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Evolution > Sex & Recombination and Asexual Reproduction > Flashcards

Sex & Recombination and Asexual Reproduction Flashcards

1
Q

What is sexual reproduction?

A

reproduction involving meiosis and fusion of gametes (syngamy) that results in new genotypes because independent assortment and crossing over occur

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2
Q

Sexual reproduction is characteristic of which domain of organisms?

A

eukaryotes

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3
Q

What is syngamy?

A

fusion of gametes

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4
Q

What is the result of sexual reproduction? By what 2 processes does this happen?

A

new genotypes by independent assortment of chromosomes and crossing over (recombination)

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5
Q

T or F: sex and recombination can be considered interchangeable terms

A

true

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6
Q

What does ‘sex’ refer to in bacteria and viruses (prokaryotes)?

A

the exchange of DNA (not sexual reproduction)

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7
Q

What is parthenogenesis?

A

asexual reproduction in eukaryotes in which a female reproduces clones of herself and there is no fertilization by sperm

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8
Q

T or F: there is a lot of diversity amongst asexual reproduction strategies

A

true, parthenogenesis has evolved independently many times and in a variety of different ways

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9
Q

What are the two ways a lineage or organism can be parthenogenetic?

A

facultative
obligate

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10
Q

Describe facultative parthenogenesis

A

When an organism or lineage can conduct both asexual reproduction via parthenogenesis and sexual reproduction

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11
Q

Describe obligate parthenogenesis

A

When an organism or lineage can only reproduce via parthenogenesis

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12
Q

Why is it likely that parthenogenesis doesn’t occur in mammals?

A

likely due to genetic and genomic imprinting = some alleles need to be turned off depending on whether they came from mother or father so if there’s no paternal source, it could cause issues if the maternal allele is turned off

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13
Q

What is an example of facultative parthenogenetic animals?

A

Hymenoptera (wasps, bees, etc) conduct parthenogenesis to produce male clones and reproduce sexually to produce females

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14
Q

What is an example of obligate parthenogenetic animals?

A

Amazon mollies?

reproduce female clones, the entire species is female, but they rely on the sperm from males of another species to trigger the development

aka ‘sperm parasites’

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15
Q

T or F: parthenogens are usually very successful and abundant

A

true, most are weeds, pests, and invasives

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16
Q

Give some examples of parthogens

A

Jumping earthworm, book louse, mud snail, dandelion, false spider mite

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17
Q

What did the study by Hoffmann et al., looking at agricultural pests and parthenogenesis find?

A

they compared the proportion of parthenogenetic species in an insect family compared to the proportion of parthenogenetic species in that family that are also agricultural pests

they found that for example, in one family, ~25% of species were parthenogenetic and of that 25%, 80% of the species were also agricultural pests

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18
Q

Where do we usually see parthenogenetic lineages on evolutionary trees? why?

A

at the tips

they are short-lived

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19
Q

What is the only vertebrate lineage that has repeatedly evolved parthenogenesis?

A

scaled reptiles (Squamata)
39 true parthenogens

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20
Q

What did Moreira et al., do in their study of parthenogenesis in scaled reptiles?

A

they phylogenetically analyzed and mapped reproduction strategies in scaled lizards

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21
Q

What did Moreira et al. find in their study? what do their results suggest?

A

they found multiple lineages of parthenogenesis (it has evolved independently multiple times) but all of those lineages are at the tips of the phylogenetic tree (they do not persist)

this suggests these lineages either speciate less or they go extinct more often and faster than other lineages that sexually reproduce

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22
Q

What is the Paradox of Sex?

A

a question that has long puzzled evolutionary biologists: why isn’t parthenogenesis more common? Especially given that it evolves so easily (multiple lineages)

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23
Q

What are some benefits of parthenogenetic reproduction (in stable environments)?

A

reproductive assurance = don’t need to spend energy seeking and courting a mate, no potential risk in not finding a mate or being injured by mate or competition

don’t need to invest in costly secondary sex characteristics to attract mates

faster rate of reproduction

100% of parental genes are passed on to 100% of offspring - preserves successful genotypes

faster rate of growth (aka 2 Fold Cost of Sex)

avoid STDs or risks of mating (eg., exposure to predators, competition)

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24
Q

Why do we often see parthenogenesis in weeds, pests, and invasives?

A

mostly because the rapid rate of reproduction

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25
Q

What is the two-fold cost of sex (or the cost of males)?

A

males are a big investment and they cannot reproduce by themselves

if 50% of the offspring from a sexually reproducing female are male, half of her genes/resources have been passed into individuals (males) that cannot bear their own offspring

much slower rate of growth

whereas:

asexually reproducing individuals are passing on 100% of their genes to all of their offspring = frequency of genes double in every generation

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26
Q

What is the disadvantage of parthenogenesis?

