Sessile, colonial invertebrates often encounter members of their own species as they grow on hard surfaces. The nature and outcomes of such encounters are mediated by highly discriminatory recognition systems. In general, close relatives fuse whereas unrelated colonies reject. These histocompatibility phenomena have been unambiguously observed in all major colonial invertebrate groups, and have attracted the attention of various biological disciplines. In particular, they are often claimed to reflect the evolutionary origins of the vertebrate immune system, and have played a prominent role in evolutionary theory and ecology as a paradigm of competitive interactions within species. Despite the prevalence of histocompatibility and their putative biological importance, the responsible genes and molecules have not been identified in any colonial invertebrate. Histocompatibility in the colonial hydroid Hydractinia symbiolongicarpus (Cnidaria; Hydrozoa) is better understood than in perhaps any other colonial invertebrate. The response results from a dose-dependent interaction of at least two gene loci encoded in a single chromosomal interval. This project aims to identify and functionally characterize the histocompatibility genes in Hydractinia. Specifically, genetic markers located at close proximity to the target genes will be used to identify the relevant genomic segment from a Hydractinia DNA library. Plausible candidate histocompatibility genes will be identified by sequencing the entire segment. Candidate genes will be subsequently knocked-out in vivo to test their role in histocompatibility responses. Characterization of the invertebrate histocompatibility molecules will prove instrumental for understanding, at its most basic level, the nature of competitive interactions amongst conspecifics. Furthermore, it will settle the long-standing debate on the evolutionary relationship between vertebrate immunity and invertebrate histocompatibility. The project will integrate research and education by developing new curricula, providing research opportunities for undergraduates, and recruiting and training underrepresented groups in science.
