Choanoflagellate

Choanoflagellata (collared flagellates) is a clade of approximately 250 to 360 described species of free-living, aquatic unicellular and facultatively colonial microeukaryotes that form the closest living sister group to Kingdom Animalia (Metazoa). Together, Choanoflagellata and Metazoa constitute the clade Choanozoa, which sits alongside the amoeboid Filasterea (Capsaspora owczarzaki, Ministeria vibrans) and the mesomycetozoan Ichthyosporea (Sphaeroforma arctica) within the Holozoa—the animal-related branch of the supergroup Opisthokonta (eukaryotes propelled by a single posterior flagellum, opposite to the fungal branch Holomycota that includes Fungi and Opisthosporidia).

First noted for their striking cellular resemblance to the feeding cells (choanocytes) of sponges (Porifera) by the American microscopist Henry James-Clark in 1866, choanoflagellates occupy a foundational position in evolutionary biology. Comparative genomics of the solitary marine choanoflagellate Monosiga brevicollis (King et al., 2008) and the colony-forming species Salpingoeca rosetta (Fairclough et al., 2013) demonstrated that many molecular signaling and cell-adhesion domains once assumed to be unique to multicellular animals—including cadherin repeats, C-type lectins, immunoglobulin domains, and receptor tyrosine kinases (RTKs)—evolved in unicellular holozoan ancestors hundreds of millions of years before the origin of the first animal body plan.

Every choanoflagellate cell is organized around a conserved apical–basal polarity axis that is structurally and functionally homologous to the collar-flagellum complex of sponge choanocytes and eumetazoan sensory-ciliated epithelial cells:

1. The Apical Feeding Collar of Actin Microvilli: Surrounding the apical pole of the ovoid cell body (3 to 10 μm in diameter) is a funnel-shaped ring of 25 to 50 rigid, evenly spaced microvilli (3 to 8 μm in length), each supported internally by a cross-linked bundle of parallel filamentous actin (F-actin) microfilaments polarized with their barbed (+) ends pointing toward the microvillar tip and anchored basally by myosin and villin-like cross-linkers. Adjacent microvilli are held at a uniform sub-micrometer spacing (~0.1 to 0.3 μm) by an delicate extracellular glycocalyx meshwork.
2. The Single Central Opisthokont Flagellum and Low-Reynolds-Number Filter Feeding: Emerging from the center of the microvillar collar is a single apical flagellum (15 to 30 μm long) containing a canonical 9+2 microtubule axoneme anchored at its base by two orthogonal centrioles (the flagellar basal body and an accessory perpendicular centriole) surrounded by a radiating ring of cortical microtubules. Operating at low Reynolds numbers (Re ≈ 10⁻⁴ to 10⁻³, where viscous forces dominate over inertia as described by Stokes flow), undulatory base-to-tip planar waves along the flagellum pump water upward away from the cell body, creating a low-pressure zone that draws surrounding water radially inward against the outer fence of the microvillar collar.
3. Phagocytic Capture of Bacterial Prey: Suspended bacteria (~0.5–1.5 μm diameter) are too large to pass through the 0.2 μm slots between adjacent microvilli and are trapped against the outer base of the collar. Lamellipodial pseudopodia rich in branched Arp2/3-nucleated actin filaments then emerge from the base of the collar to engulf the trapped bacteria into phagocytic vacuoles for intracellular digestion—making choanoflagellates one of the primary bacterivorous grazers in marine and freshwater microbial food webs, filtering 10⁵ to 10⁶ times their own cell volume of water per hour.

Whereas Monosiga brevicollis is strictly solitary in laboratory culture, the marine craspedid Salpingoeca rosetta exhibits a complex life cycle encompassing at least five differentiated cell morphologies (solitary attached thecate cells, slow swimmers, fast dispersal swimmers, linear chain colonies, and spherical rosette colonies of 4 to 50 cells), as well as a haploid–diploid sexual cycle induced by nutrient limitation and bacterial cues.

