Gene duplication and recombination in the evolution of mammalian Fc receptors.

The immunoglobulin-related chains of cell-surface receptors for the Fc region of immunoglobulins (FcepsilonRIalpha, FcgammaRI, FcgammaRII, and FcgammaRIIIalpha) are encoded by members of a gene family. Phylogenetic analysis of representative members of this family from mammals revealed that FcgammaRIIIalpha genes of human, mouse, and rat are not orthologous to one another in the region of the gene encoding the immunoglobulin C2-set domains. In phylogenetic trees of this region, FcgammaRIIIalpha and FcgammaRII clustered together. However, in trees based on both coding and noncoding regions 5' and 3' to the C2 domains, FcgammaRIIIalpha genes of human, mouse, and rat clustered together. This pattern of relationship is most easily explained as a result of two independent recombinational events occurring in the mouse and rat after these two species diverged, in each of which the exons encoding the C2 domains were donated to an FcgammaRIIIalpha gene by an FcgammaRII gene.