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The jawless vertebrates (agnathans/cyclostomes) are ancestral animals comprising lampreys and hagfishes as the only extant representatives. They possess an alternative adaptive immune system (AIS) that uses leucine-rich repeats (LRR)-based variable lymphocyte receptors (VLRs) instead of the immunoglobulin (Ig)-based antigen receptors of jawed vertebrates (gnathostomes). The different VLR types are expressed on agnathan lymphocytes and functionally resemble gnathostome antigen receptors. In particular, VLRB is functionally similar to the B cell receptor and is expressed and secreted by B-like lymphocytes as VLRB antibodies that bind antigens with high affinity and specificity. The potential repertoire scale of VLR-based antigen receptors is believed to be at least comparable to that of Ig-based systems. VLR proteins inherently possess characteristics that render them excellent candidates for biotechnological development, including tractability to recombinant approaches. In recent years, scientists have explored the biotechnological development and utility of VLRB proteins as alternatives to conventional mammalian antibodies. The VLRB antibody platform represents a non-traditional approach to generating a highly diverse repertoire of unique antibodies. In this review, we first describe some aspects of the biology of the AIS of the jawless vertebrates, which recognizes antigens by means of unique receptors. We then summarize reports on the development of VLRB-based antibodies and their applications, particularly those from the inshore hagfish (Eptatretus burgeri) and their potential uses to address microbial diseases in aquaculture. Hagfish VLRB antibodies (we call Ccombodies) are being developed and improved, while obstacles to the advancement of the VLRB platform are being addressed to utilize VLRBs effectively as tools in immunology. VLRB antibodies for novel antigen targets are expected to emerge to provide new opportunities to tackle various scientific questions. We anticipate a greater interest in the agnathan AIS in general and particularly in the hagfish AIS for greater elucidation of the evolution of adaptive immunity and its applications to address microbial pathogens in farmed aquatic animals and beyond.

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INTRODUCTION: The first vertebrates were jawless fish, or Agnatha, whose evolution diverged into jawed fish, or Gnathostomes, around 550 million years ago. METHODS: In this study, we investigated ß PFT proteins' evolutionary divergence of lamprey immune protein from Agnatha, reportedly possessing anti-cancer activity, into Dln1 protein from Gnathostomes. Both proteins showed structural and functional divergence, and shared evolutionary origin. Primary, secondary and tertiary sequences were compared to discover functional domains and conserved motifs in order to study the evolution of these two proteins. The structural and functional information relevant to evolutionary divergence was revealed using hydrophobic cluster analysis. RESULTS: The findings demonstrate that two membrane proteins with only a small degree of sequence identity can have remarkably similar hydropathy profiles, pointing towards conserved and similar global structures. When facing the lipid bilayer or lining the pore lumen, the two proteins' aerolysin domains' corresponding residues displayed a similar and largely conserved pattern. Aerolysin-like proteins from different species can be identified using a fingerprint created by PIPSA analysis of the pore-forming protein. CONCLUSION: We were able to fully understand the mechanism of action during pore formation through structural studies of these proteins.

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As the most primitive vertebrates, lampreys are significant in understanding the early origin and evolution of the vertebrate innate and adaptive immune systems. The complement system is a biological response system with complex and precise regulatory mechanisms and plays an important role in innate and adaptive immunity. It consists of more than 30 distinct components, including intrinsic components, regulatory factors, and complement receptors. Complement system is the humoral backbone of the innate immune defense and complement-like factors have also been found in cyclostomes. Our knowledge as such in lamprey has dramatically increased in the recent years. The searching for complement components in the reissner lamprey Lethenteron reissneri genome database, together with published data, has unveiled the existence of all the orthologues of mammalian complement components identified thus far, including the complement regulatory proteins and complement receptors, in lamprey. This review, summarizes the key themes and recent updates on the complement system of agnathans and discusses the individual complement components of lampreys, and critically compare their functions to that of mammalian complement components. Interestingly, the adaptive immune system of agnathans differs from that of gnathostomes. Lamprey complement components also display some distinctive features, such as lampreys are characterized by the variable lymphocyte receptors (VLRs)-based alternative adaptive immunity. This review may serve as important literature for deducing the evolution of the immune system from invertebrates to vertebrates.

