African ancestry of the population of Buenos Aires.

The population of Argentina today does not have a "visible" black African component. However, censuses conducted during most of the 19th century registered up to 30% of individuals of African origin living in Buenos Aires city. What has happened to this African influence? Have all individuals of African origin died, as lay people believe? Or is it possible that admixture with the European immigrants made the African influence "invisible?" We investigated the African contribution to the genetic pool of the population of Buenos Aires, Argentina, typing 12 unlinked autosomal DNA markers in a sample of 90 individuals. The results of this analysis suggest that 2.2% (SEM=0.9%) of the genetic ancestry of the Buenos Aires population is derived from Africa. Our analysis of individual admixture shows that those alleles that have a high frequency in populations of African origin tend to concentrate among 8 individuals in our sample. Therefore, although the admixture estimate is relatively low, the actual proportion of individuals with at least some African influence is approximately 10%. The evidence we are presenting of African ancestry is consistent with the known historical events that led to the drastic reduction of the Afro-Argentine population during the second half of the 19th century. However, as our results suggest, this reduction did not mean a total disappearance of African genes from the genetic pool of the Buenos Aires population.

Natural selection on the erythrocyte surface.

Surface glycoproteins are principal receptors used by pathogens to invade target cells. It has been suggested that mammalian erythrocyte surface glycoproteins function as decoy receptors attracting pathogens to the anucleated erythrocyte and away from their target tissues. Glycophorin A (GYPA) is solely expressed on the erythrocyte surface where it is the most abundant sialoglycoprotein, although its function is unknown. The pathogen decoy hypothesis may be relevant here, as GYPA has been shown in vitro to bind numerous viruses and bacteria, which do not infect erythrocytes. However, it is also a receptor for erythrocyte invasion by the malarial parasite Plasmodium falciparum. Analyses of gypa sequence variation among six higher primates and within a human population show that there is a large excess of replacement (nonsynonymous) substitutions along each primate lineage (particularly on exons 2-4 encoding the extracellular glycosylated domain of GYPA) and a significant excess of polymorphisms in exon 2 (encoding the terminal portion of the extracellular domain) within humans. These two signatures suggest that there has been exceptionally strong positive selection on this receptor driving GYPA divergence during primate evolution and balancing selection maintaining allelic variation within human populations. The pathogen decoy hypothesis alone is adequate to explain both these signatures of between-species and within-species diversifying selection. This has implications for understanding the functions of erythrocyte surface components and their roles in health and disease.