The bowfin genome illuminates the developmental evolution of ray-finned fishes

The bowfin (Amia calva) is a ray-finned fish that possesses a unique suite of ancestral and derived phenotypes, which are key to understanding vertebrate evolution. The phylogenetic position of bowfin as a representative of neopterygian fishes, its archetypical body plan and its unduplicated and slowly evolving genome make bowfin a central species for the genomic exploration of ray-finned fishes. Here we present a chromosome-level genome assembly for bowfin that enables gene-order analyses, settling long-debated neopterygian phylogenetic relationships. We examine chromatin accessibility and gene expression through bowfin development to investigate the evolution of immune, scale, respiratory and fin skeletal systems and identify hundreds of gene-regulatory loci conserved across vertebrates. These resources connect developmental evolution among bony fishes, further highlighting the bowfin's importance for illuminating vertebrate biology and diversity in the genomic era.

Evolution of IFN subgroups in bony fish - 1:Group I-III IFN exist in early ray-finned fish, with group II IFN subgroups present in the Holostean spotted gar, Lepisosteus oculatus

The present study helps clarify when the fish type I IFN groups/subgroups evolved, by examination of the IFN genes present in the Holostean spotted gar, Lepisosteus oculatus, in relation to the IFN genes present in the Chondrostea (sturgeon). It confirms that all three IFN groups (I-III), and group II subgroups, existed prior to the appearance of teleost fish. Preliminary expression analysis in a gar cell line (GARL) suggests these IFN genes will have a role in antiviral defence in Holostean fish, in that they are induced by poly(I:C). A refined model of IFN evolution within the actinopterygian fish is proposed.

Intestinal granular cells of a cartilaginous fish, thornback ray Raja clavata: Morphological characterization and expression of different molecules

This investigation aims to fill gaps in our understanding of the intestinal immune cells of elasmobranchs. Whole digestive tracts of fifteen thornback ray Raja clavata were provided by a trawl fleet from the Gulf of Asinara (Sardinia, western Mediterranean Sea). Histochemical, immunohistochemical and ultrastructural observations were conducted on the spiral intestine. Three types of granular cells were identified; type I in epithelium, types II and III in lamina propria-submucosa, with each of them containing cytoplasmic granules with different ultrastructural characteristics. Data on size and density of each granular cell type are provided. Immunostaining of intestinal sections showed the reactivity of the granular cells: type I cells were positive for lysozyme, mast cell tryptase and tumor necrosis factor-ɑ based on antibody staining; type III cells were immune-reactive to anti-interleukin 6 antibody, whilst type II cells were negative to all the antibodies used. Comparison of each granular cell type with immune cells of teleosts or mammals and an hypothesis on their nature and function are reported. A potential role for granular cells in intestinal cellular immunity is also discussed with respect to type I and type III cells having similarities to Paneth cells and neutrophils, respectively.

Lesions associated with plerocerci (Platyhelminthes: Cestoda: Trypanorhyncha) in the gastric wall of a cownose ray, Rhinoptera bonasus (Mitchill), (Myliobatiformes: Rhinopteridae) from the northern Gulf of Mexico

We describe lesions associated with a seemingly intense infection of trypanorhynch plerocerci (Platyhelminthes: Cestoda: Trypanorhyncha) in the gastric wall of a female cownose ray, Rhinoptera bonasus (Myliobatiformes: Rhinopteridae) captured in the northern Gulf of Mexico. Grossly, the multitude of encapsulated, encysted plerocerci imparted a bumpy and cobbled appearance to the serosa of the stomach, and none was observed in any other tissue during routine parasitological necropsy. Histologically, the plerocerci were associated with severe intramural granulomatous gastritis, vascular ectasia and mesothelial polyposis with the exclusion of the mucosa. To our knowledge, this is the first published case study documenting platyhelminth-associated histopathological changes in the gastrointestinal tract of R. bonasus as well as that of the efficacy of immunocytochemical markers for smooth muscle actin, Factor VIII, S-100, and proliferating cell nuclear antigen in Myliobatiformes. It also may serve as a potential primer for much needed ecological investigations regarding the potential role of elasmobranchs as intermediate or 'paratenic' hosts in the life cycles of trypanorhynch cestodes.