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AIM: Pacific hagfish are exceptionally tolerant to high environmental ammonia (HEA). Here, we elucidated a cellular mechanism that enables hagfish to actively excrete ammonia against steep ammonia gradients expected to be found inside a decomposing whale carcass. METHODS: Hagfish were exposed to varying concentrations of HEA in the presence or absence of environmental Na+ , while plasma ammonia levels were tracked. 14 C-methylammonium was used as a proxy for NH4 + to measure efflux in whole animals and in isolated gill pouches; the latter allowed us to assess the effects of amiloride specifically on Na+ /H+ exchangers (NHEs) in gill cells. Western blotting and immunohistochemistry were utilized to evaluate the abundance and sub-cellular localization of Rhesus glycoprotein (Rh) channels in the response to HEA. RESULTS: Hagfish actively excreted NH4 + against steep inwardly directed ENH4 + (ΔENH4 + ~ 35 mV) and pNH3 (ΔpNH3 ~ 2000 µtorr) gradients. Active NH4 + excretion and plasma ammonia hypo-regulation were contingent on the presence of environmental Na+ , indicating a Na+ /NH4 + exchange mechanism. Active NH4 + excretion across isolated gill pouches was amiloride-sensitive. Exposure to HEA resulted in decreased abundance of Rh channels in the apical membrane of gill ionocytes. CONCLUSIONS: During HEA exposure, hagfish can actively excrete ammonia against a steep concentration gradient using apical NHEs energized by Na+ -K+ -ATPase in gill ionocytes. Additionally, apical Rh channels are removed from the apical membrane, presumably to reduce ammonia loading from the environment. We suggest that this mechanism allows hagfish to maintain tolerable ammonia levels while feeding inside decomposing carrion, allowing them to exploit nutrient-rich food-falls.

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The vertebrate eye is a vital sensory organ that has long fascinated scientists, but the details of how this organ evolved are still unclear. The vertebrate eye is distinct from the simple photoreceptive organs of other non-vertebrate chordates and there are no clear transitional forms of the eye in the fossil record. To investigate the evolution of the eye we can examine the eyes of the most ancient extant vertebrates, the hagfish and lamprey. These jawless vertebrates are in an ideal phylogenetic position to study the origin of the vertebrate eye but data on eye/retina development in these organisms is limited. New genomic and gene expression data from hagfish and lamprey suggest they have many of the same genes for eye development and retinal neurogenesis as jawed vertebrates, but functional work to determine if these genes operate in retinogenesis similarly to other vertebrates is missing. In addition, hagfish express a marker of proliferative retinal cells (Pax6) near the margin of the retina, and adult retinal growth is apparent in some species. This finding of eye growth late into hagfish ontogeny is unexpected given the degenerate eye phenotype. Further studies dissecting retinal neurogenesis in jawless vertebrates would allow for comparison of the mechanisms of retinal development between cyclostome and gnathostome eyes and provide insight into the evolutionary origins of the vertebrate eye.

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Although corticosteroid-mediated hepatic gluconeogenic activity in response to stress has been extensively studied in fishes and other vertebrates, there is little information on the stress response in basal vertebrates. In sea lamprey (Petromyzon marinus), a representative member of the most basal extant vertebrate group Agnatha, 11-deoxycortisol and deoxycorticosterone are the major circulating corticosteroids. The present study examined changes in circulating glucose and 11-deoxycortisol concentrations in response to a physical stressor. Furthermore, the gluconeogenic actions of 11-deoxycortisol and deoxycorticosterone were examined. Within 6 h of exposure of larval and juvenile sea lamprey to an acute handling stress, plasma 11-deoxycortisol levels increased 15- and 6-fold, respectively, and plasma glucose increased 3- and 4-fold, respectively. Radiometric receptor binding studies revealed that a corticosteroid receptor (CR) is present in the liver at lower abundance than in other tissues (gill and anterior intestine) and that the binding affinity of the liver CR was similar for 11-deoxycortisol and deoxycorticosterone. Transcriptional tissue profiles indicate a wide distribution of cr transcription, kidney-specific transcription of steroidogenic acute regulatory protein (star) and liver-specific transcription of phosphoenolpyruvate carboxykinase (pepck). Ex vivo incubation of liver tissue with 11-deoxycortisol resulted in dose-dependent increases in pepck mRNA levels. Finally, intraperitoneal administration of 11-deoxycortisol and deoxycorticosterone demonstrated that only 11-deoxycortisol resulted in an increase in plasma glucose. Together, these results provide the first direct evidence for the gluconeogenic activity of 11-deoxycortisol in an agnathan, indicating that corticosteroid regulation of plasma glucose is a basal trait among vertebrates.

