Cyclostomata

Cyclostomata (from the Greek kyklos, "circle," and stoma, "mouth") is the clade of living jawless vertebrates (Agnatha) comprising two orders: Petromyzontiformes (lampreys, ~43 extant species) and Myxiniformes (hagfishes, ~80 extant species). Together, lampreys and hagfishes form the monophyletic sister group to all jawed vertebrates (Gnathostomata), having diverged from the gnathostome stem lineage during the Early Cambrian (~520–500 million years ago) and split from one another in the Late Ordovician–Silurian (~450–420 million years ago).

For much of the late 20th century, morphological cladistics argued that Cyclostomata was paraphyletic (grouping lampreys closer to jawed vertebrates in a clade termed "Myopterygii" or "Vertebrata *sensu stricto*" because lampreys possess rudimentary cartilaginous neural arches, image-forming eyes with extrinsic eye muscles, and ion-regulating kidneys, whereas marine hagfishes are nearly blind, osmotic conformers to seawater, and were thought to lack vertebral elements). However, five independent lines of evidence—nuclear protein-coding phylogenomics, shared microRNA families, embryonic discovery of ventral cartilaginous vertebral elements in the inshore hagfish Eptatretus burgeri (Oisi, Ota, Kuratani, & Kusakabe, 2013), non-collagenous cartilaginous piston tongues, and leucine-rich-repeat Variable Lymphocyte Receptors (VLRs)—have established Cyclostomata as a strictly monophyletic clade.

1. Absence of Hinged Jaws, Paired Fins, and Biomineralized Bone/Enamel: Both lampreys and hagfishes lack articulated mandibular jaws, paired pectoral and pelvic appendages, dermal armor plates, mineralized bone, dentine, and enamel, as well as a differentiated stomach and spleen. Instead, their axial skeleton consists of a persistent, unconstructed, fluid-turgid notochord wrapped in a thick fibrous collagenous sheath, flanked by unmineralized cartilaginous elements composed of non-collagenous elastic matrix proteins (lamprin in lampreys and myxinin in hagfishes) alongside type II collagen (Col2a1).
2. The Protrusible Lingual Apparatus ("Piston Tongue") and Keratinous Teeth: Rather than biting vertically with hinged jaws, both cyclostome lineages feed using a bilaterally folding, cartilaginous lingual ("tongue") apparatus powered by protractor and retractor muscles and armed with sharp, replaceable horny epidermal teeth made of α-keratin (not calcified dentine). In parasitic adult sea lampreys (Petromyzon marinus), the circular oral disc attaches by suction to host teleost fishes while the piston tongue rasps through scales and skin (injecting the anticoagulant lamphredin); in scavenging and predatory hagfishes (Myxine glutinosa, Eptatretus), two laterally folding dental plates evert from the mouth to tear flesh from sunken whale falls or impale burrowing polychaetes and fishes.
3. Monorhiny (Single Median Nasohypophyseal Complex): Whereas jawed vertebrates are diplorhine (possessing paired left and right nostrils that develop separately from the pituitary gland), cyclostomes are monorhine: during embryonic head development (studied extensively by Shigeru Kuratani), the olfactory placode and adenohypophyseal (pituitary) placode fuse into a single median nasohypophyseal plate. As the upper lip (posthypophyseal process in lampreys; prehypophyseal snout in hagfishes) grows forward, it carries the single median nostril either to the top of the head as a blind-ending pouch (lampreys) or to the tip of the snout as a respiratory duct opening directly into the pharynx (hagfishes).
4. Pouched Gill Sacs (Marsipobranchii): Instead of flat lamellar gills supported on jointed internal branchial arches, cyclostomes house their respiratory filaments inside 5 to 16 pairs of spherical, muscular branchial pouches derived from endoderm (endobranchiate), allowing lampreys to pump respiratory water tidally in and out of the external gill pores even while their oral disc is clamped firmly onto a host fish or river boulder.

For decades, cyclostomes were known to reject skin allografts with immunological memory and produce antigen-specific agglutinins, yet genome sequencing revealed that they completely lack the immunoglobulin (Ig), T-cell receptor (TCR), Major Histocompatibility Complex (MHC Class I/II), and RAG1/RAG2 genes of jawed vertebrates. In 2004, Zeev Pancer, Max D. Cooper, and colleagues solved this mystery by discovering that lampreys and hagfishes evolved an entirely independent adaptive immune system built not from immunoglobulin domains, but from Leucine-Rich Repeats (LRRs)—called Variable Lymphocyte Receptors (VLRs):

1. Somatic Gene Conversion via Cytidine Deaminases (CDA1 and CDA2): In the germline DNA of a lamprey or hagfish, each of the three VLR loci (VLRA, VLRB, and VLRC) consists only of a non-functional incomplete template (encoding the N-terminal cap and C-terminal stalk separated by a non-coding intervening sequence), flanked across the chromosome by hundreds of diverse LRR-encoding donor cassettes. During lymphocyte maturation, two APOBEC/AID-family cytidine deaminases (CDA1 and CDA2) direct stepwise, copy-choice somatic gene conversion that stitches 1 to 9 random LRR cassettes into the incomplete germline gene, generating a repertoire of >10¹⁴ distinct antigen receptors—matching the diversity of mammalian antibodies!
2. Parallel T-Like and B-Like Lymphocyte Lineages: Remarkably, cyclostome lymphocytes differentiate into three functional lineages that mirror the T-cell and B-cell division of labor in jawed vertebrates:
   • VLRB⁺ Lymphocytes (B-Cell Analogues): Develop in the hematopoietic typhlosole and kidney using CDA2, express membrane-bound VLRB, and upon antigen binding differentiate into plasma-like cells that secrete pentameric or decameric multivalent VLRB protein complexes directly into the blood as soluble antibodies!
   • VLRA⁺ and VLRC⁺ Lymphocytes (αβ and γδ T-Cell Analogues): Develop inside specialized thymoid gill-filament tips using CDA1, express strictly cell-surface VLRA or VLRC receptors (never secreted), and secrete pro-inflammatory cytokines (such as an IL-17 orthologue) to coordinate cellular immunity.