Purification and characterization of parvalbumins, the major allergens in red stingray (Dasyatis akajei)

Fish has received increasing attention because it induces IgE-mediated food allergy. Parvalbumin (PV) represents the major allergen of fish, and IgE cross-reactivity to PV in various teleost fish species has been shown, while little information is available about allergens in elasmobranch fish. In this study, two PV isoforms (named as PV-I and PV-II) from red stingray (Dasyatis akajei) were purified to homogeneity by a series of procedures including ammonium sulfate precipitation and column chromatographies of DEAE-Sepharose and Sephacryl S-200. Purified PVs revealed a single band on tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular masses of PV-I and PV-II were 12.29 and 11.95 kDa, respectively, as determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Western blot using antifrog PV monoclonal antibody (PARV-19) showed positive reactions to the two proteins, confirming that they were PVs, although their immunological reactivities were weaker than those of PV from silver carp. The N-terminal amino acid sequence of PV-I was determined, and comparison with PVs from other fish species showed low homology between teleost and elasmobranch fish. The isoelectric points of PV-I and PV-II were 5.4 and 5.0, respectively, as determined by two-dimensional electrophoresis (2-DE), suggesting that both isoforms belong to the α-group. IgE immunoblotting analysis showed that sera from fish-allergic patients reacted to both PV-I and PV-II from red stingray. Thermal stability revealed that PV-I easily formed oligomers than PV-II, which might contribute to the maintenance of its allerginicity during heat processing.

Influence of reproductive activity, sex steroids, and seasonality on epigonal organ cellular proliferation in the skate (Leucoraja erinacea)

In elasmobranchs, the epigonal organ, a unique leukopoietic immune tissue, is associated with the gonads. As the ovaries increase in size during reproductive activity, the overall mass of the epigonal organ does not change. However, immunohistochemistry (proliferating cell nuclear antigen Ab) demonstrated more proliferative activity and extravasation of epigonal leukocytes from blood vessels in reproductively active (RA) skates (Leucoraja erinacea) than in non-reproductively active (NRA) skates. In addition, [(3)H]thymidine incorporation was greater in epigonal leukocytes from RA skates than in leukocytes from NRA skates. Plasma from RA skates, but not from NRA skates, increased proliferation of epigonal leukocytes in vitro, an effect that was not seen using steroid-free plasma. In contrast to the stimulatory effect of plasma on leukocyte proliferation, addition of steroids (estrogen, progesterone, testosterone, and dexamethasone) in vitro decreased [(3)H]thymidine incorporation. While the inhibitory response to steroids was seasonally variable, (3)[H]thymidine incorporation was always highest in RA animals, in which plasma steroid levels were also consistently highest. These studies suggest functional interactions between reproductive and immune tissues in the skate, and that cellular turnover in epigonal tissue may be influenced by gonadal activity.

In vivo exposure of clearnose skates, Raja eglanteria, to ionising X-radiation: acute effects on the peripheral blood, spleen, and epigonal and Leydig organs

The effects of ionising radiation on the peripheral blood, spleen, and epigonal and Leydig organs of cartilaginous fishes were investigated using juvenile clearnose skates, Raja eglanteria. Skates (N = 80) were sacrificed 12 days after exposure to 0-75 Gy of X-radiation, and morphometrics (body mass, disc width, total length), mass of spleens and epigonal organs, and peripheral blood leucocyte (PBL) counts were compared to controls using ANOVA. Spleen and epigonal organ mass and PBL counts declined logarithmically as a function of radiation dose. To assess recovery from X-radiation, skates (N = 40) were exposed to 0, 9 or 18 Gy and sacrificed when moribund or on days 10, 20, 30 and 40 post-irradiation. Partial recovery of Leydig organ and splenic red pulp was evident after 40 days in skates exposed to 9 Gy, but no indication of recovery was apparent at higher doses. Median lethal dose by 30 days (LD50/30) was calculated to be 9-18 Gy, similar to that determined for other fishes.