A

loss of diversity is the main one

if 100% of one individuals genes are passed on to all offspring, those offspring are identical to one another and to their parent. When they reproduce, they will pass on 100% of the same genes to 100% of their offspring as well

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27
Q

What is the major benefit of sexual reproduction?

A

diversity

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28
Q

What are the 2 major benefits of recombination?

A
  1. brings beneficial mutations together
  2. breaks associations with deleterious mutations
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29
Q

What are two ways to make new genotypes?

A

novel mutations

novel combinations via sexual reproduction (recombination, reassortment)

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30
Q

Why is parthenogenesis less common?

A

it’s major downfall is that it generates a loss of diversity

a lineage with little diversity is more vulnerable to being knocked out

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31
Q

T or F: recombination increases the rate of adaptation

A

true

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32
Q

How does recombination increase the rate of adaptation?

A

by bringing beneficial mutations together faster

independently evolving beneficial mutations can be brought together quickly because of recombination and reassortment and allow them to continue evolving together

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33
Q

How does the lack of recombination affect the rate of adaptation in asexual lineages?

A

when there is no recombination, it takes a much longer time for independently evolving beneficial mutations to come together in the same genotype

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34
Q

What is clonal interference?

A

because different beneficial mutations spend more time evolving independently in asexual lineages, when they are brought together there can be more pronounced fitness differences between them and they compete with each other

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35
Q

Why is it uncommon to see clonal interference in sexually reproducing lineages?

A

when different beneficial mutations arise independently, they are brought together more quickly, before they can have large fitness differences and can instead evolve together

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36
Q

What is the Red Queen Hypothesis? give an example

A

a metaphor for the paradox of sex and the evolutionary arms race between sexually reproducing animals = always running but never getting anywhere = strong selection is occurring to be changing genotype constantly in order to keep up

ex. pathogens and parasites and their hosts
- immunity genes in host have to be evolving quickly because pathogens/parasites evolve quickly to overcome their defenses

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37
Q

How is sex and recombination related to the Red Queen Hypothesis?

A

recombination is a way to quickly evolve genomes and keep up in an unstable environment

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38
Q

Why is it difficult to study the differences between sexual and asexual reproducing lineages?

A

it’s hard to find lineages that are closely related but have different reproductive strategies

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39
Q

Describe the Lively et al. (1987), study of snails, sex vs parthenogenesis, and parasite resistance

A

they looked at NZ mud snails which have high diversity and a baseline sexual reproduction but some lineages have also evolved asexual reproduction

one species, Potamopyrgus antipodarum has a mix of obligate sexuals and obligate asexuals

both susceptible to Microphallus sp. parasites which causes cysts and sterilization (huge fitness decrease)

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40
Q

What did the study by Lively et al., find?

A

parasitism is driving the maintenance of sexual reproduction in these snails

they found that when parasitism is high, there are more sexually reproducing snails

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41
Q

What did the follow up study for NZ Mud snails by Vergara et al., find?

A

they found evidence supporting the Red Queen Hypothesis

sexually reproducing snails were more resistant to parasites, in all years except 1, sexually reproducing snails had lower rates of infection

42
Q

In order to test whether the sexual genotype in mud snails was just more fit in general, what was the study by Lively et al., (2000) ? what did they find?

A

they studied parasite infection rates in a specific lake in NZ

1 where they exposed asexual clones to a parasite found in their own lake = way higher rates of infection

1 where they exposed the same species of asexual clones to a parasite found in a different lake = much lower rates of infection

conclusion:
asexual clones from Lake Poerua are adapted to sympatric parasites - their genotype is known to these parasites and they are more susceptible to infection

the allopatric parasites have not evolved with that species of asexual clones (from a different lake) so they did not recognize the genotype

43
Q

How were the studies by Lively et al., & Vergara et al., evidence for the Red Queen Hypothesis?

A

they demonstrated that a species of asexual clones had co-evolved with a specific parasite from the lake habitat they were in = high infection rates

and low infection rates when the parasite was from a different lake = they didn’t evolve together

44
Q

T or F: viruses, pathogens, and microbes can recombine

A

true

45
Q

How can viruses, pathogens and microbes recombine?

A

when there are mixed infections

if there are 2 different strains infecting one organism, the genes can recombine and chromosomes can reassort in that organism to produce different strains

46
Q

What is genetic linkage?

A

when the combination of alleles at a gene is not random more than 50% of the time, the alleles are linked

ex. genotype = AaBb

linked:
AB AB
AB AB
ab ab
ab. ab

vs

unlinked:
AB. AB
Ab. Ab
aB. aB
ab. ab

47
Q

When alleles are linked they are said to be in _____

A

linkage disequilibrium

48
Q

What does it mean for alleles to be unlinked?

