High amino acid diversity and positive selection at a putative coral immunity gene (tachylectin-2)

In a survey of 244 Oculina alleles, we find high nonsynonymous variation and a signature of positive selection, consistent with a putative role in immunity. Using computational protein structure prediction, we generate a structural model of the Oculina protein that closely matches the known structure of tachylectin-2 from the Japanese horseshoe crab (Tachypleus tridentatus), a protein with demonstrated function in microbial recognition and agglutination. We also demonstrate that at least three other genera of anthozoan cnidarians (Acropora, Montastrea and Nematostella) possess proteins structurally similar to tachylectin-2.Taken together, the evidence of high amino acid diversity, positive selection and structural correspondence to the horseshoe crab tachylectin-2 suggests that this protein is 1) part of Oculina's innate immunity repertoire, and 2) evolving adaptively, possibly under selective pressure from coral-associated microorganisms. Tachylectin-2 may serve as a candidate locus to screen coral populations for their capacity to respond adaptively to future environmental change.Host immune systems must be able to recognize a wide range of rapidly evolving microbes; thus, functional variation is a hallmark of responses to potential pathogens and other non-self molecules. Such variation can arise via complex interactions leading to somatic recombination, as in vertebrate immune systems and molluscan fibrinogen-related proteins [1], or more simply via genetic diversity in the host immune system, either at the level of families of genes or alleles at a single locus. Genes used by potential hosts to distinguish self from non-self and to recognize and defend against pathogenic microbes include some of the most genetically variable known, among them those encoding the Major Histocompatibility Complex (MHC) proteins in vertebrates [2] and a histocompatibility protein in tunicates [3], as well as disease resistance (R) proteins [4] and ribonucleases of gametophytic self-