Application Evolution: Part 1.1 Basics of Coevolution Dynamics

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1 Application Evolution: Part 1.1 Basics of Coevolution Dynamics S. chilense S. peruvianum Summer Semester 2013 Prof Aurélien Tellier FG Populationsgenetik

2 Color code Color code: Red = Important result or definition Purple: exercise to do Green: some bits of maths

3 Some Definitions Hosts and parasites exert reciprocal selective pressures on each other, which may lead to rapid reciprocal adaptation For organisms with short generation times host parasite coevolution can be observed in comparatively small time periods => possible to study evolutionary change in real-time: In the field In the laboratory These interactions are examples of evolution in action It contradict the common notion that evolution can only be detected across extended time scales.

4 Types of selection Host-parasite coevolution is characterized by reciprocal genetic change and thus changes in allele frequencies within populations. These changes can be determined by two main types of selection: Overdominant selection Negative frequency-dependent selection

5 A general model of natural selection Fitness table for a simple model: one species, one locus, two alleles Genotypes A 1 A 1 A 1 A 2 A 2 A 2 Frequency in offspring p 2 2pq q 2 Relative fitness 1 1-hs 1-s Frequency after selection p 2 / w 2 (1 ) / pq hs w q 2 (1 s) / w Where 2 2 w = p + pq hs + q s 2 (1 ) (1 ) Is the mean fitness of the population Based on Fisher s fundamental theorem of natural selection With 1 being the fitness of the homozygote A 1 A 1 genotype h is the dominance coefficient (heterozygous effect) s is the selection coefficient

6 Overdominant selection Overdominance occurs if the heterozygote phenotype has a fitness advantage over both homozygotes = "heterozygote advantage" = "heterosis". Fitness Genotypes

7 A model of overdominance Fitness table for a simple model: one species, one locus, two alleles Genotypes A 1 A 1 A 1 A 2 A 2 A 2 Frequency in offspring p 2 2pq q 2 Relative fitness 1 - s t Frequency after selection p 2 (1 s) / w 2 pq / w q 2 (1 t) / w When there is overdominance (h < 0) We can calculate the change in allele frequency from one generation to the next by selection p = s [ ps] pq qt w

8 A model of natural selection: overdominance Fitness table for a simple model: one species, one locus, two alleles Genotypes A 1 A 1 A 1 A 2 A 2 A 2 Frequency in offspring p 2 2pq q 2 Relative fitness 1 - s t Frequency after selection p 2 (1 s) / w 2 pq / w q 2 (1 t) / w We can calculate the equilibrium frequencies for both alleles pˆ t = s + t qˆ s = s + t Overdominance maintains variability as heterozygotes have an advantage A famous example of overdominance?

9 Overdominant selection: sickle cell anemia It is due to a mutation (allele a) in the hemoglobin gene sickle shape formation of red blood cells => causing clotting of blood vessels, restricted blood flow and reduced oxygen transport. The mutation confers resistance to malaria, caused by Plasmodium parasites. Homozygote (aa) and heterozygote (Aa) genotypes for the sickle-cell disease allele show malaria resistance Homozygote (aa) suffers from severe disease phenotype. Homozygote (AA) is susceptible to Plasmodium. Distribution of sickle cell anemia (source Distribution of malaria (source CHU Rouen, France)

10 Negative frequency-dependent selection An allele is subject to negative frequency dependent selection if a rare allelic variant has a selective advantage. For example, the parasite should adapt to the most common host genotype, because it can then infect a large number of hosts. In turn, a rare host genotype may then be favored by selection, its frequency will increase and eventually it becomes common. Subsequently the parasite should adapt to the former infrequent host genotype. Coevolution determined by negative frequency dependent selection is rapid, potentially occurring across few generations. It may maintains high genetic diversity by favoring uncommon alleles (see Haldane)

11 Observing negative frequency-dependent selection

12 Observing negative frequency-dependent selection

13 Observing negative frequency-dependent selection

14 Negative frequency-dependent selection An allele is subject to negative frequency dependent selection if a rare allelic variant has a selective advantage. Two outcome can occur: trench warfare dynamics arms race dynamics

15 Arms race dynamics The arms race dynamics sometimes called Red Queen dynamics Source:

16 The arms race dynamics Arms race dynamics Woolhouse et al Nat Genet Holub 2001 Nat Rev Genet

17 Arms race dynamics The arms race dynamics There is recurrent fixation of host and parasite alleles Polymorphism = presence of more than one allele in a population Polymorphism is only TRANSIENT in this dynamics this means that polymorphism is short lived and the population often has only one allele What does this mean for observing natural populations?

18 Trench warfare dynamics The trench warfare dynamics (Stahl et al. 1999) Source: Imperial War Museum An aerial reconnaissance photograph of the opposing trenches and no-man's land between Loos and Hulluch in Artois, France, taken at 7.15 pm, 22 July German trenches are at the right and bottom, British trenches are at the top left. The vertical line to the left of centre indicates the course of a pre-war road or track.

19 Trench warfare dynamics The trench warfare dynamics (Stahl et al. 1999) also called fluctuating selection dynamics Woolhouse et al Nat Genet There is variation of frequencies of host and parasite alleles Polymorphism = presence of more than one allele in a population Polymorphism is PERMANENT in this dynamics this means that polymorphism is long lived and the population contains several alleles

20 Trench warfare dynamics The trench warfare dynamics (Stahl et al. 1999) Holub 2001 Nat Rev Genet What does this mean for observing natural populations?

21 Observations in natural populations JEB, 2008

22 Extension to genomic signatures? Can you guess which signatures we expect for polymorphism in these two dynamics?