Meotic drive Flashcards
What kind of division is meiosis?
A reduction division
What is a reduction division?
Transformation of the diploid state to the haploid state
What does meiosis produce?
The gametes
How many daughter cells does meiosis produce?
4
What is meiotic drive?
When a selfish genetic element distorts meiosis to enhance its own transmission
Is meiotic drive at cost to the host?
Yes
Does meiotic drive distort the sex ratio of the progeny?
Sometimes
Simple meiotic drive occurs in males. How does it work? What type of drive is it?
There are two loci involved, Killer (toxin) and Resistant (antitoxin).
Sperm with the Killer locus kills other sperm but are themselves resistant to the toxin, and so the meiotic driver is proliferated.
What is necessary for simple/male meiotic drive to work?
Tight linkage between the Killer and Resistant alleles so that they are not broken up by recombination.
The production of sperm and eggs differ. When a primary spermatocyte divides, what does it produce?
Two secondary spermatocytes
The production of sperm and eggs differ. When a primary oocyte divides, what does it produce?
One secondary oocyte and one polar body (which divides into 2)
The production of sperm and eggs differ. When a secondary spermatocyte divides, what does it produce?
Two spermatids
The production of sperm and eggs differ. When a secondary oocyte divides, what does it produce?
One ootid and one polar body
So overall, what is produced by the a) primary spermatocyte and b) primary oocyte in meiosis?
a) 4 sperm cells
b) 1 ovum and 3 polar bodies
In oogenesis, at what stage do homologous chromosomes separate?
Transition from primary to secondary oocyte
In oogenesis, at what stage do sister chromatids separate?
Secondary oocyte to ootid
What does the ootid give rise to?
The ovum (egg)
What holds the sister chromatids together?
The centromere
In female meiosis, what is the selfish element? What does it do?
The centromere, it never ends up in the polar body when the secondary oocyte divides
One study found evidence for female meiosis.
a) Who did it?
b) What year?
c) Which organisms did they study?
a) Fishman and Saunders
b) 2008
c) Monkey flowers, M. guttatus and M. nastus
In Fishman and Saunders’ 2008 experiment on monkey flowers, what happened?
In hydridisation experiments M. guttatus always showed 100% segregation, i.e. it never ended up in the polar body, due to a duplication of centromeric DNA
Suppressors have evolved to counter meiotic drive. What are the two types?
Linked and non-linked
What are meiotic drivers often referred to as?
Segregation distorters
Who worked on D. melanogaster, what year?
Larracuente and Presgraves, 2012
Larracuente and Presgraves (2012):
D. melanogaster has a segregation distorter involving two loci. What are they?
Sd = a toxin Rsp = a responder/antitoxin
Larracuente and Presgraves (2012):
What is Sd?
A partial duplication of RanGAP. Sd-RanGAP fails to leave the nucleus and accumulates as a toxin.
Larracuente and Presgraves (2012):
There are two alleles of Rsp. What are they?
Rsp-i, provides immunity to Sd
Rsp-s, sensitive to Sd
Larracuente and Presgraves (2012):
What is the wildtype condition?
No Sd and Rsp-s
Larracuente and Presgraves (2012):
What is the mutant/selfish condition?
Sd and Rsp-i, immune to own toxin
Larracuente and Presgraves (2012):
Why is there low recombination between the Sd and Rsp loci? Give 3 reasons.
- Genes are either side of the centromere, do not recombine
- Lots of local inversions
- Recombination unfavoured as produces ‘suicide’ and ‘immune’ chromosomes
Larracuente and Presgraves (2012):
Sd chromosomes have an advantage over non-Sd chromosomes. True or false?
True
Larracuente and Presgraves (2012):
What kind of suppressor occurs in the Sd-Rsp system?
Linked: the Sd and Rsp loci are linked and if recombination occurs you can get an immune chromosome (non-Sd-Rsp-i) that does not produce the toxin or get affected by it
Who described the ‘three-way cycle’ of linked suppressors?
Crow, 1988
If not confident putting the date just say ‘idea put forward by James Crow’
Crow, 1988:
What is the three way cycle in linked suppressor systems?
SD = segregation distorter
SD invades wildtype pop.
Rare recombination produces immune chr.
Immune chr invades SD pop.
Wildtype can invade immune pop.
Who studied D. pseudoobscura?
Price at al. 2008
Price at el. 2008:
The meiotic driver in D. pseudoobscura is sex-linked (SRD = sex ratio distorter). How?
The driver is on the x-chromosome
Price at el. 2008:
What does the meiotic driver in D. pseudoobscura do?
