GENETICS AND THE ENVIRONMENT IMSS BIOLOGY ~ SUMMER 2013
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SELECTIVE PRESSURE FROM THE ENVIRONMENT ACTS DIRECTLY ON (A)
the population gene pool (B) phenotypes (C) an individuals genotype
(D) allele frequencies in a species This is because____________...
(Write answer in your notebook.)
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HOW DO WE DEFINE PHENOTYPE? Phenotype is the physical
expression of a trait. Such physical expression of traits depends
on the genotype and the organisms interactions with the
environment. How would you categorize phenotypic traits? What are
some examples? Categories Examples
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GENOTYPE, THE ENVIRONMENT, & PHENOTYPE
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PHENOTYPIC PLASTICITY Genotypes produce different phenotypes in
response to different environmental conditions.
commons.wikimedia.org
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PHENOTYPIC PLASTICITY Defined as ability of individual
genotypes to produce different phenotypes when exposed to different
environmental conditions (Pigliucci et al. 2006 and includes:
ability to modify developmental pathways in response to specific
environmental cues ability of individual organism to change its
phenotypic state or activity (e.g. metabolism) in response to
variations in environmental conditions. Genetic polymorphisms also
exist that may or may not cause phenotypic variation.
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MANY EXAMPLES Seasonal polyphenism in butterflies Caste
polyphenism in social insects Environmental sex determination in
reptiles Predator-induced polyphenism in cladocerans (e.g. Daphnia)
Phenotypic changes such as acclimation, learning, and immune
response SummerFall
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GENOTYPE, THE ENVIRONMENT, & PHENOTYPE
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KEY POINTS RE. PHENOTYPIC PLASTICITY Plasticity may be
expressed at behavioral, biochemical, physiological, or
developmental levels with different degrees of reversibility. Not
always adaptivesome traits are plastic because of the physiology,
biochemistry, or developmental biology of the organism. Type &
degree of plasticity specific to individual traits and
environmental conditions, e.g. a given trait is plastic in response
to temperature, other traits are not. There is abundant genetic
variation for plastic responses in natural populationsmakes
possible evolution of plasticity by natural selection (and other
mechanisms).
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WHICH GENOTYPE SHOWS THE LEAST AMOUNT OF PHENOTYPIC PLASTICITY?
(A) Genotype 1 (B) Genotype 2 (C) Genotype 3 (D) They are all
equivalent. Explain your answer.
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An example of genotypic reaction norms illustrating the concept
of phenotypic plasticity. Pigliucci M et al. J Exp Biol
2006;209:2362-2367 2006 by The Company of Biologists Ltd
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AT WHICH STAGE IS GENE EXPRESSION REGULATED? (A) (B) (C) (D)
(E)
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REGULATION OF GENE EXPRESSION Regulation of expression can
occur at any stage in the DNA-mRNA-protein pathway
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REGULATION OF GENE EXPRESSION Modification of DNA may up or
down regulate gene expression, e.g. chemical modification (chemical
tags) of DNA, which includes DNA methylation common mode of gene
silencing; abnormal methylation patterns are involved in
oncogenesis. Histone acetylation allows transcription to proceed;
histone deacetylation works with DNA methylation in gene silencing;
also, signals DNA to be packed more densely, thus lowering gene
expression.
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When gene is tightly wound around deactylated histones and
highly methylated, gene expression is low. When gene is loosely
wound around acetylated histones and unmethylated, gene expression
is high. Gene control simulation:
http://learn.genetics.utah.edu/content/epigenetics/ control/
http://learn.genetics.utah.edu/content/epigenetics/ control/
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DNA AND HISTONE MODEL DNA is coiled around histones. Tightly
coiled DNA is inaccessible to gene reading machinery. Methyl
molecules bind to DNA and block access to genes. Acetyl molecules
bind to histones and increase access to genes. View video tutorial:
http://teach.genetics.utah.edu/content/epigenetics/
http://teach.genetics.utah.edu/content/epigenetics/
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DNA METHYLATION Stably alters gene expression as cells divide
and differentiate from embryonic stem cells specific tissues. DNA
methylation typically removed during zygote formation
(reprogramming), then re-established through successive cell
divisions during development. DNA methylation suppresses expression
of harmful DNA sequences that have been incorporated over time
(e.g., from retroviral genes). Involved in
oncogenesis/carcinogenesis. Involved in genomic imprinting.
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GENOMIC IMPRINTING For most genes, we inherit 2 functional
copiesone from mom, one from dad. Imprinted genes are different
depending on gene, either copy from mom OR copy from dad is
epigenetically silenced (occurs during egg & sperm formation).
Environment has stronger effect on imprinted genesonly one active
copy, no back up, so any epigenetic change has greater impact on
gene expression. Improper imprinting developmental defects, cancer,
etc., e.g. Angelman syndrome (1 in 10,000)-abnormally silenced gene
on Chromosome 15 from mother Demonstrated in insects, mammals,
flowering plants (in mammals, ca. 1% of genes are imprinted).
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GENOMIC IMPRINTING LIGERS AND TIGONS Lions and tigers can
produce hybrid offspring in captivity. Offspring different
depending on whether tiger or lion is the mom or dad: Male lion x
female tiger liger (largest of big cats) Male tiger x female lion
tigon (about same size as parents) Another example is horse x
donkey mule or hinny
THE EPIGENOME Includes all the information, other than DNA
sequence itself, that is heritable during cell division. While DNA
sequences remain essentially unchanged throughout the lifetime of
an organism, the epigenome changes immensely in response to
internal or external environmental cues.
