Wright Flashcards
What is sex?
Haploid gametes produced, fuse to form diploid organsim - undergoes meiosis to produce haploid gametes - recombination can happen during meiosis
What are the costs of sex?
- Effort and energy of finding a mate and mating itself
- Might expose yourself to predators when mating
- Risk of disease transmission
- Risk of producing maladaptive offspring (worse adapted than you are)
- Loss of half of reproductive output
The … of selection is weaker in ….
This is because of:
a) Background selection - … of weakly deleterious alleles
b) Muller’s ratchet - Over time, randomly lose …-free chromosomes
c) Genetic hitchhiking - … mutations linked to selected … ones
d) Ruby in the rubbish - lose … mutations due to them being linked to strongly … ones
efficacy, asexuals, fixation, mutation, deleterious, beneficial, beneficial, deleterious
Sex … the genome so genes aren’t permanently linked to others on chromosomes - selection can act … on positive and deleterious mutations. It makes selection more …
reshuffles, independently, efficiency
What are the benefits of sex?
- Increased efficiency of selection (avoidance of background selection, muller’s ratchet, genetic hitchhiking and ruby in the red
- Genetically variable offspring generated - advantageous in a variable environment - beneficial in evolutionary arms races (red queen hypothesis) - e.g. parasites vs host (some offspring more likely to better + some worse too)
Give an e.g. of evolutionary arms race
Snail species (which show asexual and sexual females) infected by trematodes (flukes) - positive correlation between no. of males and level of infection, as expected if high levels of parasitism favour sexual reproduction
As well as sex being advantageous in hosts, it is advantageous in the …
parasites (e.g. nematodes)
There is high diversity of genetic and environmental sex … mechanisms. They hove multiple and independent origins. Mechanisms have evolved …
determination, rapidly
How can the environment determine sex?
Regulatory molecule turns gene on when environmental conditions are a certain way. Gene that when turned on triggers transcription of male development genes. Triggers may be: - Temperature - Relative body size - Food/nest availability - Social status
In Blue banded gobies sex is determined by … … within a dominance …
social interactions, hierarchy
In American alligators, sex is determined by … …
nest temperature
The XY chromosomes system is called … …
Male heterogamety (e.g. mammals) - sex determined by presence or absence of sex-determining gene on Y chromosome
The ZW chromosome system is called … …
Female heterogamety (e.g. birds) - sex determined by how many copies of sexual-development gene you have (found on Z chromosome so if you have 2 you become male if you have 1 you become female)
How did sex chromosomes evolve?
- initially evolved from identical ancestral chromosomes which acquired a sex-determining gene
- over time recombination between sex chromosomes became repressed - further and further
- selection was really efficient (due to same asexual processes as above) and there was a huge degeneration of the sex chromosomes
Sex chromosomes are thought to have an important role in … … and … …
sexual dimorphism, reproductive isolation
How is the age of sex chromosomes (strata) determined?
Sequence divergent between the X and Y chromosomes (as there is very little recombination in Y this is a good proxy)
The ampliconic regions of sex chromosomes are huge areas of…
very competitive sequences
- can actually get recombination on these palindromic regions
… X and Y chromosomes are highly differentiated. The human X chromosomes contains 1000 genes but only encodes … proteins
Mammalian, 45
Due to the rapid regeneration of the Y chromosome, some people think … might go extinct. However increased power of …-… selection and the vital roles of Y chromosome in reproduction is likely to counteract this, and as the chromosome degenerates there will be fewer and fewer targets for … anyway
males, sex-specific, mutation
W genes are convergently … in chicken breeds selected for … fecundity
upregulated, female
In drosophila, many sex and reproduction-related … genes have moved onto the Y chromosome, which gives some hope to it and to males
autosomal
Across the birds there is huge variation in the extent of … between the sex chromosomes
divergence
The extreme … in sequence divergence between sex chromosomes across clades is hypothesised to be due to …, selection against … load, and reduced … conflict
heterogeneity, chance, mutation, sexual
Female … fight for males
jacanas
Male and female interests can…
conflict each other (be very different)
In guppies, female fitness (… … …) is maximised with …, less … females. This makes sense are larger females produce more … (increased …) and colourful females are more likely to be … ….
