WP Lecture Flashcards
Competing Theories of Evolution
—> Darwinian Evolution
1) Long necked giraffes have better access to food.
2) Long necked giraffes have higher chance of surviving and passing the down long-neck trait.
3) Long-necked trait evolves by natural selection.
—> Lamarckian Evolution
1) Giraffes stretch their necks to reach high leaves.
2) Their progeny have longer necks as a result.
3) Use of trait promotes that trait in progeny.
“Germ-plasm” Theory of Heredity
- proposed that only the germ-line cells (sperm and eggs) mediate heredity.
- chopped tails off of mice over 5 generations.
- observed no reduction in tail size in progeny.
- contradicted Larmarckian model.
(can stretch your neck as much as you liked, but how would this effect sperm and eggs)
(larmackian model was an example of a wrong idea in textbooks)
Larmarckism: Round 2
- in soviet union, this idea had a renaissance. (1930s-1960s)
- villain in history of science.
- if subject seeds to cold temperatures, sometimes they germinate better in the spring.
- he became convinced that this would fundamentally changing the plants in a way that was durable in further generations.
- became convinced that mandelian genetics were wrong, and organisms could be changed in how they were treated in a certain generation (even can change their species).
- had major political battles with other soviet geneticists. (good at politics and got a lot of people executed)
- any situation where political considerations lead to the support of one theory.
Lamarckism: Round 3
- popular science press became convinced that epigenetics could create Lamarickism mechanisms.
- via DNA methylation, things that happen to you could be passed on to your decendence.
DNA Methylation
- very heritable
- every-time DNA replicates, new strand
- DNMT1 comes along and replicates a new strand.
- can happen through many cycles of cell division.
Could epigenetics mediate Lamarckian evolution?
- circumstances of parent cause epigenetic change (e.g. stretching neck causes epigenetic change in germline).
- transmission of epigenetic change to next generation.
- adaptive change in next generation linked to original stimulus (e.g. epigenetic change causes longer neck in germ-line)
- meaningful on evolutionary timescale.
Examples of transgenerational Epigenetic Inheritance
- yes in plants
- Linnaeus - described essential the first mutation in scientific literature - peloric flax plant flowers.
- identical petals (rotational symmetry, each petal identical to other petal).
- this is extremely large difference to encounter within a species.
The Peloric Antirrhinum: Snapdragon is caused by a Mutation in the Cycloidia gene.
- Charles Darwin studied.
- WT x Peloric
- cross them
- progeny is all WT
- self cross the progeny.
- 1/4 Peloric progeny.
This is mandelian genetics - peloric is recessive.
Discovered a recessive triait/
Fast-forward 100 years.
found out:
mutating the Cycloidia gene leads to the peloric phenotype. End up with snapdragon!
The Cycloidia Gene is Normal in Linearia, But methylated. Explain this.
- The cycloidia gene is not cut by restriction enzymes in the Peloric plant.
- the gene is Methyated in the peloric plant, but unmethylated in the WT.
- with methylation, the restriction enzyme cannot cut.
- some of its cleavage sites are methylated.
- what is happening in peloric plant, the gene is not mutated, its primary sequence is fine, but it is methylated.
What are other examples of plant epialleles?
(1) fwa1 mutant (dominant) vs WT:
Gene is unmethylated and overexpressed.
(2) collorless non-ripening phenotype in tomatoes:
- Methylation of gene needed for ripening.
Did not turn out to be a common phenomenon.
Are plant epialleles examples of Larmackian evolution?
- the question is if they match the criteria.
(1) Circumstances of parent causes epigenetic change (does not match) (arises randomly)
(2) Transmission of epigenetic change to next generation (does match - very heritable)
(3) adaptive change in next generation (e.g. epigenetic change causes longer neck in germline (????)
(4) meaninful on evolutionary time scale (???)
An adaptive epiallele example
- Two NLR Genes: Provide resistance to clubroot; regulated by epiallelic variation.
- Resistant Plants: High expression of unmethylated NLR genes → resist clubroot but prone to autoimmunity.
