Abstract
BackgroundProbably the best example of the rise and maintenance of balancing selection as an evolutionary trend is the role of S-haemoglobin (HbS - rs334) in protecting from malaria. Yet, the dynamics of such a process remains poorly understood, particularly in relation to different malaria transmission rates and the genetic background of the affected populations.MethodsWe investigated the association of haemoglobin HbS in protection from clinical episodes of malaria in two populations/villages where malaria is endemic, but mostly presenting in mild clinical forms. Five-hundred and forty-six individuals comprising 65 and 82 families from the Hausa and Massalit villages respectively were genotyped for HbS. Allele and genotype frequencies as well as departure from Hardy-Weinberg Equilibrium were estimated from four-hundred and seventy independent genotypes across different age groups. Age-group frequencies were used to calculate the coefficient-of-fitness and to simulate the expected frequencies in future generations.ResultsGenotype frequencies were within Hardy-Weinberg expectations in Hausa and Massalit in the total sample set but not within the different age groups. There was a trend for a decrease of the HbS allele frequency in Hausa and an increase of frequency in Massalit. Although the HbS allele was able to confer significant protection from the clinical episodes of malaria in the two populations, as suggested by the odds ratios, the overall relative fitness of the HbS allele seems to have declined in Hausa.ConclusionsSuch loss of balancing selection could be due to a combined effect of preponderance of non-clinical malaria in Hausa, and the deleterious effect of the homozygous HbS under circumstances of endogamy.
Highlights
The best example of the rise and maintenance of balancing selection as an evolutionary trend is the role of S-haemoglobin (HbS - rs334) in protecting from malaria
Polymorphisms in the b-globin gene leading to sicklecell anaemia and thalassemia are among the most notable examples of balancing selection in the human genome
Over half a century ago Haldane [1], suggested that protection against malaria was conferred by an abnormal form of haemoglobin known as thalassemia
Summary
The best example of the rise and maintenance of balancing selection as an evolutionary trend is the role of S-haemoglobin (HbS - rs334) in protecting from malaria. Over half a century ago Haldane [1], suggested that protection against malaria was conferred by an abnormal form of haemoglobin known as thalassemia. It was later suggested that these deleterious mutations are maintained in the population in a state of balanced polymorphism because of the protective effect against severe forms of malaria conferred by the heterozygous states [2,3,4]. We study the behaviour of the HbS polymorphism in two village populations of different ethnic origins, in an attempt to understand the evolutionary trajectory of this mutation under weak natural selection, where malaria is endemic but has a mostly mild clinical presentation The observed overlaps in the geographic distributions of malaria with the haemoglobinopathies, HbS [5,6,7], have been cited in support of the hypothesis that malaria has been an important evolutionary force in selection of this and other variants, with
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