Ancient divergence of a complex family of immune-type receptor genes

Multigene families of activating/inhibitory receptors belonging to the immunoglobulin superfamily (IgSF) regulate immunological and other cell-cell interactions. A new family of such genes, termed modular domain immune-type receptors (MDIRs), has been identified in the clearnose skate (Raja eglanteria), a phylogenetically ancient vertebrate. At least five different major forms of predicted MDIR proteins are comprised of four different subfamilies of IgSF ectodomains of the intermediate (I)- or C2-set. The predicted number of individual IgSF ectodomains in MDIRs varies from one to six. MDIR1 contains a positively charged transmembrane residue and MDIR2 and MDIR3 each possesses at least one immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic regions. MDIR4 and MDIR5 lack characteristic activating/inhibitory signalling motifs. MDIRs are encoded in a particularly large and complex multigene family. MDIR domains exhibit distant sequence similarity to mammalian CMRF-35-like molecules, polymeric immunoglobulin receptors, triggering receptors expressed on myeloid cells (TREMs), TREM-like transcripts, NKp44 and FcR homologs, as well as to sequences identified in several different vertebrate genomes. Phylogenetic analyses suggest that MDIRs are representative members of an extended family of IgSF genes that diverged before or very early in evolution of the vertebrates and subsequently came to occupy multiple, fully independent distributions in the present day.

Expression of individual immunoglobulin genes occurs in an unusual system consisting of multiple independent loci

Humoral immunity is effected through the rearrangement of immunoglobulin (Ig) genes in individual somatic cells committed to the B lymphocyte lineage. Haplotype or allelic exclusion restricts B lymphocytes to the expression of a single Ig receptor that can sustain further somatic modification. In most species, a specific Ig chain is encoded at a single genetic locus. However, in cartilaginous fish, hundreds of independent Ig heavy- (IgH) and Ig light-chain (IgL) gene loci are present, many of which are joined in the germ line. Ig gene transcripts have been amplified from single peripheral blood lymphocytes isolated from the clearnose skate (Raja eglanteria) using reverse-transcription PCR, and a single productive IgH transcript was detected in the majority of cells analyzed. Similarly, only a single IgL transcript was detected in over half of the individual cells. Taken together, these findings suggest that a mechanism for haplotype exclusion arose early in the evolution of antibody diversity and is independent of a single genetic locus.

Evolutionary origins of lymphocytes: ensembles of T cell and B cell transcriptional regulators in a cartilaginous fish

The evolutionary origins of lymphocytes can be traced by phylogenetic comparisons of key features. Homologs of rearranging TCR and Ig (B cell receptor) genes are present in jawed vertebrates, but have not been identified in other animal groups. In contrast, most of the transcription factors that are essential for the development of mammalian T and B lymphocytes belong to multigene families that are represented by members in the majority of the metazoans, providing a potential bridge to prevertebrate ancestral roles. This work investigates the structure and regulation of homologs of specific transcription factors known to regulate mammalian T and B cell development in a representative of the earliest diverging jawed vertebrates, the clearnose skate (Raja eglanteria). Skate orthologs of mammalian GATA-3, GATA-1, EBF-1, Pax-5, Pax-6, Runx2, and Runx3 have been characterized. GATA-3, Pax-5, Runx3, EBF-1, Spi-C, and most members of the Ikaros family are shown throughout ontogeny to be 1) coregulated with TCR or Ig expression, and 2) coexpressed with each other in combinations that for the most part correspond to known mouse T and B cell patterns, supporting conservation of function. These results indicate that multiple components of the gene regulatory networks that operate in mammalian T cell and B cell development were present in the common ancestor of the mammals and the cartilaginous fish. However, certain factors relevant to the B lineage differ in their tissue-specific expression patterns from their mouse counterparts, suggesting expanded or divergent B lineage characteristics or tissue specificity in these animals.

