Lecture 8: intragenomic 2 Flashcards
Beta chromosomes are
“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
PSR chromosome =
beta chromosome
-Female Nasonia vitripennis (parasitoid wasp) carry PSR (Paternal sex ratio)
Life cycle of Nasonia vitripennis
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)
panmictic
potential to mate with any individual of opposite sex of the whole population
PSR is passed on through
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
how does PSR avoid being a dead end being passed to a female =
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
PSR in Ansonia vitripennis causes a fertilised egg to
develop into a haploid male
PSR: why doesnt it spread across the whole population? so all males have PSR
- you would have no females
- so settle for a EQUILIBRIUM PROPORTION within the population
Equilibrium proportion: for PSR PARAMETERS
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
PSR: unfertilised eggs always result in
MALES which are PSR-
as no sperm used
PSR: eggs fertilised by PCR- males
Females
PSR: eggs fertilised by PCR+ sperm
PSR+ males (were females but sperm genes got rid so MALE)
PSR: equation: pe =
(2f - 1)/f
Between half & all the eggs must be fertilised to reach equilibrium (to have PSR exist at equilibrium in population)
for PSR to be consistent and at equilibrium in a population what frequency of eggs must be fertilised
between half and all of them (more than half!)
most animals have an even sex ratio, but Nasonia vitipennis typically has
female biased sex ratio DUE TO Local mate competition
Local mate competition =
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
false assumption made in N. vitripennis
they mate panmictically (random mating)
-BUT THEY DONT,
females mate w males reared in same habitat patch
patches colonised by one female: dispersing females only occur when
males are PSR- (but PSR+ will die out) as those who mate with PSR+ all offspring will be males
Beta chromosome in Trichogramma kaykai are infected by
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
early life RNA copying =
without polymerase & error rates were high so only copied in very short lengths
hypercycle and early origin of life: how did complex life evolve?
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
for a hyper cycle to be evolutionary stable
it must be resistant to mutant replicators that don’t contribute to hyper cycle
selection for beneficial mutants in hyper cycles
- 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
possible origins of linkage: Genes are normally ___ together into chromosomes but issues w this
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
origin of linkage possibilities
- 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