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Sea lamprey (Petromyzon marinus) begin life as filter-feeding larvae (ammocoetes) before undergoing a complex metamorphosis into parasitic juveniles, which migrate to the sea where they feed on the blood of large-bodied fishes. The greater protein intake during this phase results in marked increases in the production of nitrogenous wastes (N-waste), which are excreted primarily via the gills. However, it is unknown how gill structure and function change during metamorphosis and how it is related to modes of ammonia excretion, nor do we have a good understanding of how the sea lamprey's transition from fresh water (FW) to sea water (SW) affects patterns and mechanisms of N-waste excretion in relation to ionoregulation. Using immunohistochemistry, we related changes in the gill structure of larval, metamorphosing, and juvenile sea lampreys to their patterns of ammonia excretion (Jamm) and urea excretion (Jurea) in FW, and following FW to artificial seawater (ASW) transfer. Rates of Jamm and Jurea were low in larval sea lamprey and increased in feeding juvenile, parasitic sea lamprey. In freshwater-dwelling ammocoetes, immunohistochemical analysis revealed that Rhesus glycoprotein C-like protein (Rhcg-like) was diffusely distributed on the lamellar epithelium, but following metamorphosis, Rhcg-like protein was restricted to SW mitochondrion-rich cells (MRCs; ionocytes) between the gill lamellae. Notably, these interlamellar Rhcg-like proteins co-localized with Na+/K+-ATPase (NKA), which increased in expression and activity by almost tenfold during metamorphosis. The distribution of V-type H+-ATPase (V-ATPase) on the lamellae decreased following metamorphosis, indicating it may have a more important role in acid-base regulation and Na+ uptake in FW, compared to SW. We conclude that the re-organization of the sea lamprey gill during metamorphosis not only plays a critical role in allowing them to cope with greater salinity following the FW-SW transition, but that it simultaneously reflects fundamental changes in methods used to excrete ammonia.

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The vertebrate jaw is a versatile feeding apparatus. To function, it requires a joint between the upper and lower jaws, so jaw joint defects are often highly disruptive and difficult to study. To describe the consequences of jaw joint dysfunction, we engineered two independent null alleles of a single jaw joint marker gene, nkx3.2, in zebrafish. These mutations caused zebrafish to become functionally jawless via fusion of the upper and lower jaw cartilages (ankylosis). Despite lacking jaw joints, nkx3.2 mutants survived to adulthood and accommodated this defect by: (a) having a remodeled skull with a fixed open gape, reduced snout and enlarged branchial region; and (b) performing ram feeding in the absence of jaw-generated suction. The late onset and broad extent of phenotypic changes in the mutants suggest that modifications to the skull are induced by functional agnathia, secondarily to nkx3.2 loss of function. Interestingly, nkx3.2 mutants superficially resemble ancient jawless vertebrates (anaspids and furcacaudiid thelodonts) in overall head shape. Because no homology exists in individual skull elements between these taxa, the adult nkx3.2 phenotype is not a reversal but rather a convergence due to similar functional requirements of feeding without moveable jaws. This remarkable analogy strongly suggests that jaw movements themselves dramatically influence the development of jawed vertebrate skulls. Thus, these mutants provide a unique model with which to: (a) investigate adaptive responses to perturbation in skeletal development; (b) re-evaluate evolutionarily inspired interpretations of phenocopies generated by gene knockdowns and knockouts; and (c) gain insight into feeding mechanics of the extinct agnathans.

