Friend or foe? How an invertebrate distinguishes self from non-self in a natural histocompatibility interaction. (TRAN3P.912)

Abstract

Abstract The existence of sophisticated self/non-self systems is not limited to vertebrates. Several colonial marine invertebrates have evolved systems that distinguish between conspecifics via cell-cell contact. This phenomenon, known as allorecognition, occurs when colonies encounter each other as they grow across their substrate. Compatible colonies fuse or 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 loci, which ensure that colonies are only compatible with themselves or close kin. How allorecognition molecules achieve this specificity remains unknown. Allorecognition in Hydractinia is controlled by at least two genes, alr1 and alr2, which encode highly polymorphic transmembrane proteins similar to immunoglobulin superfamily 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 alleles bind to themselves but not to other alr1 alleles. We suggest that, in vivo, compatibility between colonies is also determined by allele-specific homophilic binding of alr proteins. Since fusion is rare in nature and single populations contain hundreds of unique alr alleles, the Hydractinia allorecognition system appears to be based on a biophysical mechanism with unprecedented allelic diversity and specificity.