Male fitness (LRS) is maximised by … size and … males. This because females like to mate with more colourful males (sexual selection) despite their fitness costs from probability of predation, so colourful males will produce lots of …. Also fecundity is not correlated with … in males
lifetime reproductive success, large, colourful, eggs, fecundity, predated upon
intermediate, colourful, offspring, size
Drosophila have a negative genetic correlation for relative adult fitness between…. A genome that is positive for … decreases fitness in … and vice versa. This is sexual …. In juveniles, relative fitness is … … between the sexes
males and females, males, females, conflict, positively correlated
What are the two types of sexual conflict?
- Intralocus sexual conflict
- interlocus sexual conflict
Intralocus sexual conflict arises when a trait has a … … … between the sexes and … selection pressures act on the sexes, e.g. … … in drosophila
shared genetic basis (with exception of sex chromosomes), contradictory, body size (female flies have greater fitness when larger (eggs), not true for males)
Important to note that not all sexually selected traits result in…
sexual conflict (only when genetic basis is shared)
Interlocus conflict arises when a trait in one sex … with a different trait in the other sex. Conflict occurs over the … of the interaction between the sexes, e.g. … interactions in Drosophila (… … in male seminal fluid binds to receptor in females, causing phenotypic changes such as increased … production. Overall harmful to females as result in reduced …)
interacts, outcome, mating, sex peptide, egg, longevity (males want to prevent females mating with other males, despite that decreasing her fitness)
… sexual conflict results in an … … between the sexes
interlocus, arms race
E.g. Bean weevils - arms race in … …. It is beneficial for males to mate multiple times but not beneficial to females to remate multiple times - conflict over mating …. Females have evolved … response to males attempting to mate, which has led to males evolving spiny … organ which pierce the females … … and act like barbs to prevent her from removing him. This increases the females risk of … and reduces her … , but leads to less … competition in males, giving them a higher reproductive fitness
genitalia mutation, frequency, kicking, intromittent, body cavity, infection, longevity, sperm
Sexual conflict leads to … selection and elevated genetic …. This is positive as we know that genetic … is the basis for evolutionary change
balancing (more alleles in pop), diversity, diversity
What is the opposite of sexually concordant selection>
Sexually antagonistic selection
A study looked at 6 different bird species varying in their levels of sexual conflict. The differences in sexual conflict, mediated by … system, explained variation in … … across the species. This potentially played an important role in ..
mating (sexual dichromatism used here), genetic diversity, adaptation
What is used to measure balancing selection/genetic diveristy?
Tajima’s D
… intralocus conflict in theory acts as an evolutionary …, hindering sex-specific adaptation. However, if it can be … quickly it allows evolution to continue
unresolved, brake, resolved
How can sexual conflict be resolved (phenotypes aligned with fitness peaks for each sex)?
- Sex linkage (on sex chromosomes)
- Gene expression evolution on the autosomes
If sexually conflicting genes can move from the autosomes onto the … chromosome then sexual conflict can be …
y, resolved
In theory the … and … chromosomes should be hotspots of sexual conflict and play an important role in resolving such conflict. E.g. Z chromosome more often in … than in ….
Dominant alleles are beneficial to be present on the … chromosome, as that chromosome is more often selected for its male-benefit effects - under strong male-specific selection.
Recessive alleles on the Z chromosome are more often selected for their …-benefitting effects as only one copy required for effect in females whereas 2 required in males - less likely to take effect
X, Z, males, females, Z, female
In Ficedula flycatchers Z chromosomes are enriched with …-… alleles. e.g. the genes for male white forehead patch map disproportionately to the Z chromosome.
(Z chromosome masculinised)
male-benefit
X chromosomes - dominant alleles selected for …-specific effect, recessive for …-specific effect. Supported in humans and mice (x chromosome demasculinised).
- movement of many testic-specific genes from the…
female, male
X chromosome to the autosomes as more beneficial
Why is it believed that recombination is prevented in sex chromosomes?
- Sex-determining gene was linked to ancestral gene in sexual conflict
- recombination suppressed in region between sex chromosomes to lock together these loci
- e.g. in guppy colour only beneficial in males - if link colouration genes to sex-determining gene by repressing recombination then beneficial and sexual conflict resolved as colouration genes only expressed in males - it has been shown that there is convergent expanding of the non-combining region in colourful guppy populations - consistent with theory
- may explain rapid evolution of sex chromosomes, as act to resolve sexual conflict - WAHOO!
However, sexually antagonistic genes are distributed across the entire genome (autosomes). Theory that sexual conflict can … expression of the same genes between males and females, resulting in …-… expression
uncouple, sex-biased
- supported in data from guppies and 6 bird species (genes expressed in gonads more uncoupled between the sexes than those expressed in the spleen)