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Lamin proteins are major constituents of the nuclear lamina. They are required for fundamental nuclear activities, as evidenced by the large number of laminopathies. Mutations in the human lamin A/C gene exhibit a broad spectrum of clinical manifestations. Most non-vertebrates including the nearest relatives of the vertebrates have only a single lamin gene. In jawed vertebrates (Gnathostomata), four lamin subtypes (B1, B2, LIII, and A) are found. Lampreys and hagfish form the two orders of jawless vertebrates, Agnatha, which represent the sister group of the Gnathostomata at the base of the vertebrate lineage. Lamin sequence information of lampreys and hagfish sheds light on the evolution of the lamin protein family at the base of the vertebrate lineage. In the genomes of the lamprey (Petromyzon marinus) and the hagfish (Eptatretus burgeri), only three lamin genes are present, a lamin A gene is lacking. The presence of an LIII gene in both, lampreys and hagfish, proves that the distinguishing features of this gene had been established before the agnathan/gnathostome split. The other two agnathan lamins, LmnI and LmnII, deviate strongly in their sequences from those of the gnathostome lamins. For none of these two agnathan lamins can orthology be established to one of the gnathostome lamin types. In the direct chromosomal neighbourhood of all three hagfish lamin genes, a MARCH3 paralog is found. This can be interpreted as further evidence that the vertebrate lamin genes have arisen in the course of the two rounds of whole genome duplication that took place at the base of the vertebrate lineage.

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In this study, there was the first attempt to sex immature European river lampreys Lampetra fluviatilis, classified as Agnatha using ultrasonography. This species starts a spawning migration from seas to rivers in the autumn and reproduction is initiated in the late spring. It is recommended to collect breeders soon after the beginning of the spawning migration, however, to date no method has been developed for distinguishing the sex of individuals during this developmental period. The lampreys for the present study were caught in autumn (November) in the Vistula River (northern Poland) during the period of spawning migration and transported to the laboratory. The lampreys were anaesthetised (MS-222, dose: 0.1 g/dm3 prior to sex determinations (n = 100) using ultrasonography. The images obtained using ultrasonography were verified with post-mortem and histological examinations. The findings with this study confirmed that the sex of the European river lamprey can be effectively ascertained much earlier than can occur with assessment of external secondary sexual characteristics (e.g., sexual papilla, ovipositor, skinfold). The advantages of the method include: 100% effectiveness, survival of the fish after examination, non-invasiveness, rapid verification of the lamprey sex and the possibility of determining extent of gonadal development.

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Olfactory cues provide critical information for spatial orientation of fish, especially in the context of anadromous migrations. Born in freshwater, juveniles of anadromous fish descend to the ocean where they grow into adults before migrating back into freshwater to spawn. The reproductive migrants, therefore, are under selective pressures to locate streams optimal for offspring survival. Many anadromous fish use olfactory cues to orient toward suitable streams. However, no behaviorally active compounds have been identified as migratory cues. Extensive studies have shown that the migratory adult sea lampreys (Petromyzon marinus), a jawless fish, track a pheromone emitted by their stream-dwelling larvae, and, consequently, enter streams with abundant larvae. We fractionated extracts of larval sea lamprey washings with guidance from a bioassay that measures in-stream migratory behaviors of adults and identified four dihydroxylated tetrahydrofuran fatty acids, of which (+)-(2S,3S,5R)-tetrahydro-3-hydroxy-5-[(1R)-1-hydroxyhexyl]-2-furanoctanoic acid was shown as a migratory pheromone. The chemical structure was elucidated by spectroscopies and confirmed by chemical synthesis and X-ray crystallography. The four fatty acids were isomer-specific and enantiomer-specific in their olfactory and behavioral activities. A synthetic copy of the identified pheromone was a potent stimulant of the adult olfactory epithelium, and, at 5 × 10-13 M, replicated the extracts of larval washings in biasing adults into a tributary stream. Our results reveal a pheromone that bridges two distinct life stages and guides orientation over a large space that spans two different habitats. The identified molecule may be useful for control of the sea lamprey.

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Hagfish are capable of tolerating extreme hypercapnia (> 30 Torr) by mounting substantial plasma [HCO3-] (hypercarbia) to compensate for CO2-mediated acidosis. The goal of this study was to characterize the mechanistic hypercarbia-recovery strategies in the highly CO2 tolerant hagfish. We exposed hagfish to hypercapnia (30 Torr) for 48 h and allowed a 24 h recovery period in normocapnic seawater. Within 8 h of the recovery period, the compensatory plasma [HCO3-] load (~ 70 mmol L-1) was rapidly offloaded. While increases in both whole-animal HCO3- excretion and glomerular filtration were observed throughout recovery (2-8 h), neither can fully account for the observed rates of whole-animal HCO3- loss, which peaked at ~ 3.5 mmol kg-1 h-1. Inhibition of carbonic anhydrase via acetazolamide revealed that the restoration of plasma [HCO3-] from hypercapnia-induced hypercarbia is likely facilitated in a dualistic manner, initially relying on both carbonic anhydrase mediated CO2 offloading and Cl-/HCO3- exchange processes, both of which are likely either upregulated or further activated as recovery progresses.

