Sex and molecular evolution. Brian Charlesworth Institute of Evolutionary Biology School of Biological Sciences University of Edinburgh.
Institute of Evolutionary Biology
School of Biological Sciences
University of Edinburgh
The Bdelloid rotifer Philodina roseola
To understand the evolutionary significance of sex, we need to understand the significance of recombination.
The set of basepairs for a given DNA sequence is known
as a haplotype
Measuring the mutation rate
Initial isogenic stock
~ 200 generations to allow mutations to arise
DNA extracted and sequenced, or mutations detected by special methods
The simplest form of selection is when two alternative variants at a given site in the genome confer differences in fitness on the their carriers.
Let one variant be called A1 and the other A2.
Let the ratio of the fitnesses of A2 and A1 individuals be 1 – s .
The quantity s measures the intensity of selection, and is called the selection coefficient.
If q is the frequency of A2, the change in q is q, given approximately by sq(1 – q).
The peppered moth Biston betularia, with melanic (dark) and non-melanic forms
A T A G C T T G A C C T A T G
Parental combinations of two variants
A A A G C C T G A G C T A T A
Recombination between A and B sites or between polymorphic sites in a DNA sequence
A A A G C C T G A C C T A T G
A T A G C T T G A G C T A T A
of the fruitfly Drosophila
Numbers of copies of the bw eyecolour variant
(forward diffusion equation)
Here, x is the vector of haplotype frequencies, (x, t) is the probability density of x at time t, xi is the deterministic change in the frequency of the ith haplotype, Cij is the covariance between the random changes in frequencies of haplotypes i and j.
The Cij are all proportional to 1/Ne; this means that we can multiply both sides by Ne,and work with Net as a time unit and with Ne xi instead of xi.
3. The level of adaptation at the protein and DNA sequence level is often reduced in non-recombining genomic regions.
1.2 million basepairs (Mb) / ~80 genes
No crossing over under normal lab conditions
Characteristics of the dot chromosome:
Low silent site variability (about 10% of genomewide mean)
P. Haddrill et al. 2007 Genome Biology 8:R18
Mean silent diversities: neo-X = 0.39% , neo-Y = 0.004%
A lack of recombination among a set of genes in a genome or genomic region means that the evolutionary fates of mutations at different sites are not independent of each other, so that they can interfere with each other’s evolution.
This is the Hill-Robertsoneffect.
(Hill and Robertson 1966 Genetical Research 8: 269-294)
Fitness = 0.9
Fitness = 0.95
Fitness = 1
Maximum fitness possible with both advantageous mutations A2 and B2
Fitness = 1.05
To answer this, we need to know:
(Loewe and Charlesworth 2007 Genetics 175: 1381-1393.)
These models assumed that selection is sufficiently strong relative to drift that deleterious mutations are mostly held close to their equilibrium values for an infinitely large population.
If a large number of sites with low recombination are under selection, this does not hold because of their mutual H-R interference, which means that deleterious variants can drift to intermediate frequencies.
This reduces the strength of their HR effects.
(Kaiser and Charlesworth 2009, Trends in Genetics 25: 9-12)
Compare observed reduction in neutral diversity (π/π0) to expectation under background selection model:
→ formula overpredicts reduction in π if recombination rates are low and chromosomes long
Exponential decline of neutral diversity with chromosome length
D. miranda neo-Y
Drosophila dot chromosome