1. BIOE 109 Summer 2009 Lecture 7- Part I Linkage disequilibrium and the evolution of sex

2. Q: What distinguishes sexual from asexual reproduction?

3. Linkage disequilibrium and the evolution of sex Q: What distinguishes sexual from asexual reproduction? A: Meiosis and Syngamy

4. Linkage disequilibrium and the evolution of sex Q: What distinguishes sexual from asexual reproduction? A: Meiosis and Syngamy Male Female 2N2N

5. Linkage disequilibrium and the evolution of sex Q: What distinguishes sexual from asexual reproduction? A: Meiosis and Syngamy Male Female 2N2N Meiosis   N N

6. Linkage disequilibrium and the evolution of sex Q: What distinguishes sexual from asexual reproduction? A: Meiosis and Syngamy Male Female 2N2N Meiosis   N N Syngamy   2N

7. A model for the evolution of two sexes

8. in many species, sexual reproduction has entailed a shift from isogamy to anisogamy.

9. A model for the evolution of two sexes in many species, sexual reproduction has entailed a shift from isogamy to anisogamy. Isogamy + -

10. A model for the evolution of two sexes in many species, sexual reproduction has entailed a shift from isogamy to anisogamy. Isogamy + - Anisogamy ♂ ♀

11. A model for the evolution of two sexes Mating type (M) +/-

12. A model for the evolution of two sexes Mating Gamete type (M) size (G) +/- Small (S)/Large (L)

13. A model for the evolution of two sexes Mating Gamete type (M) size (G) +/- Small (S)/Large (L)  Linkage disequilibrium MG

14. A model for the evolution of two sexes Mating Gamete type (M) size (G) +/- Small (S)/Large (L)  Linkage disequilibrium MG  +S ♂

15. A model for the evolution of two sexes Mating Gamete type (M) size (G) +/- Small (S)/Large (L)  Linkage disequilibrium MG  - L+ S ♀ ♂

16. MG - L+ S ♀♂

17. MG - L+ S ♀♂ Recombinants:

18. MG - L+ S ♀♂ MG + L Recombinants:  fitness due to low sperm number

19. MG - L+ S ♀♂ MG + L MG - S Recombinants:  fitness due to low sperm number  fitness due to inviable eggs

20. What is linkage disequilibrium?

21. Linkage equilibrium occurs when the genotypes present at one locus are independent of the genotypes present at a second locus.

22. What is linkage disequilibrium? Linkage equilibrium occurs when the genotypes present at one locus are independent of the genotypes present at a second locus. Linkage disequilibrium occurs when genotypes at the two loci are not independent of each other.

23. Q: What causes linkage disequilibrium?

24. 1. Natural selection

25. Q: What causes linkage disequilibrium? 1. Natural selection can be produced by epistatic selection

26. Q: What causes linkage disequilibrium? 1. Natural selection can be produced by epistatic selection epistasis occurs when the fitness of a genotype at one locus depends on its genotype at another locus

27. Q: What causes linkage disequilibrium? 1. Natural selection can be produced by epistatic selection epistasis occurs when the fitness of a genotype a one locus depends on its genotype at another locus 2. Random genetic drift much weaker than selection in creating disequilibrium.

28. Q: What causes linkage disequilibrium? 3. Population admixture

29. Q: What causes linkage disequilibrium? 3. Population admixture can be as important as selection in creating disequilibrium.

30. Q: What causes linkage disequilibrium? 3. Population admixture can be as important as selection in creating disequilibrium. Q: What eliminates linkage disequilibrium?

31. Q: What causes linkage disequilibrium? 3. Population admixture can be as important as selection in creating disequilibrium. Q: What eliminates linkage disequilibrium? A: Recombination!

32. The decay of disequilibrium depends on the rate of recombination

33. How and why did sex evolve?

34. or… how is sexual reproduction maintained in the face of so many alternative strategies?

35. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? Some alternatives: 1. Parthenogenesis (both mitotic and sexual forms)

36. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? Some alternatives: 1. Parthenogenesis (both mitotic and sexual forms) organisms develop from unfertilized eggs. Examples: lizards, aphids, many plants

37. New Mexico whiptail lizard (Cnemidophorus neomexicanus)  C. neomexicanus

38. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? Some alternatives: 1. Parthenogenesis (both mitotic and sexual forms) organisms develop from unfertilized eggs. Examples: aphids, many plants 2. Hermaphroditism (obligate or sequential)

39. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? Some alternatives: 1. Parthenogenesis (both mitotic and sexual forms) organisms develop from unfertilized eggs. Examples: aphids, many plants 2. Hermaphroditism (obligate or sequential) organisms possess both male and female reproductive organs, or change sex at some point in their lives.

40. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? Some alternatives: 1. Parthenogenesis (both mitotic and sexual forms) organisms develop from unfertilized eggs. Examples: aphids, many plants 2. Hermaphroditism (obligate or sequential) organisms possess both male and female reproductive organs, or change sex at some point in their lives. Examples: many fishes, snails, worms

42. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? 3. Haplodiploidy

43. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? 3. Haplodiploidy haploid males develop from unfertilized eggs, diploid females from fertilized eggs.

44. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? 3. Haplodiploidy haploid males develop from unfertilized eggs, diploid females from fertilized eggs. Examples: ants, bees, wasps

45. How and why did sex evolve? or… how is sexual reproduction maintained in the face of so many alternative strategies? 3. Haplodiploidy haploid males develop from unfertilized eggs, diploid females from fertilized eggs. Examples: ants, bees, wasps 4. Pseudogamy contact with sperm stimulates development from unfertilized eggs. Example: some nematodes and freshwater fishes

46. Amazon molly (Poecilia formosa)

47. But… of the world’s 2 million named species less than 2,000 are totally asexual

48. … and they don’t appear to persist very long

49. Asexual species are typically found at the tips of phylogenetic trees S = sexual species A = asexual species S A S S S A S A S S

50. The exception: bdelloid rotifers – no sex for 40 million years!

51. The “costs” of sex 1. The cost of producing males (or, the two-fold cost of sex).

52. The “costs” of sex 1. The cost of producing males (the two-fold cost of sex).

53. The “costs” of sex 1. The cost of producing males (the two-fold cost of sex).

54. The “costs” of sex 1. The cost of producing males (the two-fold cost of sex).

55. The “costs” of sex 2. The cost of finding mates

56. The “costs” of sex 2. The cost of finding mates exacerbated by low population density.

57. The “costs” of sex 2. The cost of finding mates exacerbated by low population density. 3. The costs of mating

58. The “costs” of sex 2. The cost of finding mates exacerbated by low population density. 3. The costs of mating mating is a risky business!

59. The “costs” of sex 2. The cost of finding mates exacerbated by low population density. 3. The costs of mating mating is a risky business! also vulnerable to sexually transmitted diseases.

60. The “costs” of sex 2. The cost of finding mates exacerbated by low population density. 3. The costs of mating mating is a risky business! also vulnerable to sexually transmitted diseases. 4. The cost of recombination

61. The “costs” of sex 2. The cost of finding mates exacerbated by low population density. 3. The costs of mating mating is a risky business! also vulnerable to sexually transmitted diseases. 4. The cost of recombination recombination creates superb combinations of genes then quickly breaks them apart.

62. Why then does sexual reproduction persist? 1. Adaptive evolution is enhanced

63. Why then does sexual reproduction persist? 1. Adaptive evolution is enhanced in asexual species, advantageous mutations must occur in the same lineage:

64. Why then does sexual reproduction persist? 1. Adaptive evolution is enhanced in asexual species, advantageous mutations must occur in the same lineage: advantageous mutation  Abcd           Abcd’                                                Ab’cd’    advantageous mutation

65. Why then does sexual reproduction persist? 1. Adaptive evolution is enhanced in asexual species, advantageous mutations must occur in the same lineage. in sexual populations, advantageous mutations can be combined across lineages (through meiosis and syngamy).