Complex expression patterns of lymphocyte-specific genes during the development of cartilaginous fish implicate unique lymphoid tissues in generating an immune repertoire

Cartilaginous fish express canonical B and T cell recognition genes, but their lymphoid organs and lymphocyte development have been poorly defined. Here, the expression of Ig, TCR, recombination-activating gene (Rag)-1 and terminal deoxynucleosidase (TdT) genes has been used to identify roles of various lymphoid tissues throughout development in the cartilaginous fish, Raja eglanteria (clearnose skate). In embryogenesis, Ig and TCR genes are sharply up-regulated at 8 weeks of development. At this stage TCR and TdT expression is limited to the thymus; later, TCR gene expression appears in peripheral sites in hatchlings and adults, suggesting that the thymus is a source of T cells as in mammals. B cell gene expression indicates more complex roles for the spleen and two special organs of cartilaginous fish-the Leydig and epigonal (gonad-associated) organs. In the adult, the Leydig organ is the site of the highest IgM and IgX expression. However, the spleen is the first site of IgM expression, while IgX is expressed first in gonad, liver, Leydig and even thymus. Distinctive spatiotemporal patterns of Ig light chain gene expression also are seen. A subset of Ig genes is pre-rearranged in the germline of the cartilaginous fish, making expression possible without rearrangement. To assess whether this allows differential developmental regulation, IgM and IgX heavy chain cDNA sequences from specific tissues and developmental stages have been compared with known germline-joined genomic sequences. Both non-productively rearranged genes and germline-joined genes are transcribed in the embryo and hatchling, but not in the adult.

Evolution of hematopoiesis: Three members of the PU.1 transcription factor family in a cartilaginous fish, Raja eglanteria

T lymphocytes and B lymphocytes are present in jawed vertebrates, including cartilaginous fishes, but not in jawless vertebrates or invertebrates. The origins of these lineages may be understood in terms of evolutionary changes in the structure and regulation of transcription factors that control lymphocyte development, such as PU.1. The identification and characterization of three members of the PU.1 family of transcription factors in a cartilaginous fish, Raja eglanteria, are described here. Two of these genes are orthologs of mammalian PU.1 and Spi-C, respectively, whereas the third gene, Spi-D, is a different family member. In addition, a PU.1-like gene has been identified in a jawless vertebrate, Petromyzon marinus (sea lamprey). Both DNA-binding and transactivation domains are highly conserved between mammalian and skate PU.1, in marked contrast to lamprey Spi, in which similarity is evident only in the DNA-binding domain. Phylogenetic analysis of sequence data suggests that the appearance of Spi-C may predate the divergence of the jawed and jawless vertebrates and that Spi-D arose before the divergence of the cartilaginous fish from the lineage leading to the mammals. The tissue-specific expression patterns of skate PU.1 and Spi-C suggest that these genes share regulatory as well as structural properties with their mammalian orthologs.

Members of the Ikaros gene family are present in early representative vertebrates

Members of the Ikaros multigene family of zinc finger proteins are expressed in a tissue-specific manner and most are critical determinants in the development of both the B and T lymphocytes as well as NK and dendritic APC lineages. A PCR amplification strategy that is based on regions of shared sequence identity in Ikaros multigene family members found in mammals and several other vertebrates has led to the recovery of cDNAs that represent the orthologues of Ikaros, Aiolos, Helios, and Eos in Raja eglanteria (clearnose skate), a cartilaginous fish that is representative of an early divergence event in the phylogenetic diversification of the vertebrates. The tissue-specific patterns of expression for at least two of the four Ikaros family members in skate resemble the patterns observed in mammals, i.e., in hematopoietic tissues. Prominent expression of Ikaros in skate also is found in the lymphoid Leydig organ and epigonal tissues, which are unique to cartilaginous fish. An Ikaros-related gene has been identified in Petromyzon marinus (sea lamprey), a jawless vertebrate species, in which neither Ig nor TCRs have been identified. In addition to establishing a high degree of evolutionary conservation of the Ikaros multigene family from cartilaginous fish through mammals, these studies define a possible link between factors that regulate the differentiation of immune-type cells in the jawed vertebrates and related factors of unknown function in jawless vertebrates.