A

there’s no association between alleles at the genes

ex. genotype = AaBb

unlinked:
AB. AB
Ab. Ab
aB. aB
ab. ab

vs.

linked:
AB AB
AB AB
ab ab
ab. ab

49
Q

Define linkage disequilibrium (LD)

A

a non-random association of alleles at different gene loci = combinations of alleles are more or less common than expected

50
Q

What can cause linkage disequilibrium?

A

reduced recombination (most common) = alleles are physically close together and are less likely to be separated

inbreeding = reduction in variation of alleles between individuals means you often get the same combos

natural selection = incompatibility of some allele combos

51
Q

What is genetic hitchhiking?

A

if selection favours an allele at one locus, the frequency of other nearby alleles at linked loci can be increased (regardless of benefit, neutrality, or deleterious)

52
Q

What are the steps of genetic hitchhiking?

A
  1. initial population looks like
    BBBBBBBBB
    RRRRRRRRR
    PPPPPPPPP
    OOOOOOOOO
  2. a new beneficial mutation (G) arises in the population:

BBBBGBBBB
RRRRRRRRR
PPPPPPPPP
OOOOOOOOO

  1. new mutation (G) is favoured, selective sweep and genetic hitchhiking occur:

BBBBGBBBB
BBBBGBBBB
BBBBGBBBB
OOOOOOOOO

variation is decreased (it is replacing the other alleles)

  1. recombination will break up the association between the beneficial (G) mutation and other alleles, over time:

OOBBGBBOO
OOBBGBBOO
OOBBGOOOO
OOOBGOOOO
etc

variation is increased with recombination as the association is broken

53
Q

Why do the alleles next to the new beneficial mutation become so frequent in a population?

A

because the alleles nearby the beneficial mutation are close enough in proximity that recombination does not break their association, so as the new mutation is favoured and more and more individuals have it, the nearby alleles will be passed on with it = hitchhiking

54
Q

What does genetic hitchhiking tell us about the importance of recombination?

A

without recombination, or with decreased recombination, genetic diversity is lost

55
Q

Why can genetic hitchhiking be problematic?

A

the other nearby alleles that are transferred with the beneficial allele do not also need to be beneficial to increase in frequency - these alleles can be neutral or deleterious

56
Q

What is the “Ruby in the Rubbish” referring to?

A

genetic hitchhiking when beneficial mutations (the ruby) are in linkage disequilibrium with deleterious ones (rubbish)

57
Q

What is Muller’s ratchet?

A

a metaphor for the importance of recombination for breaking associations with deleterious mutations

the ratchet refers to the accumulation of deleterious mutations in small asexual populations as a result of genetic hitchhiking and no recombination

58
Q

What kinds of populations do we see Muller’s ratchet effecting most? why?

A

small asexual populations

the ratchet is a metaphor because the blade can only move forward, once the mutations exist in an asexual population, there is no recombination (no sexual reproduction) to reduce or remove them

when the linkage is stuck, the deleterious mutations will accumulate

59
Q

How does Muller’s ratchet relate to the major disadvantages preventing asexual reproduction from long-term persistence?

A

a) recombination (only in sexual reproduction) is like the reverse step to genetic hitchhiking and this doesn’t occur in asexual reproduction so there is no way to remove what has been added = reduction in diversity

b) recombination is required to break up the associations with deleterious mutations so without this, the deleterious mutations will just accumulate

60
Q

In small, asexual populations, what happens to the class of individuals with the lowest # of deleterious mutations?

A

they are more susceptible to being lost by chance and because it’s a ‘ratchet,’ they cannot be recovered

61
Q

In small asexual populations, what is the overall result relating to Muller’s ratchet?

A

as classes of individuals with the lowest number of deleterious mutations continue to be lost over time, fitness is decreased IRREVERSIBLY (no recombination to correct this) over time

62
Q

What did the study by McDonald et al., (2016) look at?

A

they used experimental evolution (induced evolution) to study how evolution occurred over 1000 generations in yeast strains that:

only reproduced asexually

reproduced sexually only every 90 generations

they are able to freeze cultures to compare early generations to evolved ones

63
Q

What did the study by McDonald et al., (2016) find? what does it mean?

A

they found that when compared to the original lines (where they sourced their treatment groups)

there was a higher fitness increase in the lines that reproduced sexually every 90 years than in the lines reproducing only asexually

conclude:

over 1000 generations of evolution, the lines of yeast that had just a few occasions of sexual reproduction had higher fitness than the lines that never sexually reproduced = recombination is increasing rate of adaptation

64
Q

What did McDonald et al., (2016) find when they compared what type of mutations get fixed in the sexual vs asexual lines?

