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What Can Evolution REALLY Do How Microbes Can Help Us Find the Answer

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1. Bacteria become more fit. 2. Most of the gain in fitness is in the first 2000 generations ... 6. These bacteria are still very much E. coli. When Evolution ... – PowerPoint PPT presentation

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Title: What Can Evolution REALLY Do How Microbes Can Help Us Find the Answer


1
What Can Evolution REALLY Do?How Microbes Can
Help Us Find the Answer
Ralph Seelke, U. Wisconsin-Superior
2
Where Were Going
  • Confessions of an experimentalist
  • What Evolution has been able to accomplish
  • What it has NOT been able to accomplish
  • Some conclusions

3
  • For Evolution to Occur You Need
  • A LARGE Population
  • and/or
  • MANY Generations
  • A Trait That Can Evolve
  • !!!!BACTERIA!!!!
  • (or yeasts!)

4
  • 4 TRILLION in a 1 Gal Milk Jug!
  • Thousands of Generations in a Year!
  • COMPLEX Traits!
  • When they evolve, we can FIND THEM!

5

Each Transfer Produces 6.6 generations of
evolution! 46 generations per week Almost 400 per
month Over 2,400 in a year 24,000 in ten years!
6
We can FIND evolution BecauseWhen the microbe
EVOLVESitGROWSor GROWS BETTER!!!!
7
Evolution Before your very eyes!
8
MORE Evolution Before your very eyes!
9
Many Traits are Irreducibly Complex
  • Example Using Lactose
  • Milk Sugar
  • Two Requirements
  • Lactase
  • Permease
  • Brings the lactose in
  • BOTH Needed

lactose
10
Lactose
outside
Permease
inside
Glucose
Galactose
FUEL!
Irreducibly complex Without BOTH the permease
AND the lactase, The cell CANNOT use lactose as a
food source
11
Case Study Evolution of a New Lactase Gene
12
Lactose
outside
Permease (Artificially present)
inside
X
X
EBG
Present but inactive!
NO FUEL!
Evolution Can Rescue this Cell one change, and
the EBG is working
13
Lactose
outside
Permease Artificially present
inside
EBG
Present and now active!
FUEL!
After Evolution EBG present, and active cell
survives
14
What Barry Hall found when he evolved this E. coli
15
LOTS of Examples Like This
  • Utilization of
  • Xylitol
  • L-arabitol
  • D-arabinose
  • L-xylose
  • L-lyxose
  • And others
  • Mortlock, R. P. (ed.). 1984. Microorganisms as
    Model Systems for Studying Evolution Plenum
    Press, New York

16
What Happens in These Cases?
  • A gene is present in the microbe, but is either
    silent (not expressed) or expressed but producing
    an inactive product.
  • A single mutation may activate the gene (no
    longer silent) or result in an active product.
  • Further mutations can then make the microbe
    better at using this new food source.

17
Evolution in Action Richard Lenskis LONG Term
Evolution Studies
18
Do this, 12X a day, for the rest of your life!
Lenskis group has evolved 12 E. coli cultures
in a low nutrient broth, transferring daily,
since 1988. He has achieved 40,000 generations
of evolution! What has he discovered?
19
What Has 40,000 Generations Produced?
  • 1. Bacteria become more fit.
  • 2. Most of the gain in fitness is in the first
    2000 generations
  • 3. They become bigger
  • 4. Most of the gain comes from five different
    genes that have mutated.
  • 5. After 20,000 generations, his group sequenced
    918,700 bases from 50 isolates- they found 10
    changes, all in ones with a mutator phenotype.
  • 6. These bacteria are still very much E. coli.

20
When Evolution CANNOT Produce a New Function
21
Lactose
outside
X
Permease gene present, but permease not produced!
inside
Lactase Gene Missing!
X
NO FUEL! Cell Starves!
EBG Gene present but silent NO Active EBG!
EBG
X
Evolution CANNOT Rescue this Cell TWO changes,
required!
22
Why Does Requiring Two Changes Make Such a
Difference?
  • Hall found that about one in a billion cells
    mutated to have an ebg gene that broke down
    lactose
  • You would expect 1 mutant in a billion cells- the
    contents of 1 milliliter of a bacterial culture
  • If the permease gene mutated at the same rate,
    then one in a billion billion cells would be
    expected to have both mutations
  • You would need a million liters of culture to
    produce that many bacteria (several swimming
    pools) !!

23
Other Examples Where the Need for Two Mutations
Stops Evolution
  • Utilization of
  • D-arabitol
  • Ethylene Glycol
  • Lactobionate
  • Synthesis of
  • Tryptophan

24
Is the Need for Two Independent Mutations REALLY
an Evolution-Stopper?Studies with the trpA Gene
of Escherichia coli
25
Testing the Two Mutation Rule
  • Find a well-studied gene, with known mutations
    that inactivate it.
  • Introduce 1,2,3, or four inactivating mutations
  • Let the gene evolve under highly selective
    conditions

26
The Gene of Choice trpA
27
Results So FarIf Evolution Requires Two or More
Independent MutationsNOTHING HAPPENS
28
Testing Large Populations of RS202-5 (two
mutations) for Evolution
  • Test in liquid culture 0.4X 1012 cells tested
    without evidence of evolution.
  • Test on agar plates 0.9-1.8 X 1012 cells tested
    without evidence of evolution.
  • RS201-2 (one mutation) routinely produced 10-20
    Trp colonies/plate, 104/ml
  • Trp cells/ml in liquid culture.
  • No evidence of evolution of RS202-5

29
Results of Serial Transfer
  • One culture lost its trpAB genes within 275
    generations.
  • Two additional cultures have been tested for
    240 transfers, or about 1600 generations.
  • No Trp evolution observed.
  • HOWEVER.

30
The cultures have evolved to be able to grow
better in the tryptophan-limited medium
31
Why Not Pursue This Approach?
  • Can long-term evolution demonstrate the evolution
    of difficult traits?
  • Reviewer
  • On this question (evolution of difficult
    traits), we certainly know that long-term
    evolution (really long-term) has created
    "difficult" (complex) traits such as
    photosynthesis, DNA replication, protein
    synthesis, cell division, nitrogen fixation,
    transformation, toxins and many more.
  • Another reviewer
  • What can be said if the answer is no?

32
Acknowledgements
  • Merck Foundation
  • UW-Superior
  • A.C. Matin Lab and Stanford University
  • NUMEROUS undergraduate students!
  • Pravien Abeywickrema,Kayo Sakaguchi
  • Robert Jennings, Ranjuna Weerasekera
  • Lynn Meyer, Sarah Rahn
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