1. Programmed Germline Chromatin Elimination: As discovered by Jeramiah J. Smith and colleagues (2009, 2018) in the sea lamprey Petromyzon marinus (and confirmed across hagfishes by Yasuhikoimoto / Kubota), cyclostomes do not keep the same genome in their somatic cells as in their germ cells! During early embryonic cleavage (at the 6th to 9th cell divisions in lampreys), presomatic blastomeres selectively jettison ~20% of their entire nuclear genome (~500 megabases and ~12–20 germline-specific chromosomes containing thousands of protein-coding genes, including oncogenes and pluripotency factors) via lagging chromosomes packaged into micronuclei, methylated with H3K9me3, and degraded (Programmed Genome Rearrangement, PGR). This discovery means that somatic cyclostome cells possess ~78–84 chromosomes while their own sperm and oocytes carry ~96–99 chromosomes, shielding somatic tissues from illegitimate expression of germline-specific proliferation genes.
2. Biophysics of Hagfish Defensive Gill-Clogging Slime: When bitten by a shark, wreckfish, or conger eel, a Pacific hagfish (Eptatretus stoutii) ejects ~0.1 grams of milky exudate from its lateral slime pores that expands 10,000-fold in volume within ~100–400 milliseconds into nearly 1 liter of cohesive, viscoelastic mucus that clogs the predator's gills and forces immediate release (while the hagfish ties its own body into an overhand sliding knot to wipe the slime off its own skin and gain mechanical leverage to tear food!). As shown by Douglas S. Fudge and Jean-Luc Gosline, hagfish slime is a two-component hydro-fiber composite:
   • Mucous Vesicles rupture upon contact with seawater ions to form a mucin network; and
   • Gland Thread Cells (GTCs) eject a single, meticulously coiled, 10–15-cm-long protein fiber (slime thread, 1–3 μm thick) composed of α-helical intermediate filament (IF) proteins (α and γ thread keratins) that unravels hydrodynamically in shear flow to trap tens of thousands of times its weight in seawater.

How many rounds of whole-genome duplication (WGD) occurred in Cyclostomata compared to jawed vertebrates?

For over a decade, genome biologists debated whether both rounds of the vertebrate 2R whole-genome duplication occurred before or after Cyclostomata diverged from Gnathostomata. High-resolution chromosome-scale genome assemblies of the sea lamprey (Petromyzon marinus), Pacific lamprey (Entosphenus tridentatus), and brown hagfish (Eptatretus atami) (Simakov et al., 2020; Marlétaz et al., 2024; Yu et al., 2024) resolved the exact evolutionary sequence:

1. Both Cyclostomata and Gnathostomata share the first tetraploidization event (1R, ~520 Ma) in the common vertebrate stem;
2. Following their divergence (~500 Ma), the gnathostome stem underwent an independent allotetraploidization (2R_JV) to yield the canonical 1:4 gnathostome karyotype; whereas
3. The cyclostome stem underwent its own independent hexaploidization (whole-genome triplication, resulting in a 1:6 ancestral ratio), explaining why lampreys and hagfishes possess six Hox clusters and high chromosome counts (2n ≈ 168 in lamprey germline).

Why are larval lampreys (ammocoetes) no longer considered a direct model for the earliest Cambrian vertebrates?

For over 150 years, the blind, worm-like, filter-feeding ammocoete larva of modern lampreys—which lives burrowed in freshwater silt for 3–7 years before metamorphosing into a toothed adult—was viewed as an evolutionary recapitulation of an amphioxus-like ancestral vertebrate. However, paleontological discoveries by Tetsuto Miyashita, Michael I. Coates, and colleagues (2019, 2021) of fossil hatchling and juvenile stem-lampreys (Priscomyzon, Mayomyzon, Pipiscius, and Hardistiella from Devonian and Carboniferous marine shales) revealed that Paleozoic baby lampreys hatched directly with large camera eyes and a toothed sucking oral disc, completely lacking an ammocoete filter-feeding phase! The prolonged blind ammocoete larva evolved secondarily in the Mesozoic when crown-group lampreys invaded freshwater rivers.

1. Pancer, Z., Amemiya, C. T., Ehrhardt, G. R. A., Ceitlin, J., Gartland, G. L., & Cooper, M. D. (2004). Somatic diversification of variable lymphocyte receptors in the agnathan sea lamprey. Nature, 430(6996), 174–180. doi:10.1038/nature02740
2. Smith, J. J., Antonacci, F., Eichler, E. E., & Amemiya, C. T. (2009). Programmed loss of millions of base pairs from a vertebrate genome. Proceedings of the National Academy of Sciences, 106(27), 11212–11217. doi:10.1073/pnas.0902358106
3. Oisi, Y., Ota, K. G., Kuratani, S., & Kusakabe, R. (2013). Development of the chondrocranium in hagfishes, with special reference to the early evolution of vertebrates. Zoological Science, 30(11), 944–961. doi:10.2108/zsj.30.944
4. Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Janvier, P. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146–2151. doi:10.1073/pnas.1814794116
5. Marlétaz, F., Timoshevskaya, N., Timoshevskiy, V. A., Parey, E., Simakov, O., Gavriouchkina, D., ... & Pascual-Anaya, J. (2024). The hagfish genome and the evolution of vertebrates. Nature, 627(8005), 811–819. doi:10.1038/s41586-024-07070-3