Lecture 8: intragenomic 2

Question 1

Beta chromosomes are

Answer 1

“parasitic” chromosomes not needed for normal function but which are good at getting passed on
-PSR is a beta chromosome in the parasitoid wasp Nasonia Vitipennis

Question 2

PSR chromosome =

Answer 2

beta chromosome

-Female Nasonia vitripennis (parasitoid wasp) carry PSR (Paternal sex ratio)

Question 3

Life cycle of Nasonia vitripennis

Answer 3

1) mated female lays multiple eggs on fly pupa inside puparial case
2) wasp larvae eat host pup & kill host
3) wasp larvae pupate inside host puparial case
4) adults emerge & mate locally, mated females find new hosts
- males can’t fly
- female bias sex ratio (due to local mate competition)

Question 4

panmictic

Answer 4

potential to mate with any individual of opposite sex of the whole population

Question 5

PSR is passed on through

Answer 5

sperm

• HOWEVER normally if passed on through sperm it would end up in a fertilised egg, which would be female. (as female wasps diploid, males haploid)
• females do not make sperm so are a dead end to PSR

Question 6

how does PSR avoid being a dead end being passed to a female =

Answer 6

PSR changes the sex of its bearer from female to male

• it makes the fertilised egg into a male by preventing the other chromosomes transmitted by the sperm unpacking
• PSR is extremely selfish, any gene it comes into contact with (i.e. travels in sperm) is destroyed

Question 7

PSR in Ansonia vitripennis causes a fertilised egg to

Answer 7

develop into a haploid male

Question 8

PSR: why doesnt it spread across the whole population? so all males have PSR

Answer 8

• you would have no females

- so settle for a EQUILIBRIUM PROPORTION within the population

Question 9

Equilibrium proportion: for PSR PARAMETERS

Answer 9

f = proportion of eggs that r fertilised
p = proportion of PSR+ males in the current generation
p* = proportion of PSR+ male in the next generation
pe = equilibrium proportion of PSR+ males
HAVE TO BE BETWEEN 0 & 1

Question 10

PSR: unfertilised eggs always result in

Answer 10

MALES which are PSR-

as no sperm used

Question 11

PSR: eggs fertilised by PCR- males

Answer 11

Females

Question 12

PSR: eggs fertilised by PCR+ sperm

Answer 12

PSR+ males (were females but sperm genes got rid so MALE)

Question 13

PSR: equation: pe =

Answer 13

(2f - 1)/f

Between half & all the eggs must be fertilised to reach equilibrium (to have PSR exist at equilibrium in population)

Question 14

for PSR to be consistent and at equilibrium in a population what frequency of eggs must be fertilised

Answer 14

between half and all of them (more than half!)

Question 15

most animals have an even sex ratio, but Nasonia vitipennis typically has

Answer 15

female biased sex ratio DUE TO Local mate competition

Question 16

Local mate competition =

Answer 16

Males compete with brother for matings, not with all males, so the ESS sex ratio for a mother is to have less than 50% sons

Question 17

false assumption made in N. vitripennis

Answer 17

they mate panmictically (random mating)
-BUT THEY DONT,
females mate w males reared in same habitat patch

Question 18

patches colonised by one female: dispersing females only occur when

Answer 18

males are PSR- (but PSR+ will die out) as those who mate with PSR+ all offspring will be males

Question 19

Beta chromosome in Trichogramma kaykai are infected by

Answer 19

the bacteria Wolbachia

• -lives in cytoplasm
• -makes host only to produce females
• • the beta chromosome cause the wasps to revert to sexual reproduction by making them males

Question 20

early life RNA copying =

Answer 20

without polymerase & error rates were high so only copied in very short lengths

Question 21

hypercycle and early origin of life: how did complex life evolve?

Answer 21

possibility is to have several proto-genes each coding for part of primitive metabolism

• each replicator is small so can be copied
• the replicators form a self-catalysing “hyper cycle” in which one catalyses the next
• Maynard-smith and Szathmary 1995

Question 22

for a hyper cycle to be evolutionary stable

Answer 22

it must be resistant to mutant replicators that don’t contribute to hyper cycle

Question 23

selection for beneficial mutants in hyper cycles

Answer 23

• mutants that help hyper cycle function should be favoured by natural selection
• occurs if hyper cycle is contained in a cell, so that helpful mutants benefit from improving the hyper cycle
• adv is retained locally, benefitting its own cell

Question 24

possible origins of linkage: Genes are normally ___ together into chromosomes but issues w this

Answer 24

LINKED together

• but linked genes are long because it takes longer to copy a long piece of DNA
• today = not a prob as cell division is tightly replicated
• but early evolution = issue

Question 25

origin of linkage possibilities

Answer 25

• linkage could lower the probability that cell division would produce a non-functional cell (lacking an essential gene)
• a cell may grow more efficiently & thereby divide more quickly is the essential genes divide at the same rate