Transcript Document

Life without Sex
Here’s what it looks like.
• Bdelloid rotifers gave up sex tens of millions of years
ago! Yet somehow, they have survived.
• There are now more than 450 female only species.
Life without Sex
• What type of reproduction do bdelloid rotifers exhibit?
Parthenogenesis Parthenos (Gk) virgin, genesis (Gk) creation.
Life without Sex
What are the benefits of non-sexual reproduction?
• Males do not produce offspring.
• Every female does produce offspring.
What are the consequences?
• The population can grow larger without males.
Life without Sex
What are the disadvantages of sexual reproduction?
• Sexual reproduction = Time and energy
• Need to find a mate
• Mate may need to provide resources (material and
nutritional) to prove worthy.
• Risk of STDs (humans, other species?)
Life without Sex
Bdelloid rotifers are susceptible to a fungal pathogen.
• If the pathogen can infects one rotifer,
what are the implications for the
population of rotifers?
The pathogen can infect the rotifer’s kin.
They are clones, and hence genetically
identical.
• How do rotifers escape this pathogen?
They outlast them
under desiccating
conditions.
Life without Sex
•Bdelloid rotifers escape their pathogens by going dormant.
•But this is not possible for all organisms.
•Asexual reproduction + susceptibility to pathogen/predator = extinction
There is a strong connection to sexual reproduction and the
evolutionary arms race between predator/prey or pathogen/host.
Life with Sex
The Red Queen hypothesis
As the Red Queen told Alice, “it
takes all the running you can do, to
keep in the same place.” Similarly,
animals and plants must continually
adapt and evolve just to avoid going
extinct. (Illustration by Sir John
Tenniel from Lewis Carroll’s
“Through the Looking-Glass,” 1871)
Evolution at Two Loci
 Evolution at Two Loci
• Linkage Disequilibrium
• Mechanism that creates
• Mechanisms that eliminate
 Ramifications of Linkage Disequilibrium
• Does it exist
• Reasons for measuring
 Significance of Sex
• Asexual Reproduction
• Advantage of Sexual Reproduction
Evolution at Two Loci
Expand on Hardy-Weinberg one locus model & consider two
loci simultaneously.
The multilocus genotype of a
chromosome or gamete is referred to
as its haplotype.
 Independent assortment is based on chromosome segregating,
not loci segregating.
 Mendel’s law of independent assortment:
• This principle states that the alleles for a trait separate when gametes are
formed.
• These allele pairs are then randomly united at fertilization.
 Some genes on a single chromosome are linked because they
are not free to undergo independent sorting.
• Tend to be passed on as a single unit.
Evolution at Two Loci
 Population Genetics is based on Hardy-Weinberg equilibrium
• Simple (Allelic Frequencies) and Accurate (predicts
evolution)
 Problem: Tracks a single gene locus
• Genomes contain thousands of loci.
• A trait may be affected by a culmination of many of these
loci.
• Selection pressures on these other loci may alter trait under
examination.
• Question: Do we need to worry about selection at other Loci?
– Sometimes… Need to know if system is in Linkage
Equilibrium or Linkage Disequilibrium.
Evolution at Two Loci
Linkage Equilibrium – When the genotype at one locus is randomly
distributed with respect to genotype at the other locus.
Allele frequencies in
both populations are
the same.
Evolution at Two Loci
Linkage Equilibrium – When the genotype at one locus is randomly
distributed with respect to genotype at the other locus.
But the populations are
not identical.
Chromosome
frequencies in both
populations are not
the same.
e.g. The frequency of
chromosome AB differs.
Evolution at Two Loci
Linkage Equilibrium – When the genotype at one locus is randomly
distributed with respect to genotype at the other locus.
Chromosome frequencies
are calculated by
multiplying allele
frequencies.
A = 0.6, B = 0.8
AB = 0.6 x 0.8 = 0.48
Chromosome frequencies
cannot be calculated by
multiplying allele
frequencies.
A = 0.6, B = 0.8
AB = 0.44
Evolution at Two Loci
First lesson of 2 locus H-W: Populations can have identical allele
frequencies, but different chromosome frequencies.
But the populations are
not identical.
Chromosome
frequencies in both
populations are not
the same.
e.g. The frequency of
chromosome AB differs.
Evolution at Two Loci
First lesson of 2 locus H-W: Populations can have identical allele
frequencies, but different chromosome frequencies.
The frequency of B on A
is the same for both
chromosomes.
The frequency of B on A
are not the same.
Evolution at Two Loci
Linkgage Disequilibrium defined: when there is a nonrandom
association between a chromosome’s genotype at one locus and
its genotype at the other locus.
