BLG 508 Lecture Notes - Lecture 8: Mutation Rate, Point Mutation, Base Pair

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25 Jul 2016
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3.Mutation & New Alleles
The fate of a new allele will: be fixed in a population, increase in frequency and get fixed, lost, or
random;
Depends on weather its beneficial, deleterious or neutral.
Mutation Accumulation Experiment
-Used in laboratory organisms to look at the effects of mutations
They took C. elegans which are hermaphrodites, then they reproduced genetically identical
lines from a single individual (74 lines genetically identical like clones)
They reproduced for 214 generations under ideal conditions for retaining non-lethal mutations
oIf a mutation is lethal then it kills the organism and mutation dies too
oNo Natural Selection because no competition for resources
o Random selection of single parent for reproduction in each generation
Compared fitness of mutation accumulation lines to controls
oThe controls lines had Natural Selection occurring in lab culture, there was competition
for resources and the largest group of parents was chosen randomly from survivors.
74 Control (selected) lines: they started genetically identical from a single parent
Accumulation of mutations = decrease in fitness of 0.5% per generation
Control line: no change in fitness
Mutation accumulation line steady decrease in percentage surviving to maturity (all mutation in
the population)
oIndividuals die at early stage without reproducing
on average a decrease in fitness of 0.5 per generation if mutations accumulate in the absence
of natural selections
mutations on average are deleterious/ decrease in fitness
mutation is happening in every generation but it tends to decrease the fitness on average but
they are still retained in the population
RECOVERY: if you introduce selection at end of the 240 generations you get a increase in
fitness, close to the control line is over many generations
oBeneficial mutations must be rare for the line to show decrease
o-even if you have terrible mutations accumulated your offspring will have traits and will
be able to reproduce and some of them will be better you are
oNatural selections removes deleterious variance
Some mutations may increase there productivity and now with natural selection present the
ones that do really badly will be removed and the ones that remain will be set back to recovery
C. elegans reproduce at high rate, even if one beneficial mutation appears it will have millions
of babies, dominating the population and go back to high fitness with selection
Why are beneficial mutations rare?
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Brewer's Yeast
Initial population: homozygous at all loci
o144 mutation accumulation lines
o1,012 generations
compare them with their ancestor population terms of their fitness
In a neutral nutrient medium
mutations are neutral: no change in fitness
beneficial mutation occurred but they were very rare about 14% above ancestor generation
mutation decreased fitness 34%/ lethal mutations too
huge comparison by the beneficial mutations to deleterious mutations that decrease fitness
and occur at high rate
you can occasionally improve an organism that is well adapted in the environment, but there
are so many more ways that you can decrease its function than ways you can increase its
function
Overall: most mutations are deleterious
oMost mutations (mildly) deleterious
oMany mutations neutral
oBeneficial mutations rare
4. Mutation Rates
Virus have high mutation rates
Humans are 10-8 / Yeast…
Mutations rates per base pair are LOW
oThe likely hood of a mutation in our genome is very low (10-8)
oEx. Substitutions per base pair per generation: C becoming T in one generation at
that one location.
Mutation rates are diverse: mutation rates can evolve
oCan we explain this diversity through evolution of mutation rates?
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