In mammals many natural killer (NK) cell receptors, encoded by the leukocyte receptor complex (LRC), regulate the cytotoxic activity of NK cells and provide protection against virus-infected and tumor cells. To investigate the origin of the Ig-like domains encoded by the LRC genes, a subset of C2-type Ig-like domain sequences was compiled from mammals, birds, amphibians, and fish. Phylogenetic analysis of these sequences generated seven monophyletic groups in mammals (MI, MII, and FcI, FcIIa, FcIIb, FcIII, FcIV), two in chicken (CI, CII), four in frog (FI-FIV), and five in zebrafish (ZI-ZV). The analysis of the major groups supported the following order of divergence: ZI [or a common ancestor of ZI and F (a cluster composed of the FcIII and FIII groups)], F, CII (or a common ancestor of CII and MII), MII, and MI-CI. The relationships of the remaining groups were unclear, since the phylogenetic positions of these groups were not supported by high bootstrap values. Two main conclusions can be drawn from this analysis. First, the two groups of mammalian LRC sequences must diverged before the separation of the avian and mammalian lineages. Second, the mammalian LRC sequences are most closely related to the Fc receptor sequences and these two groups diverged before the separation of birds and mammals.
KeywordsLeukocyte receptor complex; Chicken Ig-like receptors; Fc receptors; Ig-like domain groupsThe mammalian natural killer (NK) cell receptors fall into two categories, one category belonging to the immunoglobulin superfamily (IgSF) and the other to the C-type lectin superfamily. The Ig-like receptors occupy a genomic region called the leukocyte receptor complex (LRC; Trowsdale et al. 2001). In mammals, the LRC contains several gene families including the killer cell Ig-like receptors (KIR), the leukocyte Ig-like receptors (LILR), and the paired Ig-like receptors (PIR), which form species-specific evolutionary clusters (Martin et al. 2002). Singleton genes have been identified in the LRC of humans, artiodactyls, and rodents (Hoelsbrekken et al. 2003;Maruoka et al. 2004;Morton et al. 2004). The presence of the LRC in all mammals so far studied suggests that this region formed before the mammalian radiation. Evolution of the LRC genes has thus far been studied mainly in mammalian species for which gene sequences have been available (Volz et al. 2001;Hughes 2002;Martin et al. 2002). Here we investigate the origin of Ig-like domains encoded by the mammalian LRC genes using recently generated homologous sequences from chicken, frog, and fish. . BLAST searching was conducted by using high expected E-values (E=10) to ensure that most sequences homologous to the mammalian LRC genes would be retrieved. The resulting BLAST hits were sorted according to the E-value and the coverage of the query sequence. Only hits covering at least 85 amino acid residues (the minimum length of the C2-type Ig-like domain; Klein and Hořejší 1997) were used for further analysis. Gene structures were predicted by using GenomeScan (Yeh et ...