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IgD is considered to be a recently evolved Ig, being previously found only in primates and rodents. Here we describe, from a teleost fish (the channel catfish, Ictalurus punctatus), a novel complex chimeric Ig heavy chain, homologous, in part, to the heavy chain (␦) of IgD. In addition to alternative secretory or membrane-associated C termini, this chimeric molecule contains a rearranged variable domain, the first constant domain of , and seven constant domains encoded by a ␦ gene homolog. Identification of the catfish gene as ␦ is based on the following properties: sequence relatedness to mammalian ␦; a location within the IgH locus that is immediately downstream of the gene; separate terminal exons for the secretory and membrane forms; coexpression with the complete chain in some but not all B cells. These results (i) suggest that IgD is an ancient immunoglobulin that was present in vertebrates ancestral to both the mammals and the ray-finned fishes, and (ii) raise the possibility that this Ig isotype may have served an as yet unidentified important function early in the evolution of the immune system.

Double-stranded RNA (dsRNA) is a common by-product of viral infections and a potent inducer of innate antiviral immune responses in vertebrates.In the marine shrimp Litopenaeus vannamei, innate antiviral immunity is also induced by dsRNA in a sequence-independent manner. In this study, the hypothesis that dsRNA can evoke not only innate antiviral immunity but also a sequence-specific antiviral response in shrimp was tested. It was found that viral sequence-specific dsRNA affords potent antiviral immunity in vivo, implying the involvement of RNA interference (RNAi)-like mechanisms in the antiviral response of the shrimp. Consistent with the activation of RNAi by virus-specific dsRNA, endogenous shrimp genes could be silenced in a systemic fashion by the administration of cognate long dsRNA. While innate antiviral immunity, sequencedependent antiviral protection, and gene silencing could all be induced by injection of long dsRNA molecules, injection of short interfering RNAs failed to induce similar responses, suggesting a size requirement for extracellular dsRNA to engage antiviral mechanisms and gene silencing. We propose a model of antiviral immunity in shrimp by which viral dsRNA engages not only innate immune pathways but also an RNAi-like mechanism to induce potent antiviral responses in vivo.Double-stranded RNA (dsRNA) is a hallmark of viral infections, and thus, it is not surprising that the immune system has evolved the capacity to recognize dsRNA and respond to it by mounting antiviral responses. In vertebrates, these innate antiviral responses rely in part on the recognition of dsRNA by Toll-like receptor 3 and by RNA-dependent protein kinase (32, 47). The consequences of dsRNA recognition include activation of the interferon system, initiation of apoptosis, and inhibition of cellular protein synthesis. From an evolutionary perspective, innate immune activation by dsRNA has long been thought to be exclusive to vertebrates. This view has been encouraged by the fact that genes encoding homologues of interferons, their receptors, and most of the prominent interferon-regulated genes are absent in fully sequenced invertebrate genomes (1, 7, 10, 11). Nevertheless, it is a reasonable expectation that invertebrates should have an innate immune system capable of recognizing dsRNA as a signature of viral infection. A previous study suggested such a capability by demonstrating that exposure of a marine shrimp to dsRNA induced innate antiviral immunity in a sequence-independent manner (36). The mechanisms underlying this phenomenon as well as its occurrence in other invertebrate taxa remain unknown, but it is clear that the recognition of dsRNA by another pathway, RNA interference (RNAi), is widely distributed among invertebrates and likely an important component of the invertebrate antiviral response.RNAi comprises a set of related cellular processes by which dsRNA molecules direct the suppression of gene expression based on sequence homology between the dsRNA trigger and the target gene. The specific mechanisms u...