SCHWARZ, Ryan S.*; CADAVID, Luis F.; University of New Mexico; University of New Mexico: Somatic cell parasitism and the evolution of histocompatibility systems

Colonial invertebrates have the ability to discriminate between self-tissues and those of unrelated members of their own species. Histocompatibility reactions typically result in either fusion or rejection. A highly polymorphic gene complex that ensures fusion of kin and rejection of unrelated conspecifics controls these allorecognition phenomena. The selective pressures that led to the evolution of invertebrate histocompatibility may in part be explained by the need to prevent cell lineage parasitism by conspecifics. Here we present data toward empirical evidence for the occurrence of somatic cell lineage parasitism in a colonial invertebrate, Hydractinia symbiolongicarpus (Cnidaria; Hydrozoa). Using individuals that differ at sequence tag site (STS) alleles, chimeras were established such that the origin of a given tissue sample�s cell line could be identified. Genomic DNA was isolated from somatic tissue samples throughout the chimera and PCR amplification of STSs performed to identify the cell lineage of each somatic tissue sample. STSs were also amplified from tissue samples of rejection reactions to test the effectiveness of the histocompatibility system in preventing migration of pluripotent stem cells from one colony to the other. Our data shows trends toward two conclusions: 1) The histocompatibility system of H. symbiolongicarpus is effective in preventing cell lineage parasitism between unrelated colonies and 2) chimeras are not constituted by an equal proportion of genotypes from each chimeric partner, but typically have predominance of one genotype over the other. These observations support the role of invertebrate histocompatibility in preventing cell lineage parasitism.