Molecular evolution of a malaria resistance gene (DARC) in primates

Genes involved in host-pathogen interactions are often strongly affected by positive natural selection. The Duffy antigen, coded by the Duffy antigen receptor for chemokines (DARC) gene, serves as a receptor for Plasmo- dium vivax in humans and for Plasmodium knowlesi in some nonhuman primates. In the majority of sub-Saharan Afri- cans, a nucleic acid variant in GATA-1 of the gene promoter is responsible for the nonexpression of the Duffy antigen on red blood cells and consequently resistance to invasion by P. vivax. The Duffy antigen also acts as a receptor for chemo- kines and is expressed in red blood cells and many other tissues of the body. Because of this dual role, we sequenced a ~3,000-bp region encompassing the entire DARC gene as well as part of its 5' and 3' flanking regions in a phylogenetic sampleofprimatesandusedstatistical methodstoevaluatethe nature of selection pressures acting on the gene during its evolution. We analyzed both coding and regulatory regions of the DARC gene. The regulatory analysis showed accelerat- ed rates of substitution at several sites near known motifs. Our testsofpositiveselectioninthecodingregionusingmaximum likelihood by branch sites and maximum likelihood by codon sites did not yield statistically significant evidence for the action of positive selection. However, the maximum likeli- hood test in which the gene was subdivided into different structural regions showed that the known binding region for P. vivax/P. knowlesi is under very different selective pressures than the remainder of the gene. In fact, most of the gene appears to be under strong purifying selection, but this is not evident in the binding region. We suggest that the binding region is under the influence of two opposing selective pres- sures, positive selection possibly exerted by the parasite and purifying selection exerted by chemokines.