Application Evolution: Part 1.1 Basics of Coevolution Dynamics
S. chilense
S. peruvianum
Summer Semester 2013
Prof Aurélien Tellier FG Populationsgenetik Color code
Color code:
Red = Important result or definition
Purple: exercise to do
Green: some bits of maths 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. 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 A general model of natural selection
Fitness table for a simple model: one species, one locus, two alleles
Genotypes A1A1 A1A2 A2A2
Frequency in offspring p2 2pq q2
Relative fitness 1 1-hs 1-s Frequency after selection p2 / w 2pq (1− hs ) / w q2 (1− s ) / w
Wherewp=+22 pq (1 −+ hs ) q 2 (1 − s ) Is the mean fitness of the population
Based on Fisher’s fundamental theorem of natural selection
With 1 being the fitness of the homozygote A1A1 genotype h is the dominance coefficient (heterozygous effect) s is the selection coefficient Overdominant selection
Overdominance occurs if the heterozygote phenotype has a fitness advantage over both homozygotes = " heterozygote advantage " = " heterosis ". Fitness
Genotypes A model of overdominance
Fitness table for a simple model: one species, one locus, two alleles
Genotypes A1A1 A1A2 A2A2
Frequency in offspring p2 2pq q2
Relative fitness 1 - s 1 1 - t Frequency after selection p2 (1− s ) / w 2pq / w q2 (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
pq[ qt− ps ] ∆p = s w A model of natural selection: overdominance
Fitness table for a simple model: one species, one locus, two alleles
Genotypes A1A1 A1A2 A2A2
Frequency in offspring p2 2pq q2
Relative fitness 1 - s 1 1 - t Frequency after selection p2 (1− s ) / w 2pq / w q2 (1− t ) / w
We can calculate the equilibrium frequencies for both alleles
t s pˆ = qˆ = s+ t s+ t
Overdominance maintains variability as heterozygotes have an advantage A famous example of overdominance? 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 Distribution of malaria (source CHU Rouen, France) (source http://www.understandingrace.org) 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) Observing negative frequency-dependent selection Observing negative frequency-dependent selection Observing negative frequency-dependent selection 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” Arms race dynamics
The “arms race” dynamics sometimes called “Red Queen” dynamics
Source: www.fas.org Arms race dynamics
The “arms race” dynamics
Woolhouse et al. 2002 Nat Genet
Holub 2001 Nat Rev Genet 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? 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 1917. 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. Trench warfare dynamics The “trench warfare” dynamics (Stahl et al. 1999) also called “fluctuating selection dynamics”
Woolhouse et al. 2002 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 Trench warfare dynamics
The “trench warfare” dynamics (Stahl et al. 1999)
Holub 2001 Nat Rev Genet
What does this mean for observing natural populations? Observations in natural populations
JEB, 2008 Extension to genomic signatures?
Can you guess which signatures we expect for polymorphism in these two dynamics?