Adaptive Significance of Sex

By:Jordan Cohen, Amanda Blankinship, Kaitlan Hughes

Sex is Silly Why reproduce sexually?

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 out come a new cycle of parthenogenetic spring/ summer females!

Sexual vs. AsexualJohn 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-over2.Matings between unrelated individuals -Together, genetic recombination occursThe 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 sexAsexual 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 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.plattsburgh.edu/neil.buckley/Evolution/Chapter8a.ppt+John+Maynard+Smith+null+model+of+reproduction&cd=6&hl=en&ct=clnk&gl=us&client=safari