Linkage Equilibrium – When the genotype at one locus is randomly
distributed with respect to genotype at the other locus.
3 conditions are true for a pair of loci in linkage equilibrium.
Evolution at Two Loci
Linkage Equilibrium – When the genotype at one locus is randomly
distributed with respect to genotype at the other locus.
1. The frequency of B on chromosomes carrying allele A is equal to the frequency of B
on chromosomes carrying allele a.
Evolution at Two Loci
Linkage Equilibrium – When the genotype at one locus is randomly
distributed with respect to genotype at the other locus.
2. The frequency of any chromosome haplotype can be calculated by multiplying the
frequencies of the constituent alleles.
A = 0.6, B = 0.8
AB = 0.6 x 0.8 = 0.48
Evolution at Two Loci
Linkage Equilibrium – When the genotype at one locus is randomly
distributed with respect to genotype at the other locus.
3. The quantity D, known as the coefficient of linkage disequilibrium, is equal to “0”
D= (gAB)(gab)-(gAb)(gaB) = 0
Where: gAB = frequency of AB
gab = frequency of ab
gAb = frequency of Ab
gaB = frequency of aB
D=?
D = 0.48 x 0.08 – 0.12 x 0.32
D = 0.0384 – 0.0384 = 0
Evolution at Two Loci
D= (gAB)(gab)-(gAb)(gaB) = 0
D=?
D = 0.48 x 0.08 – 0.12 x 0.32
D = 0.0384 – 0.0384 = 0
0.48
0.12
0.32
0.08
Evolution at Two Loci
Reasons for Linkage
Disequilibrium
Selection on multilocus
genotype
Predation of certain
individuals
Result: Fewer offspring
with certain C-some
configurations
Evolution at Two Loci
Reasons for Linkage
Disequilibrium
Genetic Drift
Mutations
spontaneously occur in
a population altering
allelic frequency.
Selection that favors
this mutation may
increase degree of
disequilibrium
Evolution at Two Loci
Reasons for Linkage
Disequilibrium
Population admixture
Combining two gene
pools with different
allelic combinations
Evolution at Two Loci
Selection on Multilocus Genotype
• Predation of selective individuals in a population
• Survival of phenotypes sized >13 (65.28% population)
• Elimination of certain genotypes creates disequilibrium
Evolution at Two Loci
Selection on Multilocus Genotype
D= (gAB)(gab)-(gAb)(gaB) = 0
0.1536
0.0576
D=?
D = 0.4416 x 0.0 – 0.0576 x 0.1536
D = 0 – 0.0088 = - 0.0088
0.4416
ab = 0.0
Evolution at Two Loci
Genetic Drift
• Change in Frequency of alleles in a
population resulting from sampling error.
• Chance variation in survival and/or
reproductive success.
• Non-adaptive evolution.
• Though mutation and selection would
seem to be the forces at work,
- this process could only operate in a
finite (small ) population.
• In a large population, A -> a would occur
many times on both chromosomes.
• Selection would favor ab and aB
chromosomes.
Evolution at Two Loci
Population Admixture
Populations are in Linkage Equilibrium
Upon mixing, Locus A and B are no longer in equilibrium
AB and ab combinations are in excess
Evolution at Two Loci
What eliminates linkage disequilibrium? Sex!
Evolution at Two Loci
What eliminates linkage disequilibrium? Genetic recombination
• Crossing over and outbreeding brings together chromosomes with different
haplotypes.
• Crossing over breaks up old combinations of alleles and creates new ones.
Genetic recombination – the creation of new combinations of alleles during
sexual reproduction.
• Genetic recombination randomizes genotypes at one locus with respect to
genotypes at another locus.
• Reduces the frequency of overrepresented haplotypes.
Evolution at Two Loci
Linkage disequilibrium is reduced at a predictable rate.
The rate of decline is proportional to the rate of recombination (r)
between two loci.
Linkage disequilibrium changes by D’ = D(1-r)
Closely linked loci.
Free recombination
Evolution at Two Loci
 Genes exist in a linear fashion along the
chromosome
• Variable amount of exchange occurs between
any two genes along a chromosome
(distance).
 Linked genes may not always travel as a group,
because of crossover.
 Recombination of alleles between the
homologous chromosomes • Randomizes genotypes
• Reduces over represented combos (e.g. AB) and
increases under represented combos (e.g ab)
Evolution at Two Loci
 Disequilibrium between loci falls as a function of distance on the
chromosome.
Most pairs of loci are in linkage equilibrium.
Evolution at Two Loci
Ramifications of Disequilibrium
If loci are in disequilibrium, then
selection on one loci affects other.
