The cell also possesses a "posterior tail," a trailing extension of the cytoplasm that may help in steering or stability during swimming.

  • Locomotion: This form is motile, allowing the organism to swim through the water column or navigate the interstitial spaces of sediment to find new feeding grounds.

  • Basal Body Structure: TEM of the flagellated form reveals a flagellar basal body connected to a "striated fiber".12 Striated fibers are contractile or structural protein roots common in flagellates, used to anchor the flagellum to the nucleus or cytoskeleton.


5.3 The Stalked Extrusome: A Microscopic Harpoon


The most diagnostic feature of Solarion is its unique "stalked extrusome".3

  • Definition: Extrusomes are secretory organelles that can be triggered to discharge their contents outside the cell. They are the "weapons" of the microbial world.

  • Architecture: Unlike the toxicysts of Provora or the bottle-shaped extrusomes of Hemimastigophora, Solarion's extrusomes have a bipartite structure: a distinct "body" and a "stalk".3

  • Mechanism: The 3D reconstruction shows the extrusome body positioned near the cell membrane, ready to fire. The stalk, supported by microtubules, likely acts as an anchor or a propulsion mechanism.3

  • Function: TEM images capture the extrusome body attached to bacterial prey both outside and inside the cell.12 This suggests the extrusome is used to adhere to or paralyze the prey, facilitating phagocytosis. It is a "harpoon" or "grappling hook" for hunting bacteria.


6. Genomic Architecture: The Blueprint of an Ancient Cell


The sequencing of Solarion arienae has provided a treasure trove of data. The assembly includes not just the nuclear genes used for phylogeny, but a complete mitochondrial genome that is rewriting textbooks.


6.1 The Nuclear Genome


While the full nuclear genome details are extensive, the key takeaway from the transcriptomic data is the gene richness associated with the Disparia clade.

  • Phylogenomics: The 240-gene dataset places Solarion securely within the eukaryotic tree, resolving the polytomy that previously existed at the base of the "orphan" radiation.3

  • Gene Presence: The nuclear genome encodes the standard suite of eukaryotic cellular machinery—actin, tubulin, nuclear pore complexes—confirming its status as a typical eukaryote despite its deep divergence.


6.2 The Mitochondrial Genome


The mitochondrion of Solarion is the star of the show.

  • Cristae Structure: TEM confirms the presence of mitochondria with cristae and surrounding endoplasmic reticulum, the hallmark of eukaryotic aerobic (or facultatively anaerobic) metabolism.12

  • Genomic Content: The mitogenome is described as "gene-rich." In the context of mitochondrial evolution, "gene-rich" refers to genomes that retain more of the ancestral bacterial genes than the streamlined genomes of animals (which typically have ~13 protein-coding genes) or alveolates (often just 3).

  • Comparison: It is comparable to the mitogenomes of Jakobids (60-66 proteins) and Mantamonas (62 proteins).21 This places Solarion in an exclusive club of eukaryotes that have evolved slowly in terms of mitochondrial genome reduction.


7. The Mitochondrial SecA Anomaly: A Molecular Fossil


The most profound discovery in the Solarion genome is the SecA gene.3 To understand why this is revolutionary, we must detail the mechanisms of protein transport.


7.1 The Bacterial Sec Pathway (The Ancestor)


In bacteria (and thus in the alphaproteobacterial ancestor of mitochondria), the primary pathway for moving proteins out of the cytoplasm and across the inner membrane is the Sec pathway.

  • Components: It consists of a protein-conducting channel (SecYEG) and a motor ATPase (SecA).22

  • Mechanism: SecA binds to a pre-protein in the cytoplasm, burns ATP, and pushes the protein through the SecYEG pore into the periplasm.22


7.2 The Mitochondrial TIM/TOM Pathway (The Derived State)


When the endosymbiont became an organelle, the flow of proteins reversed. Most genes moved to the nucleus, so proteins had to be imported into the mitochondrion.

  • The Innovation: Eukaryotes evolved the TOM complex (Outer Membrane) and the TIM complex (Inner Membrane) to handle this import.7

  • The Loss: The bacterial SecA was deemed redundant or incompatible with import and was lost in almost all known eukaryotes. The SecY pore was often retained (sometimes repurposed as Tim17/23 homologs or kept for internal sorting), but the SecA motor was gone.


7.3 The Solarion Configuration


Solarion arienae retains the SecA gene in its mitochondrion.