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THE EPIGENOME Far-reaching impact
http://systemsbio.ucsd.edu/
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EPIGENETIC INHERITANCE Epigenetic tags present on parents
epigenomes are passed down to offspring. Evidence is growing for
transgenerational epigenetic inheritance, though mechanisms are not
necessarily straightforward. Some examples
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TOADFLAX (LINARIA VULGARIS) Wildtype and peloric variant are
genetically identicalone epigenetic mutation has caused the
difference in petal shape. This epimutation is passed on to
offspring.
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WILD RADISH Caterpillar attack of radish plants can generate
defense phenotypes (distasteful chemicals and protective spines)
that can last for multiple generations, even in absence of
caterpillars. Involves DNA methylation, histone modifications, and
small RNAs.
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WATER FLEA (DAPHNIA) Female Daphnia respond to chemical signals
from predators by growing protective helmets. Offspring of helmeted
Daphnia also born with helmets, even in absence of predator
signals. Effect continues to next generation, although helmet size
decreases. http://www.nature.com/nature/journal/v401/n6748/fig_tab
/401060a0_F2.html
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LAB RATS Fungicide vinclozolin is used on crops, e.g. grapes.
Feeding vinclozolin to pregnant rats causes lifelong epigenetic
changes in pups. As adults, male offpsring have low sperm counts,
poor fertility, and several diseases (prostate and kidney disease).
Persists over multiple generations. Found abnormally hi levels of
methyl tags on sperm DNA.
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DES IN HUMANS Diethylstilbestrol (synthetic estrogen) was drug
given to pregnant women to prevent miscarriages during mid-20 th C
but discontinued because it caused rare vaginal cancer. Also
associated with increased risk of breast and reproductive cancers
in daughters (and sons) and maternal granddaughters. Studies in
mice suggest DES causes abnormal methylation of genes involved in
uterine development; abnormalities present across multiple
generations. http://diethylstilbestrol.co.uk/
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NUTRITION AND THE EPIGENOME The role of diet as an
environmental factor in epigenetic change is area of active
research. Nutrients from food are turned into methyl groups (along
a metabolic pathway) that are ultimately attached to DNA.
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Diets high in methyl-donating nutrients can rapidly alter gene
expressions, especially during early development when epigenome is
first being established.
http://learn.genetics.utah.edu/content/epigenetic s/nutrition/
http://learn.genetics.utah.edu/content/epigenetic s/nutrition/
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15 min. Your Environment, Your Genome
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ACTIVITY: YOUR ENVIRONMENT, YOUR GENOME Factors from your
environment, such as diet, physical activity, and stress level,
influence the epigenome.
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IMPORTANCE OF MOMS DIET Growing evidence for influence of
mothers diet on offsprings epigenome. Agouti gene in mammals:
Unmethylated yellow fur, obese, prone to diabetes & cancer
Methylated brown, thin, low disease risk When pregnant yellow mice
fed methyl-rich diet pups were brown, thin, healthy for life
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Norbotten, Sweden a small farming village with meticulous
agricultural records back to 19 th C. Lars Bygren (1980s) uses the
records to see how much food was available to parents and
grandparents when they were kids. Discovered that famine/low food
availability for paternal grandfathers when they were 9-12 yo
associated with extended lifespan in grandchildren; early death
associated with diabetes or heart disease in grandchildren of
grandfathers who gorged themselves. ENOUGH ABOUT MOM, HOW ABOUT
DAD?
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THE ROYAL DIET Royal jelly is complex, protein-rich substance
secreted from head glands of worker bees. The larva destined to be
queen is fed lots of royal jelly inside a queen cup. All larvae
genetically identical but royal jelly diet silences Dnmt3 gene
which codes for enzyme involved in genome-wide silencing. When
Dnmt3 gene is active, queen genes epigenetically silenced and
larvae become workers. When royal jelly turns off Dnmt3 gene, queen
genes are active and larva turns into queen.
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ROLE OF ENVIRONMENTAL POLLUTANTS Increasing evidence shows the
negative impact of environmental pollutants on the epigenome, e.g.
BPA Pregnant yellow agouti females fed BPA more yellow, unhealthy
babies BPA-exposed moms fed methyl-rich diet offspring
predominantly brown (nutrient supplementation counteracted negative
effects of BPA exposure)
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ENTERTAINING SUMMARY OF EPIGENETICS
http://youtu.be/kp1bZEUgqVI
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REFERENCES/RESOURCES Learn Genetics from University of Utah
http://learn.genetics.utah.edu/content/epigenetics /inheritance/
http://learn.genetics.utah.edu/content/epigenetics /inheritance/
National Human Genome Research Institute
http://www.genome.gov/27532724 http://www.genome.gov/27532724 NOVA
ScienceNOW http://www.pbs.org/wgbh/nova/education/activiti
es/3411_02_nsn.html http://www.pbs.org/wgbh/nova/education/activiti
es/3411_02_nsn.html
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NATURAL SELECTION Mechanism of evolutionary change that
generates biological adaptation. Natural selection drives changes
in the heritable components of phenotype that are mediated by
differences in individual fitness. Phenotypes (thus genotypes) with
the greatest reproductive success (highest fitness) are favored and
become more common in the population.
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THE CONCEPT OF FITNESS An individuals fitness is the
contribution that individual makes to the gene pool of next
generation so depends on that individuals survival and
reproduction. The reproductive success of an individual indicates
its fitness but is often difficult to measure, so proxies are more
typically used, e.g., # matings, # offspring surviving to
maturity.
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VS. WHICH TASTY SNACK WILL HAVE A HIGHER GENETIC FITNESS?