Single-locus population genetic models
will make inaccurate predictions
If loci are in equilibrium, then
selection on one loci doesn’t affect
other.
 Good news: Sex is really good at reducing Disequilibrium
 Really good news: Most pairs of loci are in equilibrium most of
the time.
Single locus models will work well most of the time
Evolution at Two Loci
Case study – Clegg et al. (1980)
Fly populations set up in
disequilibria.
But quickly (than
expected) go to
equilibrium.
Evolution at Two Loci
The downside of linkage equilibrium?
Linkage disequilibrium
• Selection at locus A changes
frequency at locus B too.
• Population genetic studies
examining locus B alone will make
incorrect predictions about its
evolution.
Genetic hitchhiking. – change in the
frequency of an allele due to selection on a
neighboring allele.
• Makes it difficult to associate a particular allele with a disease.
Evolution at Two Loci
The downside of linkage equilibrium?
Mutation of the L503F allele (C -> T)
strongly associated with Crohn’s disease.
L503F mutation is adaptive. It increases the
transport of an antioxidant – ergothioneine.
Does L503F contribute to Crohn’s disease?
Probably not. L503F probably in linkage
disequilibrium with nearby genes that do
play a role in Crohn’s disease.
Significant link between L503 F and Crohn’s
No link between L503 F and Crohn’s
Better
candidates for
Crohn’s disease.
Evolution at Two Loci
Calculation of linkage disequilibrium permits estimation of when
a mutation appeared.
•
•
•
•
L503F appeared as a unique mutation.
Rose to high frequency due to selection.
Frequency of L503F high in old world.
Linkage disequilibrium decaying under the influence of recombination.
• Knowing the rate of recombination,
age of the L503F allele can be
calculated.
• Estimated to be 12,000 years old.
The Adaptive Significance of Sex
Squeezing offspring out like this
Should lead to
Lots of organisms do it.
But they also
do this. Why?
2X as many offspring in 3 generations
The Adaptive Significance of Sex
John Maynard Smith (1978) – developed the null model of reproductive mode.
Considers the fate (which outcome is more likely) of two populations that differ in
reproductive mode:
1: Females reproduce asexually.
2. Females reproduce sexually.
An Evolutionary Paradox!
Smith made 2 assumptions:
1. A female’s reproductive mode does not affect how many offspring she makes.
2. A female’s reproductive mode does not affect the probability that her offspring will
survive.
But it doesn’t! Even asexual organisms
This model should result in this –
reproduce sexually at some time.
the asexual mode should dominate
Jaquiery et al. 2013 PLOS
The Adaptive Significance of Sex
A thought experiment regarding the two assumptions. Which is likely to be violated?
1. A female’s reproductive mode does not affect how many offspring she makes.
 This is violated for any species in which the male provides some form of parental care.
• This would seem to enhance the reproductive output compared to asexual species.
• But – for most species, parental care by the male is usually lacking.
• So reproductive output of a female should not differ between sexual and asexual.
2. A female’s reproductive mode does not affect the probability that her offspring will
survive.
 This would appear to be the assumption that is most likely violated
The Adaptive Significance of Sex
The evidence. Caenorhabditis elegans. Populations used environment to select against
deleterious mutants.
Obligately outcrossing
males and females
Hermaphrodites and males
Obligate selfing
hermaphrodite
Reduced fitness likely due to
mutations passed on to
offspring.
Which offspring? All of them.
The Adaptive Significance of Sex
The evidence. Caenorhabditis elegans.
Obligately outcrossing
males and females
Hermaphrodites and males
Obligate selfing
hermaphrodite
Reduced fitness likley due to
mutations passed on to
offspring.
Increase the mutation rate
The Adaptive Significance of Sex
The evidence. Caenorhabditis elegans.
Obligately outcrossing
males and females
Hermaphrodites and males
Obligate selfing
hermaphrodite
Reduced fitness likley due to
mutations passed on to
offspring.
Increase the mutation rate
Fraction of
offspring
fathered by
males evolves.
The Adaptive Significance of Sex
The evidence. Caenorhabditis elegans.
Greater mutation rate selects
for greater outcrossing.
Take Home Message: Sexual reproducing
individuals produce offspring with a higher
degree of Fitness.
Population Genetics and Sex
 Sex results in Crossing over and Random matings in a population
• Main consequence of sex is to Reduce Linkage
Disequilibrium.
• If a population is in Linkage Equilibrium, then sex has no
effect or benefit.
 2 main events make Sex a Benefit by driving populations toward
Equilibrium.
