Origins and maintenance of sexSexual conflict

Sex ratios

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: fitnessF

req

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ncy

of

ind

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s ASEX

SEX

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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 happen and accumulate

mutation-free chromosome

Müller’s ratchetfr

eq

uen

cy

# of deleterious mutations

LLC: “least-loaded class”

fre

qu

enc

y

# of deleterious mutations

“CLICK”

Müller’s ratchet in sexuals? No.fr

eq

uen

cy

# of deleterious mutations

fre

qu

enc

y

# of deleterious mutations

fre

qu

enc

y

# 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: adaptationA1 B1

A2 B2

A1 B2

A2 B1

Low fitness

High fitness

High fitness

Low fitness

A1 B1

meiosis

A2 B2

A1 B1

A2 B2

A1 B2

A2 B1

parental

recombinant

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

A1 B1

meiosis

A2 B2

A1 B1

A2 B2

A1 B2

A2 B1

parental

recombinant

recombinant

parental

Experimental test of recombination and adaptation

Adaptation may require new combinations of alleles

Asexuality does not allow thisExperiment: 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: adaptationP

ropo

rtio

n se

xual 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

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Parasite Host

Parasites and host: Red queen

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Parasites and host: Red queenParasite Host

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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?

Par

asit

es

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

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 questions1. 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?

6. 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.