What is Hydractinia?
We study allorecognition in Hydractinia symbiolongicarpus, a colonial hydrozoan (Phylum Cnidaria) that lives on the backs of hermit crab shells with a range extending along the Atlantic coast of North America from Maine to Florida. Colonies grow by asexually propagating mat, polyps, and stolons (Figure 1). Each colony is derived from a single zygote and therefore has a distinct genotype. In Hydractinia, contact between colonies results in rejection (Figure 2), in which colonies fight until one kills the other via nematocyst discharge, or fusion (Figure 3), in which colonies fuse to create a single, chimeric colony. Fusion may be permanent or transitory, resulting in separation of the colonies.
The Allorecognition Complex (ARC)
Using inbred lines of Hydractinia, we have demonstrated that allorecognition is controlled a region of the genome called the Allorecognition Complex (ARC). The ARC contains many allorecognition genes. To date, we have described two, Allorecognition 1 (Alr1) and Allorecognition 2 (Alr2). Colonies matching alleles at both genes fuse, while colonies matching no alleles reject. Colonies that only match alleles at one gene undergo transitory fusion. Both genes encode transmembrane proteins with extracellular regions containing domains similar to Ig-like domains and large cytoplasmic tails with motifs suggesting a role in signal transduction. The Alr genes are highly polymorphic (198 unique Alr2 alleles in a single population!), and sequence variation in the extracellular domain predicts allorecognition phenotypes.