Abstract
The vertebrate immune system is distinguished by an unusual propensity for genetic invention. For example, three forms of programmed somatic DNA recombination (V-D-J recombination, class switching and a highly targeted form of gene conversion) have arisen entirely independently in the course of immunoglobulin (Ig) heavy-chain gene evolution. Similar phenomena are virtually unknown in other metazoan genetic systems. Genes that mediate immunity are further characterized by accelerated sequence divergence rates when compared to nonimmune genes in studies of mouse and human gene orthologs (Hughes 1997; Murphy 1993). Both of these attributes, the tendency towards mechanistic novelty and a high rate of sequence evolution, may emerge from the dynamic nature of host-pathogen interactions and thus be a universal characteristic of immune systems. To investigate this possibility it is necessary to characterize immunity in animal phyla where the vertebrate forms of adaptive immunity are absent. A number of molecular advances have been made in recent years in the study of arthropod immunity (e.g., Hoffmann et al. 1996; Iwanaga and Kawabata 1998). As these data accumulate, in combination with similar work on an invertebrate deuterostome that is described here, a more general understanding of immunity will emerge.
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