A

major result: different types of mutations (intergenic, synonymous, nonsynonymous) become fixed in the two lines

all mutations compared between asexual and sexual were relatively similar in amount and composition

fixed in asexual:
- overall amount that get fixed significantly higher (just under 150)
- largely nonsynonymous, few synonymous and intergenic

fixed in sexual:
- overall amount that get fixed significantly lower (<50, maybe like 25?)
- huge proportion nonsynonymous, very little intergenic, no synonymous

65
Q

Why might different types of mutations get fixed in the sexual lines vs asexual lines?

A

in sexual lines:
- mostly nonsynonymous that become fixed = synonymous mutations wouldn’t have an affect on fitness so they wouldn’t be favoured enough to outcompete existing alleles and recombination can shuffle out the deleterious nonsyn. mutations
- the ones that get fixed are ONLY beneficial nonsynonymous mutations

in asexual lines:
- any type of mutation can become fixed when there’s no recombination to prevent it from being passed on

66
Q

Why was the study by McDonald et al., (2016), SO significant (ie., why was it published in such an esteemed journal like Science)?

A

It demonstrated Muller’s Ratchet hypothesis in real life

they found that deleterious mutations become fixed in asexual lines (no recombination to remove them), but NEVER in sexual ones (recombination could remove them)

67
Q

T or F: individuals or lineages are either sexual or asexual reproducing, but not both

A

false

the combination of both has evolved independently in multiple lineages

68
Q

What are two modes of reproduction that combine both sex and asex?

A

cyclical parthenogenesis

selfing with occasional outcrossing

69
Q

What is selfing?

A

self-fertilization in organisms that are self-compatible

sometimes they need to outcross (source mates)

70
Q

What is cyclical parthenogenesis?

A

when there are alternating cycles of sex and asex

71
Q

What are some examples of organisms that experience combined sexual and asexual reproductive modes?

A

A. pisum, aphids - cyclical

D. magna, water fleas - cyclical

C. elegans, nematodes - self and some outcrossing

Arabidopsis thaliana

72
Q

Can the amount of recombination within a genome vary or is this fixed?

A

it can vary

some regions of a genome in a sexually reproducing organism will experience little or no recombination

73
Q

T or F: some regions of a genome in a sexually reproducing organism will experience little or no recombination

A

true and there is big consequences

74
Q

What are some examples of parts of the genome of sexually reproducing organisms that do little or no recombination?

A

mitochondria

Y-chromosomes

centromeres

telomeres

chloroplasts

75
Q

Give an example of the consequences experienced by the regions of a genome that do not recombine in a sexually reproducing organism

A

the Y chromosome does not recombine

Over time, the Y chromosome has deteriorated in size and quality = it consists of very few genes (codes for ~30 different proteins), a lot of deleterious mutations, and is significantly smaller than its paired X chromosome

76
Q

Why does the Y chromosome not recombine?

A

it doesn’t have a partner Y chromosome to recombine with = much smaller population size

the only part that pairs with the X chromosome is the pseudoautosomal region

77
Q

T or F: there is diversity in the modes of sexual reproduction in eukaryotes

A

true

78
Q

What is anisogamy?

A

sexual reproduction that requires the fusion of two different gametes (larger gamete from female, smaller gamete from male)

an = not
iso = same
gamy = gametes

79
Q

What has selection favoured when anisogamy is occurring?

A

selection has favoured the evolution of 2 different types of gametes (female egg/ovule and male sperm/pollen)

80
Q

What are the major modes of sexual reproduction in multicellular eukaryotes?

A

dioecy
hermaphrodite
androdioecy
gynodioecy
protogyny
protoandrogynyh

81
Q

What mode of sexual reproduction in multicellular eukaryotes is super common across different lineages?

A

dioecious

82
Q

What does dioecious mean?

A

sexual reproduction occurs between individuals with separate sexes

ie., one type of individual produces male gametes and the other produces female gametes

ex. angler fish, seed eaters, humans

83
Q

describe hermaphrodites

A

a mode of sexual reproduction in which individuals from a species can produce both male and female gametes

84
Q

What are androdioecy and gynodioecy?

A

modes of sexual reproduction in a species which has a combination of individuals that produce both gametes and individuals that produce one of the two

androdioecy = sperm producing

gynodioecy = egg producing

85
Q

T or F: some species have individuals that can produce both male and female gametes

A

true

86
Q

What are the two modes of sexual reproduction in which individuals within a species produce either one of the gametes but there are also individuals in that species that produce both gametes?

A

androdioecy = sperm producing

gynodioecy = egg producing

87
Q

What are the two modes of sexual reproduction in which individuals within a species can change sex?

A

protogynous = female first

protoandrous = male first

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90
Q

What drives sexual selection?

A
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93
Q

What is Bateman’s principle?

A
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Evolution (11 decks)

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