Transcript Adaptive significance of sex

Adaptive Significance
of Sex
By:
Jordan Cohen, Amanda Blankinship, Kaitlan
Hughes
Sex is Silly
 Why reproduce sexually?
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Energy expensive
Hazardous
Unforeseen complications
Introduction of diseases
Asexuality is Cool
 Benefits of reproducing asexually
 A good number of asexually reproducing
organisms can also reproduce sexually.
 Rapid population growth
 Production of more offspring per parent
 Genetically identical
Is it a He or a She?
 Both!
 Aphid species ( Plant lice)
Reproduction by means of parthenogenesis;
During spring and summer months
populations
are 100% dominated by asexual females.
Fall populations make the change to
produce
sexually reproductive males and females,
and
Sexual vs. Asexual
John Maynard Smith’s null model
In the same population if both modes of
reproduction are possible, will one mode
replace the other?
First things first:
-In order to use this model two assumptions must be
met
1.A female’s reproductive mode does not affect the number of offspring
she can make.
2.A female’s reproductive mode does not affect the probability that her
offspring will survive.
Asexual take over!
 If these assumptions are met, asexual
populations theoretically will dominate.
- asexual females reproduce and by the 3rd generation produce twice as
many individuals.
- ideally, in a population of both modes asexuality should dominate over
time, why doesn’t this occur in real populations?
What benefits allow sex to
stay?
 Simple biological facts..
Violation of assumptions
- first assumption does depend on whether the
reproductive female is sexual or asexual.
- violation of the second assumption holds great
importance of the advantage of sex.
Dunbrack and colleagues
suggests..
Second assumption is incorrect, at least by means of their experiement..
R.L. Dunbrack used flour beetles (Tribolium castaneum) in
mixed populations of black and red T. castaneum, distinguishing
between “sexual” and “asexual” beetles.
- Researchers introduced a challenge to the beetles
environment by adding fluctuating amounts of insecticide to the flour
they lived in. This was done to observe an evolution of insecticide
resistant generations.
- Eight replicates of this experiment were conducted, with
increasing amounts of insecticide.
Results
 Control cultures correlated with Smith’s model.
 Experimental cultures asexual strand appeared to
begin dominating, but within roughly 20 generations the
evolving sexual strand recovered, and eventually
eliminated the asexual strand.
 Speed of elimination depending on insecticide
concentrations.
Sex in Populations Means
Genetic Recombination
-Sex is reproduction involving:
1.Meiosis with crossing-over
2.Matings between unrelated individuals
-Together, genetic recombination occurs
The placement of allele copies on chromosomes
or within gametes that are different from the
multilocus genotypes they once belonged to in a
previous generation
Otto and Lenormand
 Selection and genetic
recombination
 Experiments in which populations were
exposed to artificial selection
 Looked for a degree of change in genetic
recombination during meiosis
 Found an increase in change
 Figure 8.19
Sex Is Beneficial
-Drift theory of sex
Asexual vs. sexual
-Muller’s ratchet  H.J. Muller:
linkage disequilibrium is created
by drift
-Finite asexual population
Asexual Populations
Accumulate Deleterious
Mutations
Sex Breaks the Ratchet
• Genetic load- Reduction in the
mean fitness of a population due to
the presence of deleterious alleles
• Sexual reproduction
Muller’s Ratchet
 Haigh- mathematical model of Muller’s
ratchet
 Population size
 Mutation rate
Andersson and Hughes
 Salmonella typhimurium
 444 cultures
 1,700 generations
A Bacterial Population
Subjected to Periodic
Bottlenecks
5, or 1%, had
significantly
reduced
fitness
Lambert and Moran
 Tested Muller’s ratchet in nature
 Used 9 species of bacteria living in insect
cells (obligate endosymbionts)
 Focused on stability of rRNA genes
 Found consistence with Muller’s ratchet
 15-25% less stable
Sex by Drift
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Counteracting the ratchet
Keightley and Otto- sex beneficial
Sex over the long term
Asexual females in a sexual environment
Sexual vs. asexual
Selection Imposed by a Changing
Environment Can Make Sex Beneficial
• In a constant environment asexual
reproduction is a better fit than sexual
reproduction
• In a changing environment sexual
reproduction is a better fit than asexual
reproduction
Red Queen Hypothesis:
 Evolutionary arms race between hosts
and parasites.
 Parasites and hosts struggle.
 Parasites select in favor some
multilocus host genotypes in some
generations and others in other
generations
Curtis Lively:
 Examined if parasites select in favor of
sex in hosts
 Studied snails (Potamopyrgus
antipodarum) hosts to parasitic
trematode worms
 Sexual and asexual females
 Worms castrate hosts
Lively’s Hypothesis:
 If snails and trematodes select in favor of
sex in snails then sexual snails should
have a higher trematode infection rate.
Lively’s Experiment
 Took samples of snails from lakes and
determined their sex and if they were
infected
Results
 Higher proportion of females are sexual
in heavily parasitic populations
 Results match prediction
References
 http://www.indiana.edu/~curtweb/Resear
ch/Red_Queen%20hyp.html
 http://webcache.googleusercontent.com/
search?q=cache:x_uiDSs2pAYJ:faculty.p
lattsburgh.edu/neil.buckley/Evolution/Cha
pter8a.ppt+John+Maynard+Smith+null+
model+of+reproduction&cd=6&hl=en&ct=
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