Evolution of Sex

Question 1

How does sexual reproduction play out

Answer 1

Plays out in complicated ways

***Must be important

Question 2

Sexual reproduction norm

Answer 2

Sexual reproduction seems like the norm but hard to explain why this is the case

Question 3

Example weird reproduction

Answer 3

1. Snail – hermaphroditic –> recipircally fertilize
2. Weird fish – start as female and when males leaves becomes male
3. Clown fish – Start as male and turns female when female dies
4. Male angler fish – Male supplies sperm into body
   • Parasitic that just adds sperm to body

Question 4

Commanality of sexual reproduction

Answer 4

Very common in living world

Just because we do it doesn’t mean it needs to be done

Question 5

Sexual selection

Answer 5

Fitness is determined by variation in attracting males
- Occasionally leads to outlandish phenotypes

Phenotype relates to mate choice – not just survival and # of offspring

Example – Bird + colors in monkeys + elk that has long antlers + Dopsin fly (has mandables that attracts mates)

Question 6

What are traits of many organsims driven by

Answer 6

Traits of many organisms seem to be driven by sexual selection
- Traits are there because ways selection interacts in sexual reproduction to drive trait evolution

Question 7

Sexual conflict

Answer 7

Males and females have different optima for fitness
- May be common in nature

Example – 1. Lions –> Males kill all of the females pride that she had with another male to make the female fertile
- Good for fitness of male; bad for fitness of female

2. Bed bugs = violent insemination – male stabs sperm into female body
3. Weird falas on falcan – corkskrew shapes – female has vaginal track that is corkscrew in the opposite direction –> Males change direction to control reproduction and females will chnage in response to control reproduction

Question 8

Understanding why organisms have sex

Answer 8

Still major subfield of evolutionary biology

Question 9

Sex = NS

Answer 9

Sexual reprdouction poses a paradox for adaptation by NS –> it is reproductively costly to maintain two maintaining two mating types

Question 10

Two fold cost of sex

Answer 10

It is reproductively costly to maintain two maintain two mating types

Each cost = doubling of fitness effect and 2 different aspects that come into play

Question 11

Maintaining two mating types

Answer 11

Sometimes hard to explain

Females making only females that also only make females = X2 reproduction rate = X2 fitness

Example – Each females makes 4 offsrping –> only the females will make more offspring
IF 1/2 off are female – each female makes 4 offsrping –> end get 8 Grandchildren

IF Females can make female offsrping without males –> each females makes 4 females –> End 16 grandchildren

8 VS. 16 – BIG difference – X2 replication rate in only females = big fitness advantage
- Females making only females = growth rate X2 = fitness X2

Question 12

Aescual growth rate

Answer 12

Evolve to have aesexual reproduction – #s grow fast = something else is in check

Question 13

Genes in sexual reproduction Vs. Asexual

Answer 13

Sexual reproduction – only pass 1/2 genes to individuals –> offspring is 50% related to mom

Asexual reproduction – offspring is 100% related to mom

In sexual if have a mutations that leads to asexual –> allelle will be in all = spred fast (because asecual will pass mutation to all)

Sexual –> if have gene that maintains sexual = only pass to 1/2 offspring

Question 14

Females that only make females

Answer 14

Contribute disproportionately to next genertion –> have higher fitness

***Females making only females = growth rate X2 = fitness X2

Question 15

First thing to look at in evolution of sex + why it persisted

Answer 15

First need to look at distrubution of sex in nature

Maybe it was impossible to evolove asexual

Question 16

Loss of sex

Answer 16

Diploid organisms whose ancestors had sex – sex is lost rather infrequently

Evolution of asexual out of sexual does happen enough that we know it can happen

Question 17

Reprdouction in many dilpid

Answer 17

Parthenogensis

Question 18

Parthenogenesis

Answer 18

Reproduction by females of other females via unfertilized eggs (females make other females via unfertilized egg)
- Often facultative not obligative –> organisms can repduce in either mode depending on conditions – occurs depending on contexct
- Colony of genetic recombination