Crucially, rosette colonies in Salpingoeca rosetta do not form by the aggregation of separate swimming cells (which would create genetically chimeric colonies vulnerable to somatic cheating); instead, rosettes form strictly by serial mitotic cell division (incomplete cytokinesis) from a single founding cell, mirrors the clonal cleavage of an animal zygote into a blastula:

1. Intercellular Cytoplasmic Bridges and Extracellular Matrix (ECM): Daughter cells within a dividing S. rosetta rosette remain connected via thin intercellular cytoplasmic bridges (~250 nm wide, stabilized by two electron-dense parallel plates) and an central core of secreted extracellular matrix (ECM) Containing a C-type lectin protein called Rosetteless. Mutating the rosetteless gene leaves solitary cell division and swimming completely intact but abolishes rosette colony formation.
2. Interspecies Chemical Signaling from Prey Bacteria (RIF-1, RIF-2, and EroS): In 2012, Nicole King, Jon Clardy, and colleagues discovered that S. rosetta switches from solitary division to multicellular rosette development specifically when exposed to femtomolar-to-picomolar concentrations of a sulfonolipid signaling molecule called Rosette-Inducing Factor-1 (RIF-1) (and a synergistic sulfonolipid, RIF-2) secreted by its natural marine bacterial prey, Algoriphagus machipongonensis (Phylum Bacteroidota). Subsequently, they showed that a second marine bacterium, Vibrio fischeri, secretes a chondroitin-lyase enzyme called EroS (Extracellular regulator of Sex) that digests chondroitin sulfate proteoglycans on the S. rosetta cell surface to trigger synchronized gamete fusion and meiotic recombination—demonstrating that environmental bacteria regulate both multicellular morphogenesis and sexual reproduction in the closest relatives of animals.
3. Contractile Light-Regulated Colony Bending in Choanoeca flexa: Discovered in splash pools on the Caribbean island of Curaçao in 2019 (Brunet et al., 2019), the colonial choanoflagellate Choanoeca flexa forms cup-shaped sheets of hundreds of polarized cells connected directly by apical actin–myosin (actomyosin) rings at the microvillar collars. In the light, the colony relaxes into a concave cup with all flagella beating inward ("flagella-in" feeding posture); when a shadow passes overhead (detected by a retinal-binding rhodopsin–cyclic nucleotide phosphodiesterase fusion protein), coordinated apical actomyosin constriction across all cells simultaneously inverts the curvature of the sheet within seconds into a convex sphere with all flagella pointing outward ("flagella-out" fast-swimming posture)—providing a living unicellular-to-colonial model for the evolution of embryonic epithelial folding and gastrulation.

Sequencing the genomes of Monosiga brevicollis, Salpingoeca rosetta, and filasterean/ichthyosporean outgroups revolutionized our understanding of the genetic innovations that preceded the origin of animals:

• Cadherins and Cell-Adhesion Modules: Monosiga brevicollis encodes 23 cadherin genes and Salpingoeca rosetta encodes 29 cadherins containing canonical extracellular Ca²⁺-binding cadherin repeats (EC domains), transmembrane segments, and EGF/immunoglobulin domains. Unlike metazoan classical cadherins, choanoflagellate cadherins lack the cytoplasmic β-catenin binding tail (instead localizing to the actin-filled feeding collar to bind bacterial prey or tether neighboring cells), indicating that ancestral bacterial-capture receptors were co-opted at the base of Metazoa to link with intracellular α- and β-catenin into stable epithelial adherens junctions.
• Massive Expansion of Receptor Tyrosine Kinases (RTKs): While fungi and plants lack canonical receptor tyrosine kinases (relying on serine/threonine and histidine kinases), Monosiga brevicollis encodes 128 tyrosine kinases (more than the ~90 tyrosine kinases in the human genome!), paired with SH2 phosphotyrosine-reader domains and PTP protein tyrosine phosphatases—demonstrating that phosphotyrosine signaling diversified extensively in unicellular holozoans before being repurposed for intercellular growth-factor communication in animals.
• Pre-Existence of Transcription Factor Families (p53, Myc, Sox, Runx, T-box) vs. Metazoan-Specific Morphogens: Unicellular holozoans already possess functional p53, Myc/Max, Runx, Sox, Fox, T-box (Brachyury), and NF-κB transcription factors, as well as the Hippo/YAP kinase cascade (in Capsaspora) and the Notch domain components. However, choanoflagellates completely lack Hox, ParaHox, and ETS transcription factors, lack nuclear hormone receptors, and lack secreted Wnt, TGF-β/BMP, and Hedgehog ligands—confirming that these developmental morphogen systems were assembled de novo (via exon shuffling and domain fusion) along the stem lineage of crown-group Metazoa.
• Ancestral Linkage Groups (BCnS Macrosynteny): Chromosome-level genome assemblies of Salpingoeca rosetta and Capsaspora owczarzaki played the decisive outgroup role in resolving the basal metazoan root (Schultz et al., 2023), showing that ctenophores retain ancestral choanoflagellate chromosomal linkage blocks that underwent irreversible fusion-with-mixing in the common ancestor of sponges, cnidarians, and bilaterians.