Kill all y-bearing sperm in males
Price at el. 2008:
Why are males that contain the meiotic driver weak competitors?
Because 50% of their sperm (the y-bearing sperm) die
Price at el. 2008:
What evidence is there that the SRD in D. pseudoobscura causes males to be weaker competitors?
SRD males sire less offspring than normal males, particularly when they are the second male to mate (already lots of good sperm in there, lots of competition)
Price at el. 2008:
Why can drivers not spread to fixation if there is polymorphism?
Morphs are differentially fit, normal males are fitter and thus will be selected for, counter-balances drive
Who described a non-linked suppressor system in D. simulans?
Kingan et al. 2010
Kingan et al. (2010):
In D. simulans, what are the driver and suppressor loci called?
Driver = Dox Supressor = Nmy
Kingan et al. (2010):
Why is Nmy in relation to Dox, how does it suppress Dox?
Nmy is a duplicate of Dox that was retrotransposed onto chromosome 3 (from chr2).
Nmy is read in reverse and produces antisense RNA which binds Dox RNA. Double-stranded RNA then recognised as a virus and translation prevented.
Are organisms holistic?
No, they could be viewed as containers for selfish elements, lots of competing interests
Meiotic drivers generally spread, fix then degrade. Give 2 examples of how they can degrade.
Linkage or inversions around the SD loci trap deleterious mutations, produces sterility/lower fitness, thus selected against.
Evolution of host repression means they no longer have an effect.
Who described polyandry as a solution to drive?
Price et al. (2008)
How did Price et al. (2008) propose polyandry would counteract drive? Which organism did they look at?
D. pseudoobscura
Found that females evolved increased remating rates when exposed to SRD males. Polyandry increases offspring fitness by selecting for males that are better competitors (i.e. have non-driver sperm).
Who described sexual selection as a solution to drive?
Cotton et al. (2014)
Cotton et al. (2014):
The authors studied T. dalmanni, what did they find? What does this mean?
In T. dalmanni driver is X-linked, so is a SRD.
However driver also linked to male eyespan (must also be on the X).
Driver males have a smaller eyespan and come from female-biased broods.
Authors propose sexual selection evolved/is maintained as a way for females to avoid mating w/ driver males.
It is thought that drive can lead to speciation. How?
There are Red Queen dynamics between drivers and suppressors
There are 2 types of driver. What are they?
- Segregation distorters
2. Sex ratio distorters, ALWAYS SEX-LINKED
Was Larracuente and Presgraves (2012) a review paper?
Yes
Give 2 examples that drive reduces the fitness of the host (aside from the obvious reducing transmission of host genes)
- In D. melanogaster, Sd/Sd is lethal
- In D. recens, drivers are associated w/ 2 major inversions that trap deleterious alleles. Also produces recessive female sterility.
Kingan et al., 2010:
Dox is a SRD. Why do the authors say Nmy was selected for?
Dox alters the sex ratio, makes it highly polymorphic.
There is selection on the repressor (Nmy) to re-establish the sex ratio.
Kingan et al., 2010:
Is Dox fixed in any D. simulans population?
No, because it is in an evolutionary arms race with Nmy.
Who discovered Nmy in D. simulans?
Tao et al., 2007
What is it called when antisense RNA binds and prevents translation?
RNA interference
Fishman and Saunders, 2008:
Why is SD drive not fixed in M. guttatus?
Homozygous driver individuals have reduced pollen viability
Who studied drive as a speciation force in T. dalmanni?
What did they do?
What did this show? How did we know this?
Wilkinson et al., 2014
Crossed 2 populations of T. dalmanni, one from Malaysia and one from Sumatra.
There was F1 sterility in males.
They then backcrossed F1 females.
That X-Sumatra was an X-linked driver and Y-Malaysia was a Y-linked driver. This was implied from the sex bias in the broods.
Wilkinson et al., 2014:
How do hybridisation events expose drivers?
What does this mean for speciation?
You can’t see the drivers in the wild type as suppressors have evolved.
Suppressors are different in different species.
In the F1 the drivers and their suppressors are uncoupled, leads to sterility in the heterogametic sex.
In a backcrossed F2 you can see the effects of the drivers.
Speciation occurs as drivers and suppressors evolve rapidly in different populations; if hybridisation causes sterility then it is selected against, causes reproductive isolation and speciation.
How does Wilkins et al., 2014 experiments on T. dalmanni agree with Haldane’s rule?
In the F1 the heterogametic sex is sterile as driver/suppressors been broken up by recombination and there is nothing to mask the driver.
Sterility of hybrids = reproductive isolation = speciation
