Long Term Advantages of Sex

Is sex better than Asex?

(Assume: female’s reproductive mode doe not affect number of offspringOr survival probabilities of her offspring)

TEST Dunbrack et al 1995

Used Tribolium castaneum; red morph assigned to be sexual and black morphAssigned to be asexual

Selective agent used was malathion added to flour

Fitness of blacks and reds measured each generation

Which is more fit – asexual or sexual phenotype????

CONCLUSIONS: sexual forms have long term advantages

REASONS FOR ADVANTAGES?

1.Muller’s RatchetDue to genetic drift, populations will continually accumulatemore mutations per genome that cannot be removed by selection

TEST: Anderson and Hughes . . .using Salmonella

2. Changing selection in changing environments

Short Term Advantages of Sexual Reproduction

Changing environments impose hostile and novel selection in an unpredictable manner on organisms

BENEFITS OF SEX

RED QUEEN hypothesis: B/c of constantly changing ‘selection environments’ between generations, individuals who produce morevariable offspring have higher fitness than those that reproduceless variable offspring

• Curtis Lively et al. 1992 through 2007; studied snails in New Zealand (Potamopyrgus antipodarum)

Snails infested by over 12 different trematode worm parasites

Parasite eats gonads – castrating host (0 fitness)

Two types of females in population: obligately sexual and obligatelyasexual

Proportions of sexuals/asexuals and trematode densities vary across population

Snails infested by over 12 different trematode worm parasites

Parasite eats gonads – castrating host (0 fitness)

Two types of females in population: obligately sexual and obligatelyasexual

Proportions of sexuals/asexuals and trematode densities vary across population

Why two reproductive phenotypes maintained in same species in N.Z.? -

Does varying selection influence the benefits to costs of sexual vs. asexualreproduction? (ie. Support for the Red Queen?)

Alternate Hypotheses?

1. Trematode infection rates are higher in more dense populations of snails

2. More asexual females are found in less dense snail populations b/c the costs of finding a male (as required by sexual phenotypes) are higher when fewer individuals are present

COSTS

Purging Bad Mutations Hypothesis

Harmful mutations accumulate within an individual’s genetic line; sex exposes the recessive mutations to selection and thus allows them to be removed