Origins and maintenance of sex Dr. Sally Otto, UBC

Asexuality vs. self-fertilization Self-fertilization: Asexuality:

Costs of sex

Advantages of asexuality: which sex limits population growth? Asexual lineage Sexual lineage

Advantages of asexuality: fitness Frequency of individuals ASEX SEX

So, is sex good? Sexual Asexual Cnemidophorus DaphniaDandelions 99.9% of species can’t be wrong

(but a few asexual lineages persist) Bdelloid rotifers freshwater filter feeders

Hypotheses to explain the maintenance of sex Harmful mutations: Mller’s ratchet Speed of adaptation and the Red Queen

Disadvantages of asexuality: Müller’s ratchet Mutations happenand accumulate mutation-free chromosome

Müller’s ratchet frequency # of deleterious mutations LLC: “least-loaded class” frequency # of deleterious mutations “CLICK”

Müller’s ratchet in sexuals? No. frequency # of deleterious mutations frequency # of deleterious mutations frequency # of deleterious mutations “CLICK”

Müller’s ratchet : an experiment Set up 444 cultures of Salmonella Transfer one individual every 24 hours 1700 generations Prediction: lower fitness Test: generation time: –parental: –Experimental populations:

Evidence for Müller’s ratchet : the human Y chromosome In XX females, recombination Y chromosome does not pair with X

Müller’s ratchet overall

Reason’s for sex: adaptation A1A1 B1B1 A2A2 B2B2 A1A1 B2B2 A2A2 B1B1 Low fitness High fitness Low fitness A1A1 B1B1 meiosis A2A2 B2B2 A1A1 B1B1 A2A2 B2B2 A1A1 B2B2 A2A2 B1B1 parental recombinant parental

Does recombination increase genetic variance for fitness? D = 0.25D = -0.25D = 0

If parents have higher than average fitness, what effect of recombination? The results of sex: cost of recombination A1A1 B1B1 meiosis A2A2 B2B2 A1A1 B1B1 A2A2 B2B2 A1A1 B2B2 A2A2 B1B1 parental recombinant parental

Experimental test of recombination and adaptation Adaptation may require new combinations of alleles Asexuality does not allow this Experiment: flour beetles (Tribolium) Have stock population Allow one population to evolve The other is restocked from original population, as if asexual. Asexual has 3x reproductive advantage Asexuals start 0.5 of population Selection: pesticide Malathion What proportion are sexual?

Advantage of sex: adaptation Proportion sexual Malathion concentration Generations 30 figure 7.18

Do organisms need to adapt? Red queen hypothesis Red Queen to Alice: “Now, here, you see, it takes all the running you can do, to keep in the same place.” --Lewis Carrol, Through the Looking Glass (1872) Concept: constant adaptation needed. Why?

Parasites and hosts Imagine four parasite genotypes, four host defense genotypes ParasiteHost

Parasites and host: Red queen ParasiteHost start after selection on host

Parasites and host: Red queen ParasiteHost after selection on host after selection on parasite

Evolution over time: cycling genotypes

Red queen, evidence: topminnows of Mexico Interspecific hybrids from asexual triploids or: sexual diploid fish Infected by parasites that cause spots

Data I: sexuals vs. asexuals: which has more parasites? Parasites Fish length (mm) Asexuals have more parasites

What if there is no variation in sexual population? Heart pool: dried up in 1976 Recolonized by just a few sexual minnows and a few asexual minnows. Which will have the higher fitness?

Why sex, summary

Readings and questions Reading: Freeman and Herron chapter 8. (chapter 7, 3 rd edn) 1. In the beetle evolution experiment (figure 8.18) Dunbrack et al did not actually asexual beetles, as there aren't any. Instead they used two different lines of beetles that differed in color, treating one line as if it were asexual by replacing individuals with individuals from a stock population. The researcher's simulated asexual population was not allowed to evolve at all in response to competition and the presence of the insecticide. Is this realistic? 2. In general, would you expect asexual lineages to persist longer with small population sizes or large population sizes? Why? 3. If the offspring of sexual and asexual individuals have equivalent fitness, why would asexuals take over a population? 4. Explain how Muller's ratchet affects sexual populations differently from asexual populations. 5. Explain how the Red Queen hypothesis relates to the maintenance of sex. Why might sex be advantageous in the face of parasites or disease?