Malaria has been one of the greatest challenges to human survival in the tropics and subtropical areas, which make up the bulk of our early distribution as species. In response we have developed a number of genetic-biological strategies of which the best known is the allele that causes the sickle cell disease in homozygotes but protects heterozygotes against the deadly infection in a clear example of balancing selection. This is not however the only adaption against malaria.
A new study explores a SE Asian (including Southern Chinese) adaption that also seems to be a way to fight against the disease, which is a variant of Alpha-Thalassemia:
Qin-Wei Qiu et al., Evidence of recent natural selection on the Southeast Asian deletion (–SEA) causing alpha-thalassemia in South China. BMC Evolutionary Biology 2013. Open access → LINK [doi:10.1186/1471-2148-13-63]
BackgroundThe Southeast Asian deletion (–SEA) is the most commonly observed mutation among diverse alpha-thalassemia alleles in Southeast Asia and South China. It is generally argued that mutation –SEA, like other variants causing hemoglobin disorders, is associated with protection against malaria that is endemic in these regions. However, little evidence has been provided to support this claim.
ResultsWe first examined the genetic imprint of recent positive selection on the –SEA allele and flanking sequences in the human alpha-globin cluster, covering a genomic region spanning ~410 kb, by genotyping 28 SNPs in a Chinese population consisting of 76 –SEA heterozygotes and 138 normal individuals. The pattern of linkage disequilibrium (LD) and the long-range haplotype test revealed a signature of positive selection. The network of inferred haplotypes suggested a single origin of the –SEA allele.
ConclusionsThus, our data support the hypothesis that the –SEA allele has been subjected to recent balancing selection, triggered by malaria.