Sex, Cooperation and conflict Flashcards
What is sex?
The occurrence of meiosis
Under the meiosis definition of sex, what 3 things are assumed?
Sex does not require separate sexes
Sex is not necessary for reproduction
Sex is not the same as reproduction
What is the paradox of sex?
Sex must confer some sort of fitness benefit or it wouldn’t be so pervasive
The cost of sex
Takes a long time
Risk of reproducing maladapted offspring
Cost of mating (finding a mate, sexually transmitted diseases)
Loss of half of reproductive output (2-fold cost of sex)
What is the two fold cost of sex?
When producing sexually a female must combine her offspring with a male and thus loses 50% of her gene copies.
when producing asexually she has a 2-fold advantage of passing of two copies of her genes
all asexually produced offspring are female
Time difference between yeast reproducing asexually and sexually ?
Asexually - 90mins
Sexually - several days
The cost of males (2-fold cost of sex)
The clonal offspring of an asexual female multiplies at twice the rate of the progeny descended from a sexual female
a sexual female has only 50% of the fitness of an asexual female
Why is asexual selection favoured more in stable environments?
Sex is costly
What is background selection?
Selection against strongly deleterious mutations
Fixation of weakly deleterious mutations
Accumulation of deleterious mutations
What happens to an asexually producing population under background selection?
The population gets weaker in time due to lack of chromosome mixing
Stochastic loss
Random loss
Muller’s ratchet
Stochastic loss of mutation-free chromosomes within a population
leads to an accumulation of deleterious mutations
Muller’s ratchet in an asexual population
Cannot be revered within an asexual populations
Leads to fitness decline over generations
Accumulation of deleterious mutations
Genetic hitchhiking
Deleterious mutations accumulate because they are linked to beneficial mutations
Selection for strongly beneficial mutations
Hitchhiking of linked deleterious mutations
Genetic hitchhiking in asexual populations
Recombination could de-couple but this cannot happen in asexual reproduction
beneficial and linked deleterious mutations are linked and cannot be separated
Accumulation of deleterious mutations
Ruby in the rubbish
Selection against strongly deleterious mutations
Elimination of linked beneficial mutations
Linked beneficial mutations cannot be de-coupled and so are lost
Less adaptation
The Hill-Robertson effect
Without recombination, fixation of beneficial mutations is much slower.
Selection is weaker without sex and recombination.
benefits of sex - increased efficiency of selection
Benefit of the group not the individual
This does not fit with how we understand evolution
Evolution doesn’t have foresight
Benefits of sex - Generation genetically variable offspring
Advantageous in a variable environment
Red queen hypothesis - running to stand still
What are Facultative asexuals?
Organisms that can switch between sexual and asexual reproduction
Favours environmental change
Mud snail - parasite example (sexual/asexual reproduction)
Parasite causes castration of snail (complete fitness loss)
High levels of parasite numbers correlate with high levels of sexual reproduction
Nematode example (free living and parasitic) - sexual and asexual
Parasitic form reproduce sexually to compete with changing host behaviour
Free living are much more stable
what is Isogamy ?
Same sized gametes (ancestral sexual state)
What is anisogamy?
Different sized gametes
Balanced polymorphism
What are mating types?
- First steps in sex differentiation
- Ensures gametes of same organism dont fuse
- Promotes outbreeding
- Mating types stop gametes fusing with gametes from the same organism (+/- equivalent to male/female)
Mating type examples
- Chlamydomonas reinhardtii has two mating types (+/-)
- Schizophyllum commune has thousands of mating types
- Most common mating type is two (examples up to 10)
how did mating types evolve into sexes?
trade-off between size and number of gametes (large is more viable - small and many have more chances of fusion)
Medium gamete size = mediocre (selected against)
Disruptive selection leads to evolution of balanced polymorphism for gamete sizes (Anisogamy)
Large = egg = female
Small (and many) = sperm = male
what makes zygote size
The sum of the gametes that fuse to form it
viability increases with size
zygote must be a certain size in order to be functional
positive relationship between zygote size and viability
Consequences of anisogamy
- Sets up potential for sexual selection
- Sexual selection is responsible for phenotypic differences between male and female
- E.g. courtship behaviour, ornament (antlers etc), colouration etc.
Differential gamete investment
Males produce large quantities of gametes (individual gametes ‘cheap’ to produce)
Females produce fewer larger gametes (individual gametes are more costly)
How do males increase their mating success?
Mate with many partners
success increases linearly with number of mates
Females don’t benefit from multiple matings (only one chance of fertilisation)
What is Bateman’s Principle?