A long form of the skate IgX gene exhibits a striking resemblance to the new shark IgW and IgNARC genes

Differential screening has been used to identify cDNAs encoding a long form of IgX in Raja eglanteria (clearnose skate). Comparisons of the IgX long form with the previously described short-form IgX cDNAs and the genomic IgX locus indicate that the V and two 5' C regions of the short and long forms of IgX are >90% identical at the nucleotide level. Differences between the V sequences of the long- and short-form IgX genes are concentrated in complementarity determining regions, suggesting that these forms are derived through alternative splicing of the same genomic loci or transcription of highly related loci. The extreme conservation of nucleotide sequence, including third position codons, among different cDNAs as well as the near identity of nucleotide sequence in the intervening sequences of germline IgX, IgX short-form sterile transcripts and IgX long-form sterile transcripts indicate that the multiple IgX loci are recently diverged from one another and/or are under intense gene correction. Phylogenetic analyses of the known cartilaginous fish immunoglobulin loci demonstrate that the long form of IgX is orthologous to IgW/IgNARC (NARC) and is most consistent with: 1) the divergence of the IgX/IgW/NARC and IgM-like loci from a common ancestral locus prior to the divergence of the cartilaginous/bony fish lineages and 2) the divergence of the NAR locus from the IgX/IgW/NARC gene(s) after the cartilaginous/bony fish split but prior to the shark/skate split, approximately 220 million years ago.

alpha, beta, gamma, and delta T cell antigen receptor genes arose early in vertebrate phylogeny

A series of products were amplified using a PCR strategy based on short minimally degenerate primers and R. eglanteria (clearnose skate) spleen cDNA as template. These products were used as probes to select corresponding cDNAs from a spleen cDNA library. The cDNA sequences exhibit significant identity with prototypic (alpha, beta, gamma, and delta T cell antigen receptor (TCR) genes. Characterization of cDNAs reveals extensive variable region diversity, putative diversity segments, and varying degrees of junctional diversification. This demonstrates expression of both alpha/beta and gamma/delta TCR genes at an early level of vertebrate phylogeny and indicates that the three major known classes of rearranging antigen receptors were present in the common ancestor of the present-day jawed vertebrates.

Immunohistochemical localization and biological significance of the phylogenically conserved thymus-brain antigen (UB-13 antigen) in skate, rat and human

A monoclonal antibody (UB-13) originally raised against the brain of the skate (Raja kenojei, a cartilaginous-fish) was found to react with lymphoid and brain tissues from many species when examined immunohistochemically. In rat and human thymus, UB-13 antigen was observed to be closely associated with reticular tissue in the medulla and cortex. Interestingly, a few or several thymocytes were encircled by the UB-13-reactive reticular tissue. At 14 days gestation, rat thymus consisted mainly of reticular epithelial tissue, after which strong thymocyte production started. At this stage, some of the reticular tissue was heavily stained with UB-13. In the thymus tissues of the irradiated and recovering rats, where reduction and massive reproduction of thymocytes were observed, extensive UB-13 antigen expression localized on the reticular epithelial tissue, an observation which may support the thymocyte re-population. These findings suggest that the antigen recognized by UB-13 may be important for thymocyte proliferation and maturation. UB-13 antigen was found in the fibrous structure of the molecular and granular layer of the human cerebellum. Some glial cells were also stained strongly with UB-13 in the human cerebellar or cerebral grey and white matter. In rat, glial cells, especially astroglias, and the endothelial structure of blood vessels were stained strongly with UB-13. These findings suggest that UB-13 may be a useful monoclonal antibody for analysis of brain-lymphoid antigen in many species.

