Download 1 - Section of population genetics

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
A 1A 1
A 1A 2
A2A2
Frequency in offspring
p2
2pq
q2
Relative fitness
1
1-hs
1-s
Frequency after selection
p2 / w
2 pq(1 − hs) / w
q 2 (1 − s ) / w
Where w = p 2 + 2 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
Fitness
homozygotes = "heterozygote advantage" = "heterosis".
Genotypes
A model of overdominance
Fitness table for a simple model: one species, one locus, two alleles
Genotypes
A 1A 1
A 1A 2
A2A2
Frequency in offspring
p2
2pq
q2
Relative fitness
1-s
1
1-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
∆s p =
pq [ qt − ps ]
w
A model of natural selection: overdominance
Fitness table for a simple model: one species, one locus, two alleles
Genotypes
A 1A 1
A 1A 2
A2A2
Frequency in offspring
p2
2pq
q2
Relative fitness
1-s
1
1-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?
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 http://www.understandingrace.org)
Distribution of malaria (source CHU Rouen, France)
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?