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Hagfishes are unique to the vertebrate lineage in that they acquire dissolved nutrients across multiple epithelia including the intestine, gill, and skin. This feat has been attributed to their immersive feeding behavior that likely simultaneously provides benefits (nutrient rich) and potentially adverse (hypercapnia, hypoxia, high environmental ammonia) physiological effects. Examinations have been conducted of the ex vivo transport capabilities of specific nutrients as well as in vivo effects of the hypothesized feeding environments, yet the physiological effects of feeding itself have never been elucidated. We examined the post-prandial physiology of Pacific hagfish (Eptatretus stoutii), identifying changes in oxygen consumption, acid-base balance, ammonia waste excretion, and intestinal morphology following feeding in captivity. Following voluntary feeding, post-prandial oxygen consumption was significantly elevated (1868 ± 272 µmol kg-1 h-1) 8 h following feeding when compared to control resting metabolic oxygen consumption (642 ± 51 µmol kg-1 h-1) and resulted in a factorial metabolic scope of 2.92. Changes in acid-base status were not observed following feeding in either the excreted components or the caudal blood samples; however, a significant alkalosis was observed 8 h post-feeding in the major intestinal blood vein. Significant increases (16-fold) in ammonia excretion were recorded in 36 h post-fed hagfish. Finally, significant post-prandial increases in intestinal mucosal thickness and microvilli length were observed, with mucosal thickness remaining significantly increased throughout 36 h and the microvilli length returning to fasted lengths by 36 h. These results demonstrate the post-feeding physiology of the earliest diverging extant craniate and identify correlations between physiology and hindgut morphology 8 h following feeding.

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This study examined the mechanisms of glucose acquisition in the hindgut of Pacific hagfish (Eptatretus stoutii) using in vitro gut sac techniques. The intestine was determined to have the capacity to digest maltose into glucose along the entirety of the tract, including the foregut. Glucose uptake was biphasic and consisted of a high-affinity, low-capacity concentration-dependent component conforming to Michaelis-Menten kinetics (Km 0.37mM, Jmax 8.48nmol/cm2/h) as well as a diffusive component. There was no observed difference in glucose flux rate along the length of the intestine, similar to other nutrients investigated in the hagfish intestine. A reduced sodium (<1mM) environment did not result in a change in glucose uptake rates, likely due to a functional redundancy of glucose transporters. There was no observed effect of phloretin, yet the sodium glucose-linked transporter (SGLT)-specific inhibitor phlorizin significantly reduced glucose uptake at all concentrations tested (0.0001-1mM). Additionally, the glucose transporter (GLUT) inhibitor cytochalasin b significantly reduced glucose transport rates. The effects of these pharmacological inhibition experiments suggest the presence of multiple types of glucose transport proteins. This study clarifies the uptake strategies used by hagfish to acquire glucose at the intestine and provides insight into the evolution of such transport systems in early-diverging vertebrates.

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Hagfish consume carrion, potentially exposing them to hypoxia, hypercapnia, and high environmental ammonia (HEA). We investigated branchial and cutaneous ammonia handling strategies by which Pacific hagfish (Eptatretus stoutii) tolerate and recover from high ammonia loading. Hagfish were exposed to HEA (20 mmol/l) for 48 h to elevate plasma total ammonia (TAmm) levels before placement into divided chambers for a 4-h recovery period in ammonia-free seawater where ammonia excretion (JAmm) was measured independently in the anterior and posterior compartments. Localized HEA exposures were also conducted by subjecting hagfish to HEA in either the anterior or posterior compartments. During recovery, HEA-exposed animals increased JAmm in both compartments, with the posterior compartment comprising ~20% of the total JAmm compared with ~11% in non-HEA-exposed fish. Plasma TAmm increased substantially when whole hagfish and the posterior regions were exposed to HEA. Alternatively, plasma TAmm did not elevate after anterior localized HEA exposure. JAmm was concentration dependent (0.05-5 mmol/l) across excised skin patches at up to eightfold greater rates than in skin sections that were excised from HEA-exposed hagfish. Skin excised from more posterior regions displayed greater JAmm than those from more anterior regions. Immunohistochemistry with hagfish-specific anti-rhesus glycoprotein type c (α-hRhcg; ammonia transporter) antibody was characterized by staining on the basal aspect of hagfish epidermis while Western blotting demonstrated greater expression of Rhcg in more posterior skin sections. We conclude that cutaneous Rhcg proteins are involved in cutaneous ammonia excretion by Pacific hagfish and that this mechanism could be particularly important during feeding.

