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.

This study examined the response properties of skate electrosensory primary afferent neurons of pre-hatch embryo (8-11 weeks), post-hatch juvenile (1-8 months), and adult (> 2 year) clearnose skates (Raja eglanteria) to determine whether encoding of electrosensory information changes with age, and if the electrosense is adapted to encode natural bioelectric stimuli across life history stages. During ontogeny, electrosensory primary afferents increase resting discharge rate, spike regularity, and sensitivity at best frequency. Best frequency was at 1-2 Hz for embryos, showed an upwards shift to 5 Hz in juveniles, and a downward shift to 2-3 HZ in adults. Encapsulated embryos exhibit ventilatory movements that are interrupted by a "freeze response" when presented with weak uniform fields at 0.5 and 1 Hz. This phasic electric stimulus contains spectral information found in potentials produced by natural fish predators, and therefore indicates that the embryo electrosense can efficiently mediate predator detection and avoidance. In contrast, reproductively active adult clearnose skates discharge their electric organs at rates near the peak frequency sensitivity of the adult electrosensory system, which; facilitates electric communication during social behavior. We suggest that life-history-dependent functions such as these may shape the evolution of the low-frequency response properties for the elasmobranch electrosensory system.

Summary Ampullary organ electroreceptors excited by weak cathodal electric fields are used for hunting by both cartilaginous and non-teleost bony fishes. Despite similarities of neurophysiology and innervation, their embryonic origins remain controversial: bony fish ampullary organs are derived from lateral line placodes, while a neural crest origin has been proposed for cartilaginous fish electroreceptors. This calls into question the homology of electroreceptors and ampullary organs in the two lineages of jawed vertebrates. Here, we test the hypothesis that lateral line placodes form electroreceptors in cartilaginous fishes by undertaking the first long-term in vivo fate-mapping study in any cartilaginous fish. Using DiI-tracing for up to 70 days in the little skate, Leucoraja erinacea, we show that lateral line placodes form both ampullary electroreceptors and mechanosensory neuromasts. These data confirm the homology of electroreceptors and ampullary organs in cartilaginous and non-teleost bony fishes and indicate that jawed vertebrates primitively possessed a lateral line placode-derived system of electrosensory ampullary organs and mechanosensory neuromasts.

The olfactory organ of juvenile clearnose skates (Raja eglanteria) was studied with the light and electron microscopes. The organ is ovoid in shape, and its free surface is complicated by the presence of some 20 lamellae. Each lamella has a folded surface lined by a typical neurosensory olfactory epithelium. Bipolar olfactory receptor neurons, ciliated sustentacular cells, and basal cells are the pre-eminent cellular components of the epithelium. Two types of receptor neurons, both bearing microvilli but not cilia, were identified. The type 1 neuron is similar to that previously described in other fishes. The type 2 neuron has a characteristic morphology justifying a separate description. Its dendritic knob is larger than that of type 1, and its microvilli, which are shorter and thicker, are straight and regularly arranged. Tight bundles of filaments provide a skeleton to each microvillus, and these filament bundles reach more than 5 microns down into the dendrite. Type 2 receptor neurons have a well-developed Golgi complex and sparse rough endoplasmic reticulum (rER), whereas type 1 receptor neurons have a less well-developed Golgi complex and a conspicuous system of rER lamellae. The mucous layer on the epithelial surface is provided by the secretion of goblet cells that are situated mostly in the peripheral regions of each lamella. Secretory granules in the sustentacular cells and glands in the lamina propria were not observed.

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.