Meeting Abstract
Allorecognition is the ability to distinguish between self tissues from those of conspecifics via cell-cell contact. This ability is shared by most reef-dwelling colonial invertebrates, including sponges, corals, hydroids, bryozoans, and colonial ascidians. Upon contact, compatible colonies typically fuse or peacefully coexist, while incompatible colonies compete for space, often aggressively. Despite the importance of allorecognition in determining the life history of colonial organisms, how colonies are able to distinguish self from nonself remains largely unknown. Here we report a mechanism for allorecognition specificity in a cnidarian model of allorecognition, Hydractinia symbiolongicarpus. Allorecognition in Hydractinia is controlled by at least two histocompatibility genes, alr1 and alr2, which encode transmembrane proteins. The extracellular regions of these proteins are highly polymorphic and their sequence determines allorecognition outcomes. Colonies with matching extracellular domains fuse, while colonies with different extracellular domains reject. Using in vitro assays, we demonstrate that alr1 and alr2 are capable of acting as ligands for each other across opposing cell membranes. This homophilic binding is allele-specific, meaning that protein alleles only bind to themselves but not to other variants. We hypothesize that this allele-specific homophilic recognition is what determines compatibility between colonies in vivo. This is the first plausible biophysical mechanism of allorecognition specificity in any invertebrate and provides a step toward understanding the evolution of allorecognition systems in cnidarians.