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The goal of this study was to identify whether Pacific hagfish (Eptatretus stoutii) possess glucocorticoid and mineralocorticoid responses and to examine the potential role(s) of four key steroids in these responses. Pacific hagfish were injected with varying amounts of cortisol, corticosterone or 11-deoxycorticosterone (DOC) using coconut oil implants and plasma glucose and gill total-ATPase activity were monitored as indices of glucocorticoid and mineralocorticoid responses. Furthermore, we also monitored plasma glucose and 11-deoxycortisol (11-DOC) levels following exhaustive stress (30 min of agitation) or following repeated infusion with SO42-. There were no changes in gill total-ATPase following implantation with any steroid, with only very small statistical increases in plasma glucose noted in hagfish implanted with either DOC (at 20 and 200mgkg-1 at 7 and 4days post-injection, respectively) or corticosterone (at 100mgkg-1 at 7days post-injection). Following exhaustive stress, hagfish displayed a large and sustained increase in plasma glucose. Repeated infusion of SO42- into hagfish caused increases in both plasma glucose levels and SO42- excretion rate suggesting a regulated glucocorticoid and mineralocorticoid response. However, animals under either condition did not show any significant increases in plasma 11-DOC concentrations. Our results suggest that while there are active glucocorticoid and mineralocorticoid responses in hagfish, 11-DOC does not appear to be involved and the identity and primary function of the steroid in hagfish remains to be elucidated.

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The Pacific hagfish (Eptatretus stoutii) has an exceptional ability to both withstand and recover from exposure to high external ammonia (HEA). This tolerance is likely due to the feeding behavior of this scavenger, which feeds on intermittent food falls of carrion (e.g. fish, large marine mammals) during which time it may be exposed to high concentrations of total ammonia (T(Amm)=NH3+NH4(+)) while burrowed inside the decomposing carcass. Here we exposed hagfish to 20 mmol L(-1) T(Amm) for periods of up to 48 h and then let animals recover in ammonia-free seawater. During the 48 h HEA exposure period, plasma T(Amm) increased 100-fold to over 5000 µmol L(-1) while ammonia excretion (J(amm)) was transiently inhibited. This increase in plasma T(Amm) resulted from NH3 influx down massive inwardly directed ΔP(NH3) gradients, which also led to a short-lived metabolic alkalosis. Plasma [T(Amm)] stabilized after 24-48 h, possibly through a reduction in NH3 permeability across the body surface, which lowered NH3 influx. Ammonia balance was subsequently maintained through the re-establishment of J(amm) against an inwardly directed ΔP(NH3). Calculations of the Nernst potential for ammonia strongly indicated that J(amm) was also taking place against a large inwardly directed NH4(+) electrochemical gradient. Recovery from HEA in ammonia-free water was characterized by a large ammonia washout, and the restoration of plasma TAmm concentrations to near control concentrations. Ammonia clearance was also accompanied by a residual metabolic acidosis, which likely offset the ammonia-induced metabolic alkalosis seen in the early stages of HEA exposure. We conclude that restoration of J(amm) by the Pacific hagfish during ammonia exposure likely involves secondary active transport of NH4(+), possibly mediated by Na(+)/NH4(+) (H(+)) exchange.