Might alternate like clockwork (one generation sexual; 1 generation aesexual)

Question 19

Example Parthenogenesis

Answer 19

1. Daphnia –> can grow a colony so you know they all have the same genotype
   
   • Do sometimes reproduce sexually
2. Insects –> reprduce asexual for many generations and then when stressed csan reproduce sexually

Question 20

Occasional parthinegensis

Answer 20

Parthenogensis can just occur ocasionally happen sometimes – organisms can be largley sexual and sometimes be aesexual
- Seen in complex organisms –> seen in female if away from male

Example – Komoto dragons + bords + sharks –> asexual reproduction of females without sperm

Question 21

Aesexual from sexual

Answer 21

Some organisms evolove asexual from sexual

Example –
1. Stick bugs –> all female – all parthenogensis (eggs develope to females)

2. Worms – Asexual
3. Lineages of lizards – some exlusivley female
   • Some make male offspring –> gives males to stimulate behavior not to get sperm

SHOWS – asexual can evolove from sexual

Question 22

bdelloid rotifers

Answer 22

No one has ever seen them have sex BUT genomic data shows they might on occasion

Question 23

Question in Sexual Reproduction

Answer 23

Given the 2 fold cost of sex why did sex evolve to begin with and why does it persist?

2 fold cost – decrease in reproductive rate + Lowers chance of passing on allele to offspring

Potential Answer: maybe because once evolved –> can’t go back

Question 24

Loss of sex

Answer 24

Diploid ancestors who had sex lose sex rather infrequently BUT there are exceptions

Exception – Loss of sex to have parthenogenesis –> Shows that there must be ways for asexual to evolove

Question 25

Widespread Parthenogesis

Answer 25

Can occur in widespread way

Example:
1. Can occur in normal part of lifecycle
2. Can be facultative way
3. Can be rare/spontaneous way in complex organisms (can be occasional ion some species)
- Organisms that are otherwise sexually reproducing –> Shows that might be way for asexual to evolove from sexual
4. Species can be fixed for asexual reproduction

Question 26

What does parthenogensis show

Answer 26

Shows that asexual can evolve from sexual (Have sexual –> asexual where you stay asexual)

New question = why doesn’t it sweep through given fitness advantage

Question 27

Obligate Parthenogens

Answer 27

Fixed for asexual reproduction (need parthenogenesis) – tend to be single species or very small clased (NOT large raduioations of diversity) + They tend to be young
- NOt very diverse
- One or a handful of species
- Lineages are young –> NOT long standing groups of aninals that reproduce asexually for long periods –> suggests that there are constraints that asxueal is not good long term

Question 28

Expectations to long term asexual lineages rule

Answer 28

Bdelloid rotoifers
- Several hundred soeces going back 100 million years –> NO one has ever seen them have sex BUT the latest genomic data shows that they might on occasion
- Expectations to diverges long lineages evoloving asexual
- Very diverse –> thought that they were the most diverse and longest standing asexual organism

Sequencing the genome –> shows that they likley have sexual reproduction sometimes

Question 29

How can genomic data reveal that sex has taken place?

Answer 29

Recombination between lineages – shows that sexual reproduction goes on

If a chromosome is the resulyt of reocmbination = evidencve that not purley clonal

When have recombination = varaition from 2 parents that recombine = evidence for sexual reproduction

Question 30

Why does sex persist

Answer 30

Recombination

Question 31

Benefits of recombination

Answer 31

The 2 fold cost of sex can be made up for by the benefits of recombination acting to break up linkage disequilibrium that inevitabley forms in purely asexual popultions
- Cost is counteracted by the benefit of recombination

Process = Leads to ineviatble accumulation of deletrious mutations in asexual species

Benefit in recombination = reshuffle variation to get new geentic combinations

Question 32

Issue in clonal organisms

Answer 32

All new mutations = have linkage disequilibrium with other variation = have accumulations of deleterious mutations

Process = Leads to ineviatble accumulation of deletrious mutations in asexual species

