Title: What Can Evolution REALLY Do How Microbes Can Help Us Find the Answer
1What Can Evolution REALLY Do?How Microbes Can
Help Us Find the Answer
Ralph Seelke, U. Wisconsin-Superior
2Where 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!
6We can FIND evolution BecauseWhen the microbe
EVOLVESitGROWSor GROWS BETTER!!!!
7Evolution Before your very eyes!
8MORE Evolution Before your very eyes!
9Many Traits are Irreducibly Complex
- Example Using Lactose
- Milk Sugar
- Two Requirements
- Lactase
- Permease
- Brings the lactose in
- BOTH Needed
lactose
10Lactose
outside
Permease
inside
Glucose
Galactose
FUEL!
Irreducibly complex Without BOTH the permease
AND the lactase, The cell CANNOT use lactose as a
food source
11Case Study Evolution of a New Lactase Gene
12Lactose
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
13Lactose
outside
Permease Artificially present
inside
EBG
Present and now active!
FUEL!
After Evolution EBG present, and active cell
survives
14What Barry Hall found when he evolved this E. coli
15LOTS 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
16What 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.
17Evolution in Action Richard Lenskis LONG Term
Evolution Studies
18Do 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?
19What 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.
20When Evolution CANNOT Produce a New Function
21Lactose
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!
22Why 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) !!
23Other Examples Where the Need for Two Mutations
Stops Evolution
- Utilization of
- D-arabitol
- Ethylene Glycol
- Lactobionate
- Synthesis of
- Tryptophan
24Is the Need for Two Independent Mutations REALLY
an Evolution-Stopper?Studies with the trpA Gene
of Escherichia coli
25Testing 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
26The Gene of Choice trpA
27Results So FarIf Evolution Requires Two or More
Independent MutationsNOTHING HAPPENS
28Testing 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
29Results 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.
30The cultures have evolved to be able to grow
better in the tryptophan-limited medium
31Why 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?
32Acknowledgements
- 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