66. Why then does sexual reproduction persist? 1. Adaptive evolution is enhanced in sexual populations, advantageous mutations can be combined across lineages (through meiosis and syngamy): advantageous mutation  Abcd    Abc’d x  Abc’d’   Abcd’    Abcd’  advantageous mutation

67. Why then does sexual reproduction persist? 2. The Red Queen hypothesis

68. Why then does sexual reproduction persist? 2. The Red Queen hypothesis originally proposed by Leigh Van Valen in 1973.

69. Why then does sexual reproduction persist? 2. The Red Queen hypothesis

70. Why then does sexual reproduction persist? 2. The Red Queen hypothesis originally proposed by Van Valen in 1973. species must continuously “run” (evolve) to track changing environments.

71. Why then does sexual reproduction persist? 2. The Red Queen hypothesis originally proposed by Van Valen in 1973. species must continuously “run” (evolve) to track changing environments. if species fail to adapt, they may go extinct.

72. Why then does sexual reproduction persist? 2. The Red Queen hypothesis originally proposed by Van Valen in 1973. species must continuously “run” (evolve) to track changing environments. if species fail to adapt, they may go extinct sexual reproduction facilitates this process.

73. The Red Queen process is an evolutionary arms race

74. Target species “Enemies” (parasites, predators, competitors)

75. The Red Queen process is an evolutionary arms race  Target species “Enemies” (parasites, predators, competitors) Adaptation

76. The Red Queen process is an evolutionary arms race   Target species “Enemies” (parasites, predators, competitors) Adaptation Counter- adaptation

77. 3. Muller’s ratchet

78. Hermann Muller (1890 – 1967)

79. A simple ratchet crank  pawl  pawl 

80.       Muller’s ratchet Mutation

81.       Muller’s ratchet deleterious mutations occur in asexual lineages… Mutation

82.       Muller’s ratchet deleterious mutations occur in asexual lineages… … causing the least mutated class to dwindle… Mutation 

83.       Muller’s ratchet deleterious mutations occur in asexual lineages… … causing the least mutated class to dwindle… … and be lost by random drift Mutation       Mutation ?

84.       Muller’s ratchet deleterious mutations occur in asexual lineages… … causing the least mutated class to dwindle… … and be lost by random drift now the ratchet has “clicked” forward once. Mutation       Mutation ?

85.       Muller’s ratchet deleterious mutations occur in asexual lineages… … causing the least mutated class to dwindle… … and be lost by random drift now the ratchet has “clicked” forward once. now the ratchet has “clicked” forward again. Mutation           Mutation Mutation ? ?

86. Muller’s ratchet Asexual populations can only evolve towards ever greater loads of deleterious mutations!

87. Muller’s ratchet Asexual populations can only evolve towards ever greater loads of deleterious mutations! Does Muller’s ratchet occur in sexual populations?

88. Muller’s ratchet Asexual populations can only evolve towards ever greater loads of deleterious mutations! Does Muller’s ratchet occur in sexual populations? NO! Sex breaks the ratchet.

89. Muller’s ratchet Asexual populations can only evolve towards ever greater loads of deleterious mutations! Does Muller’s ratchet occur in sexual populations? NO! Sex breaks the ratchet. How? By reconstituting the least mutated classes (by recombination).

90. Muller’s ratchet Asexual populations can only evolve towards ever greater loads of deleterious mutations! Does Muller’s ratchet occur in sexual populations? NO! Sex breaks the ratchet. How? By reconstituting the least mutated classes (by recombination). SEX IS RECOMBINATION!

91. Q: So why are asexual species at the tips of phylogenetic trees? S = sexual species A = asexual species S A S S S A S A S S

92. Q: So why are asexual species at the tips of phylogenetic trees? A: Because the short-term benefit of asexual reproduction is countered by the long-term advantage of sex. S = sexual species A = asexual species S A S S S A S A S S