The Hydractinia Allorecognition Complex encodes a novel family of immunoglobulin superfamily proteins

Invertebrate histocompatibility—also known as allorecognition—has long interested marine ecologists, population geneticists, evolutionary biologists, and immunologists, but its genetic basis remains enigmatic in most species. Here, we report the nearly complete sequence of a histocompatibility complex from the colonial cnidarian, Hydractinia symbiolongicarpus . This sequence reveals that the two known Hydractinia allorecognition genes, Allorecognition 1 ( Alr1 ) and Allorecognition 2 ( Alr2 ) are part of a large family of immunoglobulin superfamily (IgSF) genes, several of which are candidates for new allodeterminants. These genes encode transmembrane proteins with domain architectures similar to cell-adhesion molecules and immune receptors. Several also contain cytoplasmic immunoreceptor tyrosine-based activation motifs (ITAMs) and immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Our data, which include highly accurate protein structure predictions, reveal that these proteins have V-set and I-set domains with unusual sequence signatures. This suggests the last common ancestor of cnidarians and bilaterians had distinct V-set and I-set Ig domains. ### Competing Interest Statement The authors have declared no competing interest.