Evolution and Maintenance of Sex

Evo Exam 2 :((

What is sex?

Two parts

1. Meiosis w/ recombination between homologous chromosomes

2. Mating: association of homologous chromosomes from different individuals

Paradox of Sex

Why does sex happen if asexual reproduction is more efficient?

What is the benefit?

Potential Costs of Sex

• Finding and securing partners

  • can be challenging or dangerous

  • no partner = no reproduction

  • finding + securing partner = exposure to danger

• Increased risk of predation

• Time lost from other activities

  • ex: foraging

• Sexually transmitted infections

• Chance of hybridization and inviable offspring

  • ex: mules

• Break up fit allele combinations for multi-locus genotypes

• Genome dilution

• 2- fold cost of sex

Genome Dilution

sexual females lose genetic representation in the next generation relative to asexual

• females need to produce two offspring to replace themselves evolutionarily

Two-fold cost of sex

1. Reproductive mode (sexual/asexual reproduction) does not affect the # of offspring a female produces

2. Does not affect offspring survival

• asexual females have a 2-fold advantage over sexually reproducing females

  • also avoid genome dilution!

List and describe the hypothesis discussed in lecture which seek to explain the evolution and maintenance of sex.

• Weismann hypothesis

• Negative Selection

• Positive Selection

  • Fisher-Muller hypothesis

• Complex environments

  • Tangled Bank, Red Queen

Weismann hypothesis

• sex produces diversity that speeds the response to selection

  • adaptive evolution more efficient

• selection responses are stronger w/ recombination ( increase in recombination rate)

• sex speeds evolutionary response to selection

• Connect to LD:

  • recombination breaks up linked alleles, reducing LD and allowing selection to act on additive alleles at different loci independently

• selection can indirectly target recombination rate

Negative selection

• sex slows down the accumulation of deleterious mutations/ purges bad mutations

  • ex: Muller’s Ratchet

Muller’s ratchet

Accumulation of mutations in small asexual populations leads to eventual local extinction

• in small populations, high fitness genotypes can be lost through drift

  • edging population closer to extinction

• asexual populations accumulate deleterious mutations in an irreversible manner

  • causes fitness to decline each generation

    • producing a ‘mutational meltdown’ extinction

What breaks Muller’s ratchet?

SEX BREAKS THE RACHET!!!!!!

• recombination recovers individuals with no mutations, purges harmful mutations

• get a mutation free chromosome

Positive selection

Sex speeds up the spread of beneficial mutations

• Fisher-Muller hypothesis

Fisher-Muller hypothesis

Sex allows the horizontal transfer of beneficial mutations through the population

• AB novel mutations

  • asexual: AB can only rise if both mutation occur in same individual

  • sexual:

    • A + B can arise in different individuals, but AB can form through sex

  • makes sex spread good mutations faster

Complex environments

Sex enhances the ability to exist in complex, changing environments

Ex: Red queen

Red Queen Hypothesis

• environmental heterogeneity results from interactions among species

  • sex is favored bc genetically diverse offspring can withstand disease or parasites

• recombination should be greatest in long-lived species where # of parasites generations far exceeds host generations

RQH: Experiment: Does selection pressure by parasites increase the frequency of sexually reproducing individuals?

YES

• parasites infect the host to they are more adapted to

• novel hosts have more resistance

  • resistance by snails conferred by novel immune system

  • snails more easily infected by local parasites; shows advantage of rare host genotype

  • genetic diversity advantage

What are the most importation disadvantages of sex?

• Two Fold Cost (Cost of males)

  • females produce offspring but spend resources producing sons who cannot

• Genome Dilution

  • asexual passes 100% of genes

  • Sexual passes 50%

    • diluting genetic contribution

Why isn’t reproductive mode plasticity more common?

• cost of maintenance

  • organism has to predict environment is going to change

  • timing is hard

• phylogenetic constraints

  • once a certain way for generations, its hard to switch back

• Asexual phase can accumulate mutations

  • Mullers Ratchet

  • too long, genetic quality is bad

Do the direct benefits of sex explain it’s occurrence?

• no strong evidence of direct benefits in most species

• insufficient to explain the phylogenetic ubiquity of sex

• need other explanations

Direct benefits of sex

• offspring from sexual reproduction have higher mean fitness than those produced by asexual reproduction

• two parents better than one IF males step up and give resources that increase fitness

  • co-parenting can work

    • doesn’t explain systems where males provide no parental care

Indirect benefits of sex

Offspring from sexual reproduction have higher variance in fitness

• diversity = phenotypic variance

• some are matched well to environment

• Weismann

• Negative selection

• Positive selection

• Complex environments

Wesimann’s hypothesis example

• Treatment flies

  • recombination suppressed over 80% of genome by inversions = no new haplotypes

• Control flies

  • recombination normal = create novel haplotypes

• Selection for decreased bristle number

Why can selection work more efficiently in areas of high recombination?

• recombination breaks up linked alleles, reducing LD and allowing selection to act on additive alleles at different loci independently

What explains the persistence of sex?

• genetic diversity for changing environments

• Red Queen Hypothesis

  • new combination of genes prevent parasites from killing hosts

• All indirect benefits