Male mating success is more variable than female mating success
Females in most mating interactions are the limiting factor
Why is male mating success more variable? (Bateman’s principle)
Males mating success is variable because they may or may not be allowed to mate
Females will always have the same mating success because they are the limiting factor
What does Bateman’s principle lead to?
Variance leads to competition between males and choosiness of females
known as the parental investment theory
Parental investment theory
Variance leads to competition between males and choosiness between females
basis of sexual selection
Why are separate sexes not inevitable?
94% of flowering plants (angiosperms) are bisexual (have both male and female sex organs)
produce different sized gametes but fertilise themselves
Example of male variance in mating success
Rough-skinned newts: Females don’t benefit as much as males from mating with multiple partners
Example of choosy males
- Sea horses
- Poison arrow dart frogs
Males show all parental care
females have multiple partners and males are choosy
how does female choice incur a direct benefit?
Direct fitness increase
Choose a male that will directly provide resources to the female and her offspring
e.g.
Fertile or fecund males
Good parenting ability
Access to resources (food, nesting sites etc.)
What does fecundity mean?
potential number of offspring that could be produced
Direct benefits of female mate choice (mottled sculpin fish example)
males defend females eggs
females choose larger males
larger males have a higher proportion of hatchings
Direct benefit = parental care
Signal = larger size
Examples of direct benefit of female mate choice
Nuptial gifts
- gifting a snack or something (better the gift more likely to mate) many insects and great grey shrike (bird).
Territory defence
e.g. male zebras defend watering holes - females benefit.
Indirect benefit
Genetic benefit → choose the male that will provide genes to make fitter offspring
Examples of indirect benefit
Bird of Paradise dance
no direct benefit to the female
Is a signal of good sperm
sign that male will produce fitter offspring
Fisherian runaway selection model
Pick sexy traits to have sexy sons that get chosen by choosy females etc
Runaway selection
E.g. Swordtail fish - females love the sword
Good genes model
Traits have evolved as an honest signal for males genetic quality.
Only males with good genes can afford to invest in the trait
Example of good genes (stalk-eyed fly)
- Larger stalks = honest signal of their good health
- Study showing how different food eaten can cause variation in stalk length (poor nutrition - more chance of shorter stalks)
- Male selection ornaments: Traits are often condition-dependent
Sexual Vs. Natural Selection
Sexual selection can sometimes be costly and bad for survival
Sexual Vs. Natural Selection Example (marine iguanas)
Galapagos marine iguana
Males are often much larger than optimum size (costly, often causes death)
Males attract females by defending territories (bigger body = better territory defence)
female choice is driving sexual selection to outweigh the survival cost
What is sex determination?
Mechanisms that determine what sex a developing embryo ends up being
What is the name for X&Y (as a sex-determining system?
Male heterogamety
e.g. humans, mammals, drosophila
What is female heterogamety?
Z & W chromosomes
e.g. birds, butterflies
Male heterogamety
X & Y chromosomes
XX = female
XY = male
Why is Y a sex limited chromosome?
It only shows up in one of the sexes
W is a sex-limited chromosomes in a W&Z
ZZ = male
ZW = female
(There can sometimes be more than 2 chromosomes)
In some animals, sex is determined by other triggers
- Sex determined by temperature
American alligator:
higher temp = males
lower temp = females
issue with climate change
- Social factors
blue banded gobi:
Sequencial hermaphrodites
remove male, largest female becomes male and takes over role
what are sequencial hermaphrodites ?
Born one sex but have the ability to change sexes at some stage in their life
sex determination in animals is incredibly diverse
- XY sex chromosomes
- ZW sex chromosomes
- Haplodiploidy
- Paternal genome elimination
- Genetic sex determination
- Environmental sex determination
- Hermaphroditism
Who discovered sex chromosomes?
Nettie Stevens
Recombination suppression
Area on chromosome (strata) recombination is turned off
Older strata have fewer similar genes
Degeneration of the sex-limited sex chromosome
what is recombination suppression?
- Recombination suppression is key in the evolution of sex chromosomes, sex chromosomes are key drivers in sexual dimorphism and reproductive isolation.
- Recombination has become suppressed between sex chromosmes.*
What causes recombination suppression?
Is recombination suppression a cause or a consequence of sex chromosome evolution?
Step in the process leading to the evolution of sex chromosomes
Sex chromosome degradation - what is X-degenerate?