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Lampreys possess two T-like lymphocyte lineages that express either variable lymphocyte receptor (VLR) A or VLRC antigen receptors. VLRA(+) and VLRC(+) lymphocytes share many similarities with the two principal T-cell lineages of jawed vertebrates expressing the αß and γδ T-cell receptors (TCRs). During the assembly of VLR genes, several types of genomic cassettes are inserted, in step-wise fashion, into incomplete germ-line genes to generate the mature forms of antigen receptor genes. Unexpectedly, the structurally variable components of VLRA and VLRC receptors often possess partially identical sequences; this phenomenon of module sharing between these two VLR isotypes occurs in both lampreys and hagfishes. By contrast, VLRA and VLRC molecules typically do not share their building blocks with the structurally analogous VLRB receptors that are expressed by B-like lymphocytes. Our studies reveal that VLRA and VLRC germ-line genes are situated in close proximity to each other in the lamprey genome and indicate the interspersed arrangement of isotype-specific and shared genomic donor cassettes; these features may facilitate the shared cassette use. The genomic structure of the VLRA/VLRC locus in lampreys is reminiscent of the interspersed nature of the TCRA/TCRD locus in jawed vertebrates that also allows the sharing of some variable gene segments during the recombinatorial assembly of TCR genes.

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Lamin proteins are found in all metazoans. Most non-vertebrate genomes including those of the closest relatives of vertebrates, the cephalochordates and tunicates, encode only a single lamin. In teleosts and tetrapods the number of lamin genes has quadrupled. They can be divided into four sub-types, lmnb1, lmnb2, LIII, and lmna, each characterized by particular features and functional differentiations. Little is known when during vertebrate evolution these features have emerged. Lampreys belong to the Agnatha, the sister group of the Gnathostomata. They split off first within the vertebrate lineage. Analysis of the sea lamprey (Petromyzon marinus) lamin complement presented here, identified three functional lamin genes, one encoding a lamin LIII, indicating that the characteristic gene structure of this subtype had been established prior to the agnathan/gnathostome split. Two other genes encode lamins for which orthology to gnathostome lamins cannot be designated. Search for lamin gene sequences in all vertebrate taxa for which sufficient sequence data are available reveals the evolutionary time frame in which specific features of the vertebrate lamins were established. Structural features characteristic for A-type lamins are not found in the lamprey genome. In contrast, lmna genes are present in all gnathostome lineages suggesting that this gene evolved with the emergence of the gnathostomes. The analysis of lamin gene neighborhoods reveals noticeable similarities between the different vertebrate lamin genes supporting the hypothesis that they emerged due to two rounds of whole genome duplication and makes clear that an orthologous relationship between a particular vertebrate paralog and lamins outside the vertebrate lineage cannot be established.

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Agnathans (jawless vertebrates) occupy a key phylogenetic position for illuminating the evolution of vertebrate anatomy and physiology. Evaluation of the agnathan globin gene repertoire can thus aid efforts to reconstruct the origin and evolution of the globin genes of vertebrates, a superfamily that includes the well-known model proteins hemoglobin and myoglobin. Here, we report a comprehensive analysis of the genome of the sea lamprey (Petromyzon marinus) which revealed 23 intact globin genes and two hemoglobin pseudogenes. Analyses of the genome of the Arctic lamprey (Lethenteron camtschaticum) identified 18 full length and five partial globin gene sequences. The majority of the globin genes in both lamprey species correspond to the known agnathan hemoglobins. Both genomes harbor two copies of globin X, an ancient globin gene that has a broad phylogenetic distribution in the animal kingdom. Surprisingly, we found no evidence for an ortholog of neuroglobin in the lamprey genomes. Expression and phylogenetic analyses identified an ortholog of cytoglobin in the lampreys; in fact, our results indicate that cytoglobin is the only orthologous vertebrate-specific globin that has been retained in both gnathostomes and agnathans. Notably, we also found two globins that are highly expressed in the heart of P. marinus, thus representing functional myoglobins. Both genes have orthologs in L. camtschaticum. Phylogenetic analyses indicate that these heart-expressed globins are not orthologous to the myoglobins of jawed vertebrates (Gnathostomata), but originated independently within the agnathans. The agnathan myoglobin and hemoglobin proteins form a monophyletic group to the exclusion of functionally analogous myoglobins and hemoglobins of gnathostomes, indicating that specialized respiratory proteins for O2 transport in the blood and O2 storage in the striated muscles evolved independently in both lineages. This dual convergence of O2-transport and O2-storage proteins in agnathans and gnathostomes involved the convergent co-option of different precursor proteins in the ancestral globin repertoire of vertebrates.

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