• Genetic Drift and Mutation
- Sex restores lost genotypes
• Changing Environments
- Sex recreates favorable combinations for new
environments
Population Genetics and Sex
Muller’s Ratchet (1964)
 Argues that asexual populations are doomed
to accumulate deleterious mutations.
 Each mutation group (e.g. 0 mutations, 1
mutations) is a subpopulation.
 If they are small, they may “drift” to
extinction. (zero mutation in this example)
 This leaves small populations with greater
number of mutations. “click” – the ratchet
 Loss of a group due to drift more likely than
re-creation of a group (e.g. 0 mutations) via a
back mutation.
 Genetic load – burden imposed by the
accumulation of mutations.
 Load becomes so great that population goes
extinct via selection against multiple
deleterious alleles.
Population Geneticists and Sex
Muller’s ratchet (1964)
 Sex breaks the ratchet by recreating
favorable multilocus genotypes
• If a no-mutation group is lost, it can
be reconstituted by outcrossing and
recombination.
 The crux of Muller’s ratchet is that
linkage disequilibrium is created by
genetic drift.
• Sex reduces linkage disequilibrium
by re-creating missing genotypes.
Selection, Parasites, Environmental Change and Sex
 In a Constant Environment:
• Asexual offspring have the same fitness as mother (proven
design works again, and again, and again, and again……)
• Sexual offspring may not survive well (they are new
combinations – unproven designs)
 In a Changing Environment:
• Asexual offspring are the same design in a changing world. If
the single combination of alleles is not successful, then
extinction.
• Sexual offspring are new combinations. At least one
combination may work in a new environment. Persistence.
Selection, Parasites, Environmental Change and Sex
C. elegans experiment. 3 Treatments
1) Control – challenged with heat killed
bacteria.
2) Evolution - challenged with pathogenic
bacteria.
3) Coevolution - challenged with pathogenic
bacteria from dead worms in treatment
#2.
Outcrossing advantageous
to a point.
But sex breaks up
advantageous genotypes.
Not good in a constant
environment.
Selection, Parasites, Environmental Change and Sex
C. elegans experiment. 3 Treatments
1) Control – challenged with heat killed
bacteria.
2) Evolution - challenged with pathogenic
bacteria.
3) Coevolution - challenged with
pathogenic bacteria from dead
worms in treatment #2. Selection for
successful bacteria and successful
worms.
Both host and pathogen are under
selection – a constant arms race. Sex is
beneficial in constantly generating new,
potentially successful combinations.
Selection, Parasites, Environmental Change and Sex
The Red Queen Hypothesis: posits that the role of sex is to preserve alleles which are
currently disadvantageous, but which will become advantageous against the
background of a likely future population of parasites.
From Lewis Carroll’s “Alice Through the Looking Glass”, Alice finds herself hand-in-hand with
the Red Queen, running faster and faster but without getting anywhere. The Red Queen
explains, "Now, here, you see, it takes all the running you can do, to keep in the same
place. If you want to get somewhere else, you must run at least twice as fast as that."
Selection, Parasites, Environmental Change and Sex
Red Queen Hypothesis - Most favored changing-environment theory of sex
 Selection for resistance to competitor/parasite/predator #1
may favor some multi-locus genotype.
E.g. AABB and aabb in a two-locus model.
Thus Aabb and aaBB may become temporally rare.
 In the absence of resistance to competitor/parasite/predator
#2, they become more successful, and more common.
 Selection pressures now switch for resistance to
competitor/parasite/predator #2, thus favoring the two-allele
combinations Aabb and aaBB.
• These alleles would have been lost in an asexual system
because of linkage disequilibrium. But sex is able recreate
these rare genotypes.
Selection, Parasites, Environmental Change and Sex
Selection, Parasites, Environmental Change and Sex
Male frequency
as an index of
sexual females.
Check 4th edition
for alternate
explanations.
• Study by Lively (1992) was observational, hence two alternate explanations
are possible.
1- Trematode infection rates are higher in more dense populations of snails (true),
because high host density facilitates parasite transmission.
2 – The frequency of parthenogenetic females is higher in less dense (false) populations
of snails, because the real benefit of parthenogenesis is that it allows females to
reproduce even when mates are hard to find.
Selection, Parasites, Environmental Change and Sex
Male frequency
as an index of
sexual females.
Infection rate by
trematode parasite.
• Study by Lively (1992) was observational, hence two possible explanations:
1- Red Queen hypothesis - Sexual reproduction permits constant evolution by constantly
recreating multilocus genotypes that may have been eliminated by selection or drift.
2 – Males are more susceptible to infections (ruled out by lab experiments).