Balancing selection, sickle cell anemia, and the African slave trade as an evolutionary test of overdominant selection
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Balancing selection, sickle cell anemia, and the African slave trade as an evolutionary test of overdominant selection
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Balancing selection, sickle cell anemia, and the African slave trade as an evolutionary test of overdominant selection
- 1. 1 Dan Graur University of Houston
- 2. 2 Overdominance Genotype A1A1 A1A2 A2A2 Fitness w11 w12 > w11,w22 w22 Frequency p2 2pq q2
- 3. 3 Underdominance Genotype A1A1 A1A2 A2A2 Fitness w11 w12 < w11,w22 w22 Frequency p2 2pq q2
- 4. 4 The change in the frequency of A2 from generation to generation is:
- 5. 5 At equilibrium, i.e., when Δq = 0. qˆ = s 2s- t
- 6. overdominance underdominance s = 0.04 and t = 0.02 s = - 0.02 and t = - 0.01 6
- 7. 7 Overdominant selection is inherently inefficient, even if the two homozygotes are not viable. Very expensive dog Cheap dog; mostly discarded Powderpuff Chinese Crested aa Aa AA
- 8. 8 The peculiar case of sickle-cell anemia
- 9. 9
- 10. 10 Glutamic acid Valine
- 11. mvhltpeeksavtalwgkvn vdevggealgrllvvypwtq rffesfgdlstpdavmgnpk vkahgkkvlgafsdglahld nlkgtfatlselhcdklhvd penfrllgnvlvcvlahhfg keftppvqaayqkvvagvan alahkyh 147aa 11
- 12. mvhltpveksavtalwgkvn vdevggealgrllvvypwtq rffesfgdlstpdavmgnpk vkahgkkvlgafsdglahld nlkgtfatlselhcdklhvd penfrllgnvlvcvlahhfg keftppvqaayqkvvagvan alahkyh 147aa 12
- 13. 13 “sticky” b globin
- 14. 14
- 15. 15
- 16. 16
- 18. 18 Intuitive Model: Directional Selection normal fitness HbHb =normal HbHs =sickle cell trait HsHs =sickle cell anemia somewhat reduced fitness reduced fitness
- 19. 19
- 20. 20 In theory, the end result should have been directional selection — a drastic reduction in HS allele frequency in the population.
- 21. 21 In practice, the frequency of the HS allele may reach enormous values in some populations. >20%
- 22. Worldwide distribution of sickle-cell anemia 22
- 23. 23
- 25. How to test the hypothesis that sickle cell anemia is under balancing selection by using a the African slave trade as an evolutionary experiment
- 26. 26 An evolutionary “experiment”: Slave trade
- 27. 27 Surinam endemic malaria HS frequency = 20% Curaçau no malaria HS frequency = 5% West Africa Frequency >20% 300 years = 10-15 generations
- 28. Genotype HH HHS HSHS Fitness 1 1.25 0.25 •With malaria in the background, heterozygotes have a huge advantage over the wild type homozygotes. • In the absence of malaria, the heterozygotes have a slight disadvantage in comparison to wild type homozygotes. • The fitness of the HsHs homozygotes is not affected by the presence or absence of malaria. 28
- 29. Modiano D, Luoni G, Sirima BS, Simpore J, Verra F, Konate A, Rastrelli E, Olivieri A, Calissano C, Paganotti GM, D'Urbano L, Sanou I, Sawadogo A, Modiano G, Coluzzi M. 2001. Haemoglobin C protects against clinical Plasmodium falciparum malaria. Nature 414:305-308. 29 A newly discovered allele that protects against malaria but does not have deleterious effects in homozygotes
- 30. E to V = HS E to K = HC 30 Hemoglobin C codon position 6! Glutamic acid Lysine
- 31. Genotype HH HHS HSHS Fitness 1 1.25 0.25 Genotype HH HHC HCHC Fitness 1 1.27 0.75 31 In the long term and in the presence of malaria, allele HbC is expected to replace alelle HbS in central West Africa.