Pathgenomics of the Oyster and Transcriptomic Infection Responses Reveals a Highly Evolved Innate Immune System

The Pacific oyster (Crassostrea gigas) belongs to one of the most species-rich but genomically poorly explored phyla, the Mollusca. Comparative analysis of the Pacific oyster genome has broad implications for the state of invertebrate genetics underpinnings of immunity. Both the deuterostome amphioxus and the sea urchin has been reported with a complex innate immune gene repertoire in their genome, and we found the protostome C. gigas genome also encodes large number of innate immune gene repertoire. C. gigas has an expanded innate receptor repertoire including 83 Toll-like receptors, 71 scavenger receptors, 11 RIG-I-like receptors, 273 C-type lectins, 333 ghC1q domain containing proteins and 205 fibrinogen-related proteins. Besides receptors, genes involved in signal transduction including Myeloid differentiation primary response gene (88), TNF receptor associated factors were also over-represented in oyster genome. The Pacific oyster genome also has a complex TNF and apoptosis systems, the expand genes including with TNFs, Inhibitor of apoptosis proteins, effector caspase. Transcriptomic analysis of C. gigas genes response to vibrio bacterial, perkinsus marinus and virus revealed common and distinct reponses to these pathogen types. Expressional profile analysis of immune-related genes in different tissues suggested gill and digestive gland were major tissues of immunity in C. gigas. Our finding provides support for the hypothesis that species lack adaptive immunity expanded vast number of innate immunity repertoire no matter in deuterostome or protostome. Besides, most of the immune-related genes annotated by homolog alignment showed differential expression under pathogen challenge, suggesting functional conservation may exist in protostome and model vertebrates.