Generation of immunoglobulin light chain gene diversity in Raja erinacea is not associated with somatic rearrangement, an exception to a central paradigm of B cell immunity

In all vertebrate species examined to date, rearrangement and somatic modification of gene segmental elements that encode portions of the antigen-combining sites of immunoglobulins are integral components of the generation of antibody diversity. In the phylogenetically primitive cartilaginous fishes, gene segments encoding immunoglobulin heavy and light chain loci are arranged in multiple clusters, in which segmental elements are separated by only 300-400 bp. In some cases, segmental elements are joined in the germline of nonlymphoid cells (joined genes). Both genomic library screening and direct amplification of genomic DNA have been used to characterize at least 89 different type I light chain gene clusters in the skate, Raja. Analyses of predicted nucleotide sequences and predicted peptide structures are consistent with the distribution of genes into different sequence groups. Predicted amino acid sequence differences are preferentially distributed in complementarity-determining versus framework regions, and replacement-type substitutions exceed neutral substitutions. When specific germline sequences are related to the sequences of individual cDNAs, it is apparent that the joined genes are expressed and are potentially somatically mutated. No evidence was found for the presence of any type I light chain gene in Raja that is not germline joined. The type I light chain gene clusters in Raja appear to represent a novel gene system in which combinatorial and junctional diversity are absent.

Structural conservation of hypervariable regions in immunoglobulins evolution

Analysis of human and mouse immunoglobulins has shown that five of six hypervariable regions that form the antigen binding site have a small repertoire of main chain conformations (canonical structures). Cartilaginous fishes are the most distantly related species to humans known to have an immune system, their evolutionary lines having diverged 450 million years ago. An analysis of VH and V kappa sequences from these fishes shows that all the main chain structures in their L1, L2, H1 and H2 hypervariable regions, and one of those in the L3 region, are the same as those most commonly found in human and mouse. This implies that the canonical structures occurring most commonly in hypervariable regions arose very early in the stages of the evolution of the immune system.

Complete genomic sequence and patterns of transcription of a member of an unusual family of closely related, chromosomally dispersed Ig gene clusters in Raja

Ig genes in cartilaginous fish are organized in clusters. This unique form of organization suggests major differences in the regulation of the segmental rearrangement mechanism from that found in mammals and other higher vertebrates. The complete DNA sequence of an IgX-type cluster in the species Raja eglanteria is defined, and shown to consist of four rearranging segmental elements and three constant region exons. Using fluorescence in situ hybridization it is shown that Raja clusters are present at multiple sites within the genome, and that there is no apparent relationship between the chromosomally dispersed IgX clusters and a second isotype (IgM type) in this species. Comprehensive examination of sequence motifs associated with transcription regulation reveals an abundance of short sequences closely resembling those found in higher vertebrate Ig and other genes. However, the linear relationship of these motifs differs markedly from that associated with regulation of expression of the mammalian Ig gene locus. Additional studies of the transcription products of the IgX gene loci emphasize the diversity of transcription and processing of these genes. Considerable variation was noted in the processing of putative IgX transcripts, including the detection of a heretofore unrecognized form containing at least four additional Ig-like domains. These results have profound significance in terms of understanding the selective expression and the evolutionary diversification of Ig genes.