Susceptible Plants: Methylated NLR genes → low expression → vulnerable to clubroot. - Key Difference: Resistance vs. susceptibility depends on methylation state, not DNA sequence.
- Evidence:
1001 Arabidopsis methylomes show variation in methylation correlates with clubroot vulnerability.
Suggests epigenetic inheritance (partially Lamarckian). - Implication: Epigenetics can influence adaptive traits passed to the next generation.
Obstacles to epialleles in mammals
- Zygotic demethylation at fertilization AND when primordial germ cells are specified in mammals prevents the germline transmission of epialleles.
- At every life cycle in mammal, almost all demethylation gets erased and then rebooted.
- this does not happen in plants, but it happens in humans.
Agouti Mice (Agouti Allele Gene)
- Agouti gene produces protein which causes nearby melanocytes to produce phaeomelanin (yellowish) instead of eumelanin (black).
- produces mice with “agouti” fur.
- Agouti viable yellow allele.
- a yellow mouse (Avy) arises at Jackson labs.
- was stripped pattern on hair, but then all yellow.
- certain amount of variation (some were bright yellow, some dimmer) - genetically identical but variation in phenotype.
What is the reasoning behind Agouti Viable Yellow Allele?
- In “pseudoagouti”, IAP is methylated, and the Agouti gene has restricted expression.
- In yellow: IAP is unmethylated and Agouti gene has high expression.
demonstrated this using methylation sensitive blocks.
Parental Effect in Coulouration
- Avy/a progeny only (aa are black)
- yellow mother, more likely to have yellow progeny than a pseudoagouti mother.
- pattern of demethylation could be transmitted throughout the lineage (evidence)
- conclusion: methylation is transmitted from mother to progeny by via oocyte.
did this by doing oocyte transfer.
Summary of the Avy allele
- Expression of Agouti gene driven from IAP retrotransposon.
- in some genetically identical mice, IAP is methylated and gene is silent. In other mice, gene is highly active.
- methylation status of maternal allele can be transmitted to offpsring.
- resetting is incomplete upon maternal transmission.
in Avy allele, resetting is incomplete upon maternal transmission. but how?
- Does anything resist genome-wide demethylation??
Look to see if any Loci escape reprogramming during DNA methylation. (during germ-cell development)
IAP elements are not fully demethylated.
(can be inherited through multiple cycles, at least for the mother)
After:
the search for more transgenerational inheritance (that behaved like the agouti gene)
had to meet many criteria:
1) IAP must be variably methylated across mice.
2) IAP must control expression of nearby genes.
3) Methylation status must be heritable.
Often Variably Methylated IAPs exist
- some even control the expression of adjacent genes
- in some mouse, IAP methylated, some almost completely de-methylated. show variability across individual mice.
-but only one shows parental transmission and pretty week…
if mom is methylated, children more likely to be methylated.
extremely rare phenomenon.
in all of biology only found two moderatly inheritable epialleles in mice.
what lessons did we learn from this experiment/lecture?
- sometimes a phenomenon is real and reproducible but does not change out understanding of biology very much.
- the popular science press gets a lot wrong.
- its unclear to what extent trans-generational epigenetic inheritance occurs in humans.
A predicament in biology
- in most years, its ideal for Plant X to germinate after the first warm day.
- plant X evolves to germinate with 100% reproducibility after first warm day.
- one year, there is a warm day followed by a frost that kills all the seedlings.
- goodbye plant X.
“Bet-hedging”
- Bet-Hedging: Maintaining variation within a species to prepare for unpredictable environments.
- Plant Example:
–> Some plants germinate late → survive unexpected events (e.g., frost) → repopulate.
—> Weaker immune systems grow faster → advantage in no disease. Stronger immune systems survive when disease strikes. - Epialleles as Bet-Hedging:
–> Random switches (e.g., methylated ↔ unmethylated) create diverse traits.
—> Stochastic epigenetic changes (e.g., DNMT1 failure) drive adaptation and resilience. - Significance: Promotes survival, development, evolution, and response to disease in varying conditions.
