Meeting Abstract

10.6  Saturday, Jan. 4 09:15  Binding specificities of Hydractinia allorecognition proteins KARADGE, U; GOSTO, M; NICOTRA, ML*; University of Pittsburgh; University of Pittsburgh; University of Pittsburgh nicotraml@upmc.edu

Colonial marine invertebrates—animals such as sponges, corals, and sea squirts—are capable of distinguishing between their own tissues and those of conspecifics via cell-cell contact. This phenomenon is known as allorecognition and occurs whenever colonies encounter each other as they grow across their substrate. Compatible colonies typically fuse or peacefully co-exist, while incompatible colonies reject and often aggressively compete for space. In all taxa studied to date, allorecognition phenotypes are determined by highly polymorphic genetic loci. These loci ensure colonies are only compatible with themselves or close relatives. How allorecognition molecules achieve this specificity remains unknown. Here we report data suggesting a biophysical mechanism for allorecognition specificity in the hydroid, Hydractinia symbiolongicarpus. Allorecognition in Hydractinia is controlled by at least two histocompatibility genes, alr1 and alr2. Both genes encode highly polymorphic transmembrane proteins similar to immunoglobulin superfamily (IgSF) molecules. Colonies with matching alleles at alr1 and alr2 fuse, while colonies with no matching alleles reject. Using in vitro assays with recombinant proteins, we demonstrate that alr1 is capable of binding to itself and that alr1 alleles bind to themselves but not to other alr1 alleles. These results suggest that, in vivo, compatibility between colonies is also determined by allele-specific homophilic binding of alr proteins. Given the fact that fusion is exceedingly rare in nature and that single populations can maintain hundreds of unique alr alleles, the Hydractinia allorecognition system appears to be based on homophilic protein-protein interactions with unprecedented allelic diversity and specificity.