The evolution of immunity: a low-life perspective.

Several of the key genes and pathways of vertebrate immunity appear to have much earlier origins than has been assumed previously and are present in some of the simplest of true animals. Surveys of recently released whole-genome sequences and large EST (expressed sequence tag) datasets imply that both the canonical Toll/Toll-like receptor (TLR) pathway and a prototypic complement-effector pathway, involving C3 and several membrane attack complex-perforin proteins, are present in corals and sea anemones, members of the basal phylum Cnidaria. However, both pathways are likely to have degenerated substantially in Hydra, leaving open the molecular mechanism by which antimicrobial activities are induced in this cnidarian. Surprisingly, the cnidarian genomes also encode a protein related to deuterostome RAG1 (recombination activation gene 1). The finding that RAG1 is likely to have originated from a Transib transposase implies that it might be possible to use in silico approaches to identify its target loci in 'lower' animals.