Are animals evolved from sponges that simplified into choanoflagellates, or from choanoflagellate-like ancestors?

In the mid-20th century, a few zoologists speculated that choanoflagellates might be secondarily simplified, free-swimming sponge choanocytes that escaped from a multicellular sponge. Whole-genome phylogenomics, mitochondrial genome architecture (choanoflagellate mitochondrial genomes are 55–76 kb and encode 25–39 protein-coding genes including ribosomal proteins lost in all metazoan mtDNA), and ancestral chromosomal synteny (BCnS) rule out secondary simplification: choanoflagellates retain ancestral eukaryotic genes, introns, and syntenic blocks that were lost or fused prior to the last common ancestor of all animals. Instead, the last common ancestor of Choanozoa was a bacterivorous collared flagellate capable of facultative colonial life, from which the sponge choanocyte was inherited directly as a homologous feeding cell type.

How can unicellular choanoflagellates undergo amoeboid migration without a multicellular body?

When a choanoflagellate cell (Salpingoeca rosetta or Monosiga brevicollis) is mechanically compressed beneath a sediment grain or trapped in a narrow < 2 μm interstitial pore where flagellar swimming is impossible, it rapidly retracts its flagellum and microvillar collar and switches within minutes into a blebbing amoeboid cell (Brunet et al., 2021). This escape response is driven by myosin II cortical contractility activated by intracellular Ca²⁺—the exact same bleb-based amoeboid motility program used by vertebrate leukocytes and zebrafish primordial germ cells migrating through tight embryonic tissues—indicating that animals inherited both ciliary/epithelial morphology and amoeboid mesenchymal motility as environmentally regulated cell states from their unicellular holozoan ancestors.

1. King, N., Westbrook, M. J., Young, S. L., Kuo, A., Abedin, M., Chapman, J., ... & Rokhsar, D. (2008). "The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans." Nature, 451(7180), 783–788. https://doi.org/10.1038/nature06617
2. Alegado, R. A., Brown, L. W., Cao, S., Dermenjian, R. K., Zuzow, R., Fairclough, S. R., ... & King, N. (2012). "A bacterial sulfonolipid triggers multicellular development in the closest living relatives of animals." eLife, 1, e00013. https://doi.org/10.7554/eLife.00013
3. Fairclough, S. R., Chen, Z., Kramer, E., Zeng, Q., Young, S., Robertson, H. M., ... & King, N. (2013). "Premetazoan genome evolution and the regulation of cell differentiation in the choanoflagellate Salpingoeca rosetta." Genome Biology, 14(2), R15. https://doi.org/10.1186/gb-2013-14-2-r15
4. Brunet, T., Larson, B. T., Linden, T. A., Vermeij, M. J., McDonald, K., & King, N. (2019). "Light-regulated collective contractility in a multicellular choanoflagellate." Science, 366(6463), 326–334. https://doi.org/10.1126/science.aay2346