EB13

Question 1

Maynard Smith quote

Answer 1

sex is the hardest problem in evo bio

Question 2

Describe sex

Answer 2

sex is meiosis and out crossing

The DNA is recombined during meosis (crossing over) and independent assortment of chromosomes

Question 3

where does recombination happen

Answer 3

across the tree of life
eukaryotes: animals, fungi and plant under meiotic sex
archea and bactera undergo non meiotic recombination aND hgt

Question 4

What are the 4 types of asexual reproduction

Answer 4

vegetative
apomixis
automixis
autogamy

Question 5

what is vegetative reproduction

Answer 5

asexual reproduction
budding or proliferation without formation of seeds etc.
spider plantlets etc.

Question 6

what is apomixis

Answer 6

spores seeds or eggs develop to be genetically identical to parent.
(effectively miotisis)
Bdelloid rotifers
Dandelions

Question 7

what is automixis

Answer 7

parthenogenesis
new individual arise from fusion of two cells from a single meiotically dividing cell.
Original ploidy is restored by suppression or fusion of intermediate products.
Can be seen in Orbatid mites and whiptail lizards

Question 8

what is autogamy

Answer 8

self fertilization of eggs by sperm from the same hermphrodite animal
or seeds by pollen form the same plant (selfing)
cat tapeworm
peanut plant

Question 9

what is amphimixis

Answer 9

sexual reproduction
fertilization occurs between gametes of different individuals.
sometimes individuals have seperate sexs (gonochorism in animals) and diocey in plants (yew trees)
others both expressed in hermaphordite orgnaims,
which outbreeds rather than self fertilisies. such as leopard slugs

Question 10

how does amphimixis occur in fungi

Answer 10

not straight forward parallels with animals and plants but results in similar genetic outcomes

Question 11

What are three other sexual habits

Answer 11

haplodiploidy
heterogony
gynogenesis/hybridogenesis

Question 12

What is haplodiploidy

Answer 12

haploid males are produced via unfertilised eggs, diploid females arise via amphimixis e.g. hymenoptera

Question 13

what is heterogony

Answer 13

apomixes alternates eiyh smphimixes on a yearly cycle

e.g. aphids, monogonont rotifers

Question 14

what is gynogenesis/hybridogenesis

Answer 14

pretend to have sex - apomictic or automictic eggs require sperm to develop but there is no paternal contribution - hangover from sex. frogs in the Rana genus

Question 15

What is the two fold cost of sex

Answer 15

asexual populations grow trice as fast as sexuals (varies by system

Question 16

what is the cost of anisogamous amphimixis

Answer 16

full cost because it involved males

anisogamy = union of two gametes which differ in size and form

Question 17

what is the cost of autogamous hermaphrodites

Answer 17

eliminates the two fold cost as male and female function in same organism

Question 18

what is the cost of isogamy

Answer 18

elimates two fold cost as gametes the same size, everyone is making an energetic contribution
Isogamy: sexual reproduction via fusion of similar gametes

Question 19

What are 6 other costs of sex

Answer 19

1. recombination breaks up co-adapted gene complexes
2. inability to reproduce alone, cost and risk of finding mates and copulating
3. sexual selection (inter sexual) an its attendant costs: mate choice, sexual conflict, sexually antagonist arms race
   peacocks, elephant seal fights, brighter colours more obvious to prey
4. STDs, chlamydia
5. meiosis itself is complicated and time consuming with risk of error
6. intracellular paraastes and transposons spread more readily in sexual organims

Question 20

How common is obligate apomixis (asexual reproduction)

Answer 20

rate
<1% of animal species
<1% if seed plants (~10% in ferns)
>15% of fungi have no described sexual stage
* in almost all groups where it occurs asexuality has a scattered distribution at the tips of phylogenetic trees = dead end. `

Question 21

What are the three main hypotheses for the maintenance of sex

Answer 21

1. sex increases efficiency for NS to remove deleterious mutations *mullers ratchet and mutational determination hypothesis
2. sex helps beneficial mutations from different backgrounds combine reducing interferenace from selection or drift at other loci. Hill robertson effect
3. sex is constantly producing rare or novel genotypes which can resist parasites that are adapted to current common genotypes

Question 22

describe mullers ratchet

Answer 22

asexual organisms accumulate deleterious mutations over time, if lost by drift the genome with the fewest mutations can not easily be reconstructed in asexuals
(lose class of organism by chance)
leads to mutational meltdown. 
in sexuals less mutated genotype can be restored by sex.

Question 23

what are the problems with mullers ratchet

Answer 23

1. deleterious mutation rates hard to measure, but cna be inferred
2. water snails and stick insects phylogenies show higher accumulation of deleterious mutations in asexuals than sexual lineages but no direct evidece that these mutations loads substantially affect fitness
3. ratchett is slow in comparison with large immediate advantages of asexuals. and only works in small populations. acting alone its not significantly powerful to prevent replacement of sexual populations by clones.
4. even a small occasional amount of sex would be enough to fix these problems - so there would be no need for organisms to have so much sex.

