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Diversification of innate immune genes: lessons from the purple sea urchin

Pathogen diversification can alter infection virulence, which in turn drives the evolution of host immune diversification, resulting in countermeasures for survival in this arms race. Somatic recombination of the immunoglobulin gene family members is a very effective mechanism to diversify antibodies and T-cell receptors that function in the adaptive immune system. Although mechanisms to diversify innate immune genes are not clearly understood, a seemingly unlikely source for insight into innate immune diversification may be derived from the purple sea urchin, which has recently had its genome sequenced and annotated. Although there are many differences, some characteristics of the sea urchin make for a useful tool to understand the human immune system. The sea urchin is phylogenetically related to humans although, as a group, sea urchins are evolutionarily much older than mammals. Humans require both adaptive and innate immune responses to survive immune challenges, whereas sea urchins only require innate immune functions. Genes that function in immunity tend to be members of families, and the sea urchin has several innate immune gene families. One of these is the Sp185/333 gene family with about 50 clustered members that encode a diverse array of putative immune response proteins. Understanding gene diversification in the Sp185/333 family in the sea urchin may illuminate new mechanisms of diversification that could apply to gene families that function in innate immunity in humans, such as the killer immunoglobulin-like receptor genes.