Multigene families of immunoglobulin domain-containing innate immune receptors in zebrafish: Deciphering the differences

• Ig domain-containing innate immune receptor multi-gene families are present in bony fish.
• Orthologs for many of these receptors cannot be identified in higher vertebrates.
• Receptors include inhibitory, activating, functionally ambiguous and secreted forms.
• The genes encoding these receptors display polymorphic and haplotypic variation.

Five large multigene families encoding innate-type immune receptors that are comprised of immunoglobulin domains have been identified in bony fish, of which four do not possess definable mammalian orthologs. The members of some of the multigene families exhibit unusually extensive patterns of divergence and the individual family members demonstrate marked variation in interspecific comparisons. As a group, the gene families reveal striking differences in domain type and content, mechanisms of intracellular signaling, basic structural features, haplotype and allelic variation and ligand binding. The potential functional roles of these innate immune receptors, their relationships to immune genes in higher vertebrate species and the basis for their adaptive evolution are of broad interest. Ongoing investigations are expected to provide new insight into alternative mechanisms of immunity.