Question 24

what is epistasis

Answer 24

the effect of one gene being dependent on the presence of one or more genes

Question 25

Describe the mutational deterministic hypothesis

Answer 25

if the combo of two deleterious mutations decreases fitness more drastically than would be expected from independent effects of each one (synergistic epistasis)
AND genomic deleterious mutation rate is greater than 1.
THEN sex is favoured because it combines multiple slightly deleterious mutations into highly unfit genotypes that are selected out of the popualtions leaving healthier genotypes over represented.

Question 26

what are the problems with the mutational deterministic hypothesis

Answer 26

1. many estimates suggest mutation rate is not generally high enough >1 to male this model work
2. deleterious mutations dont show a general tendency towards synergistic epistasis. *can be seen in both E.coli and yeast experiments, most double mutatnts show little or no epistasis. - combined harm less than expected. no general synergistic pattern
3. current consensus are that the conditions required are not widespread in nature.

Question 27

what are the merits of the mutational deterministic hypothesis

Answer 27

very easily testable model

Question 28

sex helps beneficial mutations from different backgrounds combine reducing interferenace from selection or drift at other loci.
how has this changed over the years

Answer 28

from weismanns hypothesis to muller fisher hypothesis to hill-robertson effects

• all facets of same idea that sex breaks apart or combines mutations art different site and therefore improves population response to selection, especially in finite popualtions, where drift is a problem
• focuses on how recombination helps bring good mutations together but could also split up bad.

Question 29

What is the experiment for the hill robertson effect

Answer 29

sexual and aseuxal yeast populations:
genetically engineered to have the same background but sex is KO.
fitness differences observed under lab conditions.
evolve strains over time to see how quickly they adapt.

Question 30

what did the hill robertson yeast experiment show

Answer 30

1. adaptation was faster in sexual than asexual
2. yeast is isogamous so there was no two fold cost.
3. sex can fix good mutations indepndently of bad.
4. if selection stops - e.g. environment not constantly changing than asexuals regain the advantage.

Question 31

Describe the premise for the benefits of sex due to parasitsm

Answer 31

1. parasites and pathogens are ubiquitous and are rapdily and relentlessly selected to infect the most common host genotype
   - they have short generation times, large population and high mutation rates - strong selection pressure to infect.
2. any successful combo of alelles soon becomes disproportionally susceptible, favouring new and different combos
3. sex breaks down common genotypes and replaces them with novel ones for the parasite.
4. therefore sexuals have adv. over asexuals as asexuals inherit the same vulnerabilities as their parents.

Question 32

what kind of selection do parasites cause

Answer 32

A time lagged negative frequency dependent selection

Question 33

The red queen hypothesis quote

Answer 33

now here you see it takes all the running you can do just to keep in the same place

Question 34

what is the circumstansial evidence for the RQH

Answer 34

1. taxa with both sexuals and asexuals, asexuals tend to be found in physically harsh environments with low biotic diversity. (less in biotically rich habitats, more harsh).
2. this is consistent with advantage of sex linked to co-evo interactions, rather than changing physical conditions.

Question 35

what evidence do new Zealand mud snails provide for the RQH

Answer 35

sexual snails co-exist with apomictic females (two fold cost of sex)

1. proportion of sexual snails within lakes correlated with frequency of sterilizing parasitic worms.
2. across multiple years asexual snails were more heavily infected than sexual.
3. proportion of asexuals over time is inversely linked with overall prevelance of infection and not generally increasing despite two fold advantage.
4. frequency of asexuals decreased over a decade as they became disproportionally susceptible to parasites.
5. meanwhile frequency of sexuals was roughly stable so parasites prevented any given clone from becoming successful enough to replace sexuals.

Question 36

what evidence is there for the RQH from experimental evo of C. elegans

Answer 36

1. c. elegans hermaphrodites can self-fertilise or outcross with males.
2. adaptation to uncahinging serratia strain increased the rate of outcrossing but highe rates were only sustained by co-evolution with the parasite.
3. obligately selfing. c. elegans all went extinct in the coevolution treatment, but not in others. as parasite adapted to infect the selfing strain more strongly
4. obligately outcorssing strains became more resistant to the parasite during coevolution experiment.
   * ** clear evidence of selective benefits of sex with parasites

Question 37

what are 4 main problems with the RQH

Answer 37

1. effects of sex on coevolution difficult to pindown in a model. - unsure how general adv. may be.
2. unsure effect if many parasites involved, if parasites have sex?
3. major concern: parasites select for genetic diversity not sex per se. a diverse mix of genetically different clones could potentially cycle up and down in frequency and simualte benefit of sex.
4. dont know whether beneift of sex in RQ models comes from fluctating rare advantage or from a generally improved response to selection. Hill roberston effect?

Question 38

what is the current consensus on the evolution of sex

Answer 38

mix of both hill robertson and RQH.
neither has definitive theoretical or empiral support
* RQ has empiral support in some cases but hard to make theoretical models are dynamics are so complicated.
whereas HR are simple and general to model but hard to find in nature.
** mutational hypothesis could still be important in helping to overcome the costs of sex.