Origins and maintenance of sex Dr Sally Otto

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 SEX ASEX

So, is sex good? 99. 9% of species can’t be wrong Cnemidophorus Sexual Asexual Dandelions Daphnia

(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 and accumulate Mutations happen mutation-free chromosome

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

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

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 A 1 B 1 A 1 B 2 A 2 B 1 A 2 B 2 A 1 B 2 meiosis A 2 B 1 A 2 B 2 Low fitness High fitness Low fitness parental recombinant parental

Does recombination increase genetic variance for fitness? D = 0. 25 D = -0. 25 D=0

If parents have higher than average fitness, what effect of recombination? The results of sex: cost of recombination A 1 B 1 A 2 B 2 A 1 B 2 meiosis A 2 B 1 A 2 B 2 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 3 x reproductive advantage • Asexuals start 0. 5 of population • Selection: pesticide Malathion • What proportion are sexual?

Advantage of sex: adaptation Proportion sexual Malathion concentration 30 Generations 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 Parasite Host

Parasites and host: Red queen Parasite start after selection on host Host

Parasites and host: Red queen Parasite after selection on host after selection on parasite Host

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 Asexuals have more parasites Fish length (mm)

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?