Cytoplasmic incompatibilty Flashcards
What causes cytoplasmic incompatibility?
Wolbachia
In diploid organisms, what effect does CI have?
It causes offspring death
What does ‘bidirectional incompatibility’ mean?
When you cross organisms infected with different strains of Wolbachia, it produces CI in both directions, i.e. 2 combinations, neither of which are compatible :
male1 x female2 = NO
female1 x male2 = NO
BASICALLY regardless of which sex is carrying the Wolbachia, if both sexes have different strains it will cause incompatibility.
Both sexes can transit Wolbachia. True or false?
False; only the female can as it resides in the cytoplasm which is matrilineally inherited
Can male sperm be infected?
Yes; they can be infected and so cause incompatibility but not pass on the infection to offspring
In diploid organisms, infected male sperm kill the offspring of which females?
Uninfected females
Females infected with a different strain of Wolbachia
In diploid organisms, only 2 gametes carrying the same strain of Wolbachia can produce viable offspring. True or false?
True
In diploid organisms how does the killing mechanism work?
It causes loss of the paternal genome at the first mitotic division due to abnormal H3.3. histone deposition.
Diploid organisms need both parental genomes for survival and so the offspring dies.
Which diploid genera are commonly affected by Wolbachia CI?
Culex (moqsuitos) and Drosophila (flies)
In diploid organisms, what happens if an uninfected male mates with an infected female?
The offspring is viable and carries the maternal Wolbachia strain
What does CI cause in haplodiploidy organisms?
Masculinisation of offspring
In haplodiploidy organisms, what happens when 2 gametes of the same strain come together?
Offspring are viable daughters
In haplodiploidy organisms, what happens when an uninfected male meets an infected female?
Offspring are viable and carry female Wolbachia
In haplodiploidy organisms, what happens when an uninfected female meets an uninfected male?
Offspring are male
In haplodiploidy organisms, what happens when an infected female meets a male infected with a different strain?
Offspring are male
In haplodiploidy organisms, infected males masculinise the offspring of?…
Uninfected females
Infected females of a different strain
What is the masculinising mechanism in haplodiploidy organisms?
At the first mitotic division there is loss of the paternal genome due to abnormal H3.3 histone deposition. Loss of the paternal genome from a fertilised egg produces male offspring.
Which genus is masculinising CI commonly observed in?
Nasonia (parasitoid wasp)
How does Wolbachia cause CI?
Infected sperm of a particular strain carries the MOD+ locus, a toxin. Uninfected females/those infected with a different strain DO NOT CARRY the complementary antitoxin and so offspring are killed/masculinised.
Why are organisms infected with the same strain of Wolbachia compatible?
Females infected with the same strain carry the RES+ locus, an antitoxin. This counteracts the MOD+ toxin and produces normal offspring.
What is the kin selection hypothesis for CI in diploid organisms?
Males are an evolutionary dead end as do not transmit cytoplasm, so no cost to ‘suicide’ sperm that produce incompatible offspring.
However benefit off killing offspring is large as it reduces local resource competition for compatible offspring, thus enhancing spread of Wolbachia.
The kin selection hypothesis for CI in diploids depends on what?
A viscous population; limited dispersal ensures a high degree of relatedness between organisms and thus their Wolbachia which is identical by descent.
Intense local resource competition as it increases the benefit of killing incompatible offspring for compatible ones.
What is the kin selection hypothesis for CI in haplodiploid organisms?
Parasitoid wasps lay their eggs en masse in a host.
Compatible eggs are diploid, produces a female-biased sex ratio which is optimum for Wb transmission.
Incompatible eggs produce a male-biased sex ratio which reduces transmission of Wb. However, males enhance spread of Wb by providing mates of the same strain for infected females, which will produce more infected daughters. Also males die after reproduction so do not hang around and use up resources for infected females.
BASICALLY MALE-BIASED SR ENHANCES SPREAD as it provides correct strain males for females to mate with
Invasion of Wb into a population depends on which 4 factors?
- Cost of carrying Wb to females
- Transmission rate
- Strength of Wb-induced mortality
- Benefit of CI crosses
How does the cost of carrying Wb to females affect invasion?
If the cost is too great Wb cannot invade a population as Wb females will just die.
How does transmission rate affect invasion of Wb into a population?
If transmission rate is not high enough then Wb will not invade
How does the strength of Wb-induced mortality affect invasion of Wb?
If the strength of mortality is weak then the effects of CI will not be strong enough to allow for fixation
How does the benefit of CI crosses affect the invasion of Wb into a pop.?
If the benefit of CI killing/masculinising is not great enough the trait will nto fix as it will not be selected for
So to allow for Wb invasion into a population, what level must
Cost to females
Transmission rate
Strength of mortality
Benefit of CI
be?
Low enough to not be removed by selection
High
High
High enough to be selected for
Wb-induced CI can degrade in a population under 3 conditions. What are they?
- When Wb invades and reaches a high frequency; as everyone is infected they are al compatible and CI breaks down
- There is invasion of CI-resistant Wb that do not cause CI themselves
- There is a spread of benign Wb; once MOD+ is lost MOD-RES- spreads as there is no cost to females. Then the lineage can be invaded by another strain.
k = transmission rate z = strength of mortality
When does a benign Wb strain evolve?
If k is initially high, all females become infected and CI is no longer an advantage (as everyone is compatible)
k = transmission rate z = strength of mortality
When does a virulent Wb strain evolve?
If k is initially low, there are more females in the population to be exploited and so transmission rate must increase, strain evolves to be virulent
Give a referenced example of host coevolution with Wb.
Sinkins et al., 2005
What did Sinkins et al., 2005 do?
Looked at C. pipiens
Strain of Wb from China = Bei
Strain of Wb from Sri Lanka = Pel
These are incompatible
Using introgression the authors created a cured strain P(wB) that had PEL GENOME and BEI CYTOPLASM (Wb)
What did Sinkins et al., 2005 find between cured individuals and Bei individuals?
Cured individuals showed bidirectional compatibility w/ Bei individuals as same Wb strain in cytoplasm
What did Sinkins et al., 2005 find between Pel females and cured males?
Pel females showed only partial incompatibility w/ cured males as differing cytoplasm
What did Sinkins et al., 2005 find between cured females and Pel males?
Cured female and Pel male expected to be incompatible BUT were compatible. Shows the Pel genome is resistant to Bei strain Wb
What does Sinkins et al.’s (2005) work show about the host and its Wb?
Co-evolution is possible
