PATTERNS OF RETICULATE EVOLUTION FOR THE CLASSICAL CLASS-I AND CLASS-II HLA LOCI

Some alleles of the major histocompatibility complex (MHC) genes have a reticulate pattern of evolution, probably resulting from the exchang e of segments by gene conversion or recombination. Here we compare the extent and patterns of reticulate evolution among the classical class I and class II loci of the human MI-IC using the recently developed c ompatibility and partition matrix methods. A complex pattern is reveal ed with substantial differences among loci in the extent and pattern o f reticulation, Extremely high levels of reticulation are observed at HLA-B and HLA-DPB1 high levels at HLA-A and HLA-DRB1, moderate levels at HLA-C and HLA-DQB1, and low levels at HLA-DQA1. The reticulate even ts are concentrated in the exons encoding the highly variable, peptide -binding domains, suggesting that the sequence combinations produced b y these events are maintained by natural selection.