Inevitable accumulations of deleterious alleles = Muller’s ratchet

Question 33

Ratchet

Answer 33

A mechanism that can only rotate in one direction

Muller’s ratchet – names that way because it only goes in one direction –> unidirectional increase in deletrious mutations in asexual organisms

Question 34

Muller’s Ratchet

Answer 34

Start: Well adapted high fitness genome – well adapted asexual organism

OVER time – some mutations will arise
- Some of the mutations will be good BUT most are nuetral or bad –> Most will make fitness decrease

Mutations in lineage of asexual –> All offspring will have the mutations –> once mutation pops up it is there for good
- The deletrious mutation that affects fitness a little but not enough to kill you –> Over time lose high fitness alleles until all that is left is deletrious mutations

If only slightly deleterious = not bad enough that NS can act on it

OVER time – initial high fitness genotype can drift out of existence

Question 35

What drives the ratchet fowards

Answer 35

Combination of mutation and drift drives the ratchet forward

Question 36

Why doesn’t NS act on deletrious mutations in ratchet

Answer 36

Because the mutations are only slightley deleterious = not bad enough that NS can act
- - Mutation occurs in copy of high fitness genotype BUT only sloightly deletrious so NS can’t act well = deeleyeious affect can build up

Over time = accumulate deleterious mutation – THEN get individuals with enough deleterious mutations that you weigh down the fitness = get individuals that NS can work on

Question 37

Fitness change in Ratchet

Answer 37

Start with high fitness genotype –> get copies of self = have high fitness offsrping THEN have a mutation that decreases fitness a little but not a lot –> THEN pass mutation to offspring because clonal –> THEN have another mutations
- We expect mutation in almost all generations

HAVE more mutations in the background of the first mutations

Each round the new mutations that occur stay for good in all decendents of mutated indiviauls –> the orginal high fitness genotypes become rare

End = lose the zero mutation genotype – NOW only have 2 mutation vs. 1 mutation genotype –> ability of NS to seperate mutations get weaker

End = the best fitness is 1 mutation (no longer have 0 deleterious) = have one click on ratched

Question 38

NS acting in ractch over time

Answer 38

Lose high fitness = ability of NS to purge deletrious is weaker over time because NS is weaker with less varaition (when lose highest fitness genotype lose varaition)

Over time = accumulate deleterious mutation – THEN get individuals with enough deleterious mutations that you weigh down the fitness = get individuals that NS can work on

Perhaps NS can favor the no mutation high fitness genotype gainst the lineages with 4 mutations BUT if the high fitness genotype is lost due to drift OR of another mutations occurs in an individual with that genotype –> The high fitness genotype is gone for good and the best genotypes all have at least one deletrious mutations = the ratchet has moved foward a click = less varaition = NS can’t act

Question 39

Mutation in asexual

Answer 39

Once mutations pops up it is there for good

Question 40

Ratchet moving foward a click

Answer 40

Perhaps NS can favor the no mutation high fitness genotype gainst the lineages with 4 mutations BUT if the high fitness genotype is lost due to drift OR of another mutations occurs in an individual with that genotype –> The high fitness genotype is gone for good and the best genotypes all have at least one deletrious mutations = the ratchet has moved foward a click

Question 41

Genetic load

Answer 41

The accumulation of deletrious mutations weighs doen the fitness of a popultions
- Over time popultion fitness decreases – decrease in fitness = genetoc load
- Decrease in optimal fitness
- Decrease fitness based on accumalation of deletrious alleles

Major probelm in asexual organisms –> even driving them to extsiction

Question 42

What breaks the ratchet

Answer 42

IN sexual popultions recombination breaks the ratchet
- Sexual reproduction breaks ratchet (can go back and forth) –> can get high fitness genotype back

If have a popultion that lost the high fitness genotyoe –> Can have crossing over in meiosis can give back the best genotype

Example – if the highest fitness genotype is 1 mutations
- IN asexual all mutations go to offspring
- In sexual – by combining gametes in sexual reproduction + recombination –> we can get back the fitness with 0 mutations