  • Phylogeny of the Gene: Phylogenetic analysis confirms this SecA groups with Alphaproteobacteria.8 It is a direct descendant of the endosymbiont's gene, not a horizontal transfer.

  • Structure: The Solarion SecA protein has a unique transmembrane domain not found in typical bacteria, suggesting it is tethered to the membrane in a specific way.19 It also has a divergent C-terminal domain (CTD).

  • Implication: Solarion mitochondria likely possess a "hybrid" transport system. They almost certainly have TIM/TOM for importing nuclear-encoded proteins (as Solarion has a nucleus and imports proteins), but they also retain the ancestral bacterial export capability powered by SecA. This might be used for assembling complexes in the inner membrane or exporting proteins to the intermembrane space in a bacterial-like fashion.


7.4 The Connection to Mantamonas


The only other eukaryote known to have a mitochondrial SecA is Mantamonas (CRuMs supergroup).21 Mantamonas has a complete system: SecA, SecY, SecE, SecG.

  • The Bridge: The presence of SecA in Solarion (Disparia) and Mantamonas (CRuMs)—two very distant lineages—triangulates the state of LECA. It proves that LECA had a mitochondrial SecA. The loss of SecA happened independently in the ancestors of Amorphea (animals/fungi) and SAR, rather than SecA being absent in the beginning.


8. Metabolic Versatility and the "Switch" Hypothesis


The habitat of Solarion (anaerobic culture) vs. its ultrastructure (cristate mitochondria) presents a paradox that illuminates the metabolic flexibility of early eukaryotes.


8.1 Anaerobiosis vs. Aerobiosis


The "Great Oxidation Event" predated eukaryotes, but the ocean remained chemically stratified for eons. Early eukaryotes likely evolved in environments with fluctuating oxygen levels.

  • Solarion's Adaptation: Found in an anaerobic ciliate culture, Solarion must be capable of surviving anoxia. Yet, it retains the machinery for respiration (cristae). This suggests it is a facultative anaerobe—capable of respiration when oxygen is present but able to ferment or use alternative electron acceptors when it is not.

  • Comparison: This is similar to Blastocystis or certain ciliates that have adapted to anoxia, but Solarion does so while retaining a "primitive" genomic architecture.


8.2 The "Switch" Mechanism


Recent research into deep-branching protists like Heterolobosea suggests that early eukaryotes developed regulatory "switches" to toggle between metabolic modes.

  • LYRMs: Proteins known as LYRMs act as chaperones or assembly factors for mitochondrial complexes. In deep lineages, these may serve as "orchestrated switches," regulating the assembly of the respiratory chain in response to oxygen availability.3

  • Implication for Solarion: It is hypothesized that Solarion utilizes similar ancient regulatory networks to maintain its mitochondria in a "standby" or modified state during anoxia, preventing the degradation of the organelle, a fate that befell strict anaerobes like Giardia (which possess mitosomes).


9. Phylogenetic Implications: The Architecture of Disparia


The grouping of Solarion, Provora, Hemimastigophora, and Meteora into Disparia is a robust finding supported by multi-gene phylogenomics.2


9.1 Breaking the Polytomy


The "base" of the eukaryotic tree has often been depicted as a polytomy—a comb-like structure where all supergroups emerge simultaneously. This is mathematically unlikely; it reflects a lack of resolution.

  • Disparia's Role: Disparia acts as a new distinct branch. It is not nested within Amorphea or SAR. It is its own "empire."

  • Sisterhoods: Debate continues on the exact placement of Disparia. Some analyses place it sister to the "Diaphoretickes" (SAR + Archaeplastida + Haptista) 26, while others suggest a more independent position. Regardless, it represents a massive, ancient radiation of predatory forms.


9.2 Evidence from Gene Concordance


Phylogenomic studies typically use metrics like Gene Concordance Factors (gCF) to measure support. For the sister group relationship between Meteora and Hemimastigophora, the gCF is ~8.65%, which is comparable to or higher than accepted supergroups like CRuMs or Amorphea.27 This statistical robustness confirms that Disparia is not a phylogenetic artifact but a real evolutionary signal.


10. Ecological Significance: The Rare Biosphere


Solarion arienae belongs to the "rare biosphere"—the "tail" of the species abundance distribution curve.


10.1 Why So Rare?


Why is Solarion not as abundant as the ciliates it lives with?