Relics of shared ancestry with X chromosome
Sex chromosome degradation - what is ampliconic
- Areas where there are multiple copies of the same gene
- Many have testes-specific expression, suggesting they may be involved in spermatogenesis
Sex-specific selection: in chickens
Chickens → intense female trait selection and relaxed female trait selection
W genes are convergently up or down-regulated
Gene conversion on the Y (slowing degredation)
Non-reciprocal intra-chromosomal recombination
Palindromic regions
Human genome has 8 palindrome
Gene conversion from one copy to another
Preventing degeneration of important genes by having back ups
Not swap →is a copy
Palindromic regions
Velcro
DNA sequence composed of two inverted repeats (arms) separated by a short spacer (can bind to one another like velcro)
causes a loop of dna
DNA can be exchanged within loop
Gene movement onto the Y
Duplication of autosomal genes onto the Y chromosome
refreshing of genetic material to counter degeneration
Divergent fitness optima
Fitness landscapes
Peaks and valleys of fitness landscapes dont always align between sexes
Different evolutionary interests
Drosophila show negative correlation between adult male and female but not with juveniles
Types of sexual conflict
- Intra-locus conflict
- Inter-locus conflict
Intra-locus conflict (definition)
- Tug of war over trait with shared genetic basis
- Conflict arises when a trait has a shared genetic basis (between males and females) and contradictory selective pressures act on the sexes.
Inter-locus conflict (definition)
interacting traits on different loci
Trait within one sex interacts with trait in the other sex.
Conflict occurs over the outcome of the interaction between the sexes.
Intra-locus conflict Drosophila body size example (sexual conflict)
- Males want to be larger
- females smaller
- Due to the shared genetic basis of the trait expression overall size of males and females is in the middle
- Unresolved conflict (tug of war prevents either sex from reaching their optimum size)
- Compromise that doesn’t suit either sex
How does intra-locus conflict effect diversity?
Can lead to balancing selection and elevated genetic diversity
Ongoing sexual conflict prevents purging of genetic diversity - instead it maintains it.
Inter-locus sexual conflict examples
Drosophila mating:
Sex peptide → connects to sex-peptide receptor in female: Makes female less susceptible to future males & increases egg laying - good for males not good for females.
Bean weavils:
traumatic penis - spiny penis - ensures successful mating, injures female so she can’t mate again
Hard to resolved → leads to arms race between males penis structure and female evolve to reduce direct cost (kicking).
Sexually concordant selection
→ alleles rise in population to fixation
Sexually antagonistic selection
→ allele never fixed, maintained at
What are the two routes for intra-locus conflict resolution?
- Sex chromosomes
- Gene expression on the autosomes
Sex-Linkage (conflict resolution)
Genes on autosomes are exposed to male-specific selection 50% of the time and female 50%.
Dominant alleles on X should be more often selected for female-specific effects (because there are two x’s)
Recessive alleles on X should be more often selected for male-specific effects.
Sex linkage prediction
The X chromosomes should be enriched for dominant female-benefit alleles and recessive male-benefit alleles.
Mammalian X sex-linkage example
Prediction: X chromosomes is demasculinised (fewer dominant male-benefit alleles)
Evidence: Exodus of many testis-specific genes from the X chromosome to the autosomes in humans and mice
Sex-linage: Z&W
- Two Z chromosomes in males only one is females.
- Prediction:
- The Z chromosome should be enriched for dominant male-benefit alleles and recessive female-benefit alleles.
- (opposite to mammals X&Y)
Avian Z sex-linkage examples
Male wing patch on flycatchers - Map disproportionately to the Z chromosome
The Z chromosome is masculinised in chickens
New sex chromosomes?
Prediction: there is a correlation between colour and the size of the recombination strata (recombination suppression) on the Y chromosome.
Convergent expansion of the non-recombining region in colourful populations - consistent with selection to resolve sexual conflict
Resolving conflict: Gene expression.
gene expression can decouple male and female phenotypes and act to resolve conflict
The difference between intra- and inter-locus conflict
Inter-locus conflict leads to an evolutionary arms race whereas intra-locus conflict leads to elevated genetic diversity through balancing selection
How can intra-locus conflict be resolved?
By several routes
e.g. sex linkage, sex chromosome formation and sex-biased gene expression.
There is debate around the relative roles of these different routes
Genomic conflict and sex
- Genomic conflict arises more commonly in sexually reproducing organisms
- Not all genes from one parent are inherited
- Sex allows greater scope for genomic conflict
Different levels of selection (genomic conflict)
Cells
Nucleus
Mitochondria
Cells and organelles show variation, reproduction and heredity so all subject to natural selection
