Allorecognition in Ciona intestinalis: molecular basis for self-sterility in a hermaphroditic urochordate

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How ancestral chordates discriminate between self and nonself is not known. In an unbiased screening approach to identify Botryllus genes whose expression is altered in response to allogeneic contacts we recently identified a homolog of the CD94 / NKR-P1 receptor in Botryllus (Khalturin et al, 2003) as well as in the solitary tunicates Ciona intestinalis and C. savignyii (Khalturin et al., 2004a). In vertebrates, ligands to CD94 belong to the HLA-E group of nonclassical MHC I. Since the genomes of urochorda tes do not contain any MHC-like genes, the nature of the ligand of the CD94-like receptor protein in urochordates remains enigmatic. Here we propose to further analyse the mechanisms of allorecognition in ancestral chordates. Specifically, we suggest to (i) identify the ligand for CD94-like proteins in Botryllus and Ciona; (ii) analyse the putative NK locus in Ciona and Botryllus; and (iii) screen for proteins with high degree of interindividual variability in Ciona. The results will shed light on the evol ution of the innate immune system in deuterostomes as well as on the emergence of adaptive immunity.