We are studying allorecognition in the invertebrate chordate, Botryllus schlosseri. Botryllus undergoes a self/non-self recognition reaction when terminal projections of an extracorporeal vasculature, called ampullae come into contact between two individuals. Compatibility is determined by a single, highly polymorphic locus called the fuhc- individuals sharing one or both fuhc alleles are compatible, and the vessels will fuse; while those sharing no alleles are incompatible, causing an inflammatory rejection reaction that blocks fusion. Thus allorecognition in Botryllus uses a missing-self recognition strategy, and we had found that- similar to NK cells, polymorphic discrimination is not mutually exclusive, rather due to the integration of signals from two allorecognition receptors, called uncle fester- which activated the rejection response, and fester- which inhibited the rejection response and initiated fusion, although neither encoded signal transduction motifs. Here we show that fester genes are members of an extended family consisting of >37 loci, and co-expressed with members of an even more diverse gene family- the fester co-receptors (FcoR). The FcoRs are related to fester but encode conserved tyrosine signal transduction motifs, including ITIMs and hemITAMs. Individual FF and FcoR loci are encoded and expressed as functional gene pairs, and found in large polymorphic haplotypes, with gene content variation of the fester/FcoR pairs. Individual pairs are conserved, the functional role of the FcoR partner is consistent with our previous studies, and both are co-expressed at the site of allorecognition. Remarkably, several FcoR loci can switch between putative activating and inhibitory roles by alternative splicing in a genotype specific manner, suggesting a cell-autonomous quantitative education process is required for specificity. Our results indicate that the dynamic range of ITAM/ITIM signal integration allows a cell to quantitate cell surface binding events, is required for polymorphic discrimination, and that this conservation allows the rapid, convergent evolution of allorecognition systems.