Immunoglobulin light chain class multiplicity and alternative organizational forms in early vertebrate phylogeny

The prototypic chondrichthyan immunoglobulin (Ig) light chain type (type I) isolated from Heterodontus francisci (horned shark) has a clustered organization in which variable (V), joining (J), and constant (C) elements are in relatively close linkage (V-J-C). Using a polymerase chain reaction-based approach on a light chain peptide sequence from the holocephalan, Hydrolagus colliei (spotted ratfish), it was possible to isolate members of a second light chain gene family. A probe to this light chain (type II) detects homologs in two orders of elasmobranchs, Heterodontus, a galeomorph and Raja erinacea (little skate), a batoid, suggesting that this light chain type may be present throughout the cartilaginous fishes. In all cases, V, J, and C regions of the type II gene are arranged in closely linked clusters typical of all known Ig genes in cartilaginous fishes. All representatives of this type II gene family are joined in the germline. A third (kappa-like) light chain type from Heterodontus is described. These findings establish that a degree of light chain class complexity comparable to that of the mammals is present in the most phylogenetically distant extant jawed vertebrates and that the phenomenon of germline-joined (pre-rearranged) genes, described originally in the heavy chain genes of cartilaginous fishes, extends to light chain genes.

Two distinct immunoglobulin heavy chain isotypes in a primitive, cartilaginous fish, Raja erinacea

Immunoglobulin heavy chain genes in Raja erinacea (little skate) are organized in clusters consisting of VH, DH, JH segments and CH exons (1). An immunoglobulin heavy chain mu-like isotype that exhibits 61-91% nucleotide sequence identity in coding segments to the Heterodontus francisci (horned shark) mu-type immunoglobulin is described. The overall length of the mu-type clusters is approximately 16 kb; transmembrane exons (TM1 and TM2) are located 3 to CH exon 4 (CH4). In three of four TM-containing genomic clones, a significant deletion is present in TM1. A second isotype of Raja immunoglobulin heavy chain genes has been detected by screening a spleen cDNA library with homologous Raja VH- and CH1-specific probes complementing the respective regions of the mu-like isotype. Weak hybridization with VH-specific probes and no discernable hybridization with C mu-specific probes were considered presumptive evidence for a second immunoglobulin isotype that nominally is designated as X-type. The Vx region of the X-type cDNA is approximately 60% identical at the nucleotide (nt) level to other Raja VH segments and thus represents a second VH family. Putative Dx and Jx sequences also have been identified. The constant region of the X-type immunoglobulin heavy chain gene consists of two characteristic immunoglobulin domains and a cysteine-rich carboxy terminal segment that are only partially homologous with the mu-like isotype. Genomic Southern blotting indicates that the V and C segments of both immunoglobulin heavy chain isotypes are encoded by complex multigene families. Vx- and different Cx-specific probes hybridize to different length transcripts in northern blot analyses of Raja spleen RNA suggesting that the regulation of expression of the X-type genes may involve differential RNA processing.

The second immunoglobulin class is commonly present in cartilaginous fish belonging to the order Rajiformes

Six species of cartilaginous fish distributed into four orders, Rajiformes (skates and guitarfishes), Myliobatiformes (rays), Heterodontiformes (sharks) and Carcharhiniformes (sharks), were investigated for the possible presence of a second class of immunoglobulin (Ig) other than IgM. Among those orders, fish belonging to the order Rajiformes were found to have a second Ig (IgR) with a non-covalently associated dimeric structure in which the H chain was different from that of IgM in mol. wt and antigenicity. Cartilaginous fish belonging to the other orders investigated had only one class of IgM.

Surface immunoglobulins on the lymphocytes of the skate Raja naevus

Living lymphocytes of the Elasmobranch fish, Raja naevus, have been examined for surface immunoglobulin (Ig) by treatment with a fluorescent anti-Ig system. Large numbers of Ig-positive cells (60-80%) were found in peripheral blood, spleen and thymus. Following modulation of the surface Ig with anti-Ig, resynthesis occurred, showing that the surface Ig is a product of the individual lymphocytes rather than material passively absorbed from the serum. Formation of caps was independent of temperature, occurring as readily at 4 degrees C as at 20 degrees C, a finding which presumably reflects the environmental conditions normally experienced by the skate. The presence of Ig-bearing lymphocytes in the adult skate thymus suggests a similarity to the amphibian larval thymus, which may be a primary lymphoid organ for the production of both T and B lymphocyte analogues.