  • Trophic Level: As a predator of bacteria, its population is limited by prey availability and competition with faster-growing bacterivores (like the ciliates themselves).

  • Niche Specialization: It may thrive only in specific micro-oxic transition zones (ecotones) that are transient in nature. The laboratory culture provided a stable, artificial analog of this niche.


10.2 The Role of Predation in Eukaryogenesis


The members of Disparia—Solarion, Provora, Hemimastigophora—are all complex predators.

  • The "Nibblers": Provora use a "nibbling" mechanism to bite chunks out of large prey.3

  • The "Harpooners": Solarion uses stalked extrusomes to grapple bacteria.12

  • The "Trappers": Hemimastigophora use bottle-shaped extrusomes to immobilize prey.2

  • Conclusion: This strongly supports the "Phagotrophy First" hypothesis. The engine of eukaryotic complexity was the arms race between predatory eukaryotes and their bacterial prey. The mitochondrion itself was likely acquired by a phagotrophic host.


11. Future Directions


The discovery of Solarion is a call to action.

  1. Targeted Cultivation: We must stop throwing away "contaminants." The background noise of culture collections likely harbors other members of Caelestes or Disparia.

  2. Metagenomic Mining: With the genome of Solarion now available, we can mine global metagenomic datasets (like TARA Oceans) to find where else SecA-bearing eukaryotes exist. Are they in the deep ocean? Hydrothermal vents?

  3. Cell Biology of SecA: Experimental work is needed to determine exactly what the mitochondrial SecA is doing in Solarion. Is it essential? What proteins does it transport?


12. Conclusion


The discovery of Solarion arienae is a landmark event in evolutionary biology. It has taken a microscopic, unseen contaminant and elevated it to the status of a phylum (Caelestes), anchoring a new supergroup (Disparia) that rivals the kingdoms of animals and fungi in evolutionary depth.

More importantly, Solarion has provided a window into the deep past. Its mitochondrial SecA gene is a "living fossil," a direct line to the alphaproteobacterial ancestor that initiated the eukaryotic lineage. Its existence proves that the transition from bacterium to mitochondrion was a slow, graded process involving the long-term retention of bacterial export machinery alongside the innovation of eukaryotic import systems.

Solarion teaches us that the "Tree of Life" is still under construction. In the anaerobic muds and the overlooked corners of our petri dishes, ancient lineages are waiting to tell us who we are and where we came from. The reshaping of the tree is not over; with Disparia, it has only just begun.


Table 1: Comparative Characteristics of the Disparia Supergroup Members



Feature

Caelestes (Solarion)

Provora

Hemimastigophora

Meteora

Primary Morphology

Sun-like (axopodial) & Flagellate

Flagellates with ventral groove

Multi-flagellated (kinetids)

Gliding with lateral arms

Predatory Organelle

Stalked Extrusome (Bipartite)

Toxicysts ("Nibbling" complex)

Bottle-shaped Extrusomes

Distinct/Cryptic

Mitochondrial Gene

Retains SecA

Lost SecA (Standard)

Lost SecA (Standard)

Lost SecA (Standard)

Ecological Niche

Anaerobic/Microaerophilic

Marine/Freshwater Predators

Soil/Sediment Predators

Marine Sediments

Support for Clade

Phylogenomic (240 genes) 3

Phylogenomic 3

Phylogenomic 2

Sister to Hemimastigophora 5


Table 2: Evolutionary Trajectory of Mitochondrial Protein Transport



Evolutionary Stage

Transport Machinery Configuration

Representative Organisms

Alphaproteobacterial Ancestor

Sec System (Export): SecA (ATPase) + SecYEG (Pore). No Import System.

Free-living Bacteria (Rickettsiales relatives)

Proto-Mitochondrion (LECA)

Hybrid System: SecA + SecYEG (Retained for export/assembly?) + Early TIM/TOM (Innovated for import).

Inferred Ancestor (Based on Solarion/Mantamonas)

Disparia / CRuMs Relicts

retained Hybrid: SecA + SecY (Retained). Functional TIM/TOM present.

Solarion arienae 3, Mantamonas sphyraenae 21

Modern Mitochondria

Derived System: SecA LOST. SecY retained/diverged. Complex TIM/TOM dominates.

Humans, Fungi, Plants, most Protists


Report compiled by the Evolutionary Biology Desk. Based on research materials from 3 through.27


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