Question 43

Recombination + Novel phenotypes

Answer 43

Recombination via sex might speed up the rate at which novel phenotypes appear

Ability of recombination to make novel genotyoes = important

Question 44

Benefical mutations in Asexual

Answer 44

Purley clonal popultions have to wiat for benefical mutations to occur in sequntial order in the same lineages to get novel phenotyopes

Question 45

What happens when lose high fitness genotype in Asexual

Answer 45

High fitness is gone for good unless have exact mutation back

Mutation rate = 10^-8 –> chance of exact mutation to high fitness is small

Question 46

2 mutations + 5 mutations Vs. 0 mutations + 5 mutations

Answer 46

Harder to week out differencve between 5 mutations and 2 mutations than weeding out with 5 mutations and 0 mutations

NS can act on 0 vs. 5 mutations = allow high fitness to be maintained in popultion

Question 47

Reason most asexual lineages are young

Answer 47

Because of genetic load

As mutations occur = fitness decreases over time

Question 48

Adaptation in more than 1 gene

Answer 48

Adaptations often require more than one gene

In asexual – need to wait for mutations to ccur in sequntial order in the same lineagese to get high fitness
- To get high fitness need mutation in one indiviudal –> need to occur in occsrping in the background of the other mutations

Sexual – Do not need mutation to occur in offspring in background of othe rmutations –> in sexual you can get the beneficial mutation in one organism and one beneficial in another organism and then they can combine through sex = get high fitness

IMAGE – AB = high fitness
- Top – Sexual – can get mutation in different lineagse and then mix lineages = get high fitness –> OCCURS much more rapidly – get more AB (more high fitness) – increase probability of polygenic adpatation
- Bottom = Asexual – aB = dead end –> Need A and B –> Need ab THEN Ab –> THEN AB – need B mutation to occur in backgroun of A – need mutation in same genetic line

Question 49

Sex + rapid evolution

Answer 49

Sex might be important beyond steady accumulation of deleterious mutations – might be needed for rapid evolution

Question 50

Other use of sexual reproduction

Answer 50

Might be needed for constanty adapations that organisms need all of the time

Organisms need to constantly adapt each generation – constantley need new genetic combinations
- NEED to reshuffle alleles to get new geneotypes

To deal with threat from other popultions = ability to evolve fast is important

Question 51

What is constant evolution driven by

Answer 51

Driven by co-evolution – evolove to envirnmemnta + evolove to threats from other evolving organsims

Overall – popultions don’t evolove in static envirnmmnets – have major evolutionary pressures coming from other species (predators + Prey + parasites + Competators)

Example – evolove to parasites or pathogens (evolove fast)

To deal with threat from other popultions = ability to evolve fast is important

Question 52

Red Queen Hypothesis

Answer 52

Popultions are constantley evoloving to chnaging biologic chalelemnged and sex is crucial way for popultions to keep pace woth their eneminies

To deal with defeinsive stredegy –> get niovel genetics in all generations (only can occur in sexual)

Overall – popultions don’t evolove in static envirnmmnets – have major evolutionary pressures coming from other species (predators + Prey + parasites + Competators)
- Threats to fitness from othe rorganisms that themsleves chnage (other organisms have chnaging adaptive topographies) –> Change in one organsims = have shift in other organisms adaptive topography
- Sexual reproduction = gives organims ability to keep pack in co-evolutionary arms race

Question 53

Co-evolution

Answer 53

Reciporcal evolution across species interactions

PLays a role in maintaining sexual reprodyction

Question 54

Sexual vs. Asexual experiments

Answer 54

Studies = done with organisms that can do both

Low predator or parasites = asexual is maintained at higher rate

Add predators –> Sexual reproductions = sweeps through population

Example – Snails + fish –> vary in degree of sexual reproduction
- High predatores = increase rate of sexual
- Low predators = maintain asexual

SHows red queen hypothesis

Slides:
A number of natural and
experimental systems appear to back this up- asexual populations thrive in the absence of parasites, but sexual populations do better with intense coevolution

More than making up for the 2-fold cost