Origins and maintenance of sex Sexual conflict Sex ratios Dr. Sally Otto, UBC

Asexuality vs. self-fertilization Give examples of the mechanism – pseudo copulation footage in snakes Video footage?

Costs of sex

Advantages of asexuality: which sex limits population growth? Sexual lineage Asexual lineage Note: no figure, but attempted to convey same idea with letters. What would happen if an asexual mutant occurred in a population? Would it increase or decrease?

Advantages of asexuality: fitness of individuals Frequency

99.9% of species can’t be wrong 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 This implies that asexuality should have some major problems, or sex some major advantages. What exactly?

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 happen and accumulate mutation-free chromosome

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

Müller’s ratchet in sexuals? No. “CLICK” frequency frequency # of deleterious mutations # 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 B1 A2 B2 Low fitness High fitness A1 B1 parental A1 B2 A1 B1 recombinant meiosis A2 B1 A2 B2 recombinant A2 B2 parental

Does recombination increase genetic variance for fitness?

The results of sex: cost of recombination If parents have higher than average fitness, what effect of recombination? The results of sex: cost of recombination A1 B1 meiosis A2 B2 parental recombinant

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 Malathion concentration Proportion sexual 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 start after selection on host

Parasites and host: Red queen 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? Asexuals have more parasites 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

Why two sexes? Most species have two sexes Some have multiple sexes (mating types): mating type 1 can mate with anyone except mating type 1, etc. Advantage: higher proportion of population available for mating Why only two mating types??

Implication of two sexes Sexual selection Sexual conflict

Intrasexual competition: sperm competition When females mate with multiple males, sperm can compete Example: yellow dung fly Scathophaga stercoraria

Yellow dung flies Females mate with multiple males Last male to mate fertilizes most ovules Selection experiment: compare wild to 10 generations monogamous, 10 generations polyandrous Expected effect on sperm competition? Monogamous ♀ + ♂ Polygamous ♀ + 3♂ 10 gen. 10 gen.

Yellow dung flies: polyandrous males vs. monogamous males Mate each female with two males: one from polyandrous line, one from monogamous line Hosken & Ward 2001. % offspring sired by second male to mate

Sexual selection and intersexual conflict evolution Traits that favour male success may harm female success Example: sperm competition faster sperm win: higher fitness for male too many sperm: polyspermy – egg is lost human spontaneous abortions: 3% of conceptions end due to triploid embryos (1 egg, 2 sperm)

Female evolutionary response: slow the sperm Yellow dung fly experiment

Evolution of sex ratios Which sex should be more common? Why aren’t there many females per male? imagine monogamous species females become more common which would be better to have has offspring: male or female?

Sex bias? If females could choose: What if polygamous species? some males have many mates most males do not mate If in excellent condition, should a female produce male or female offspring? If in poor condition, should a female produce male or female offspring?

Sex bias: data Case 1: Kakapo (NZ parrot) Captive breeding, plenty of food. When well fed: 70% sons Adequately fed: 50% sons

Sex bias in humans: data Mormon second (or higher) wives Sons Daughters Ratio One co-wife 4,217 3,994 105.6 More than one 712 548 129.9

References Hoskins, Garner, and Ward. 2001. Sexual conflict selects for male and female reproductive characters. Current Biology 11:489-493. Hoskins and Ward. 2001. Experimental evidence for testes size evolution via sperm competition. Ecology Letters 4:10-13. Lively, Craddock, & Vrijonhoek. 1990. Red queen hypothesis supported by parasitism in sexual and clonal fish. Nature 344:864-866. Ridley, M. 1993. The red queen: sex and the evolution of human nature. Harper. Nice job reviewing Muller’s ratchet and other hypotheses for the maintenance of sex. Goes boldly into explaining human nature, far beyond the evidence. Robertson et al. 2006. Sex allocation theory aids species conservation. Biology Letters 2:229-231.

Readings and questions 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. Explain how the Red Queen hypothesis relates to the maintenance of sex. Why might sex be advantageous in the face of parasites or disease? In most species, the sex ratio is 50 / 50 male: female. Why wouldn’t evolution favor a higher proportion of females, since one male could mate with many females? Discuss using the idea of frequency dependent selection.