BIOB51 Lec 07 & 8 Notes p. 387-389, 147-161, 163,
169-174
387-389
10.5
Phenotypic Plasticity
-
Another
plastic to say that an individual’s phenotype is influenced by its environment
is to say that its phenotype is plastic
-
Phenotypic
plasticity is itself a trait that can evolve, and it may or may not be adaptive
Phenotypic Plasticity in the Behaviour of Water Fleas
Daphnia magna experiment w/ DeMeester (cute little fleas in BIOA02)
Reproduce asexually most
of the time which makes them beneficial for researchers because they can
manipulate them to be genetically identical individuals in different
environments and compare their phenotypes
-
Plasticity in
phototactic behaviour
o
Phototactic if
they swim towards light and negatively phototactic if they swim away from light
o
10 genetically
identical individuals in a graduated cylinder, illuminated them from above and
gave them time to adjust to the change in environment
o
Most
of them tend to avoid light and even more so when they are in their habitats
Most
of them tend to avoid light and even more so when they are in their habitats
-
Genetic
variation for phenotypic plasticity is called genotype-by-environment interaction
New hypothesis: predation
by fish selects in favour of Daphnia that avoid well-lit areas when fish are
present
-
Tested by
using Daphnia eggs that are still viable even after being buried for decades
-
Took sediments
of 3 different depths = episodes in the history of the pond, hatched the
Daphnia clones
-
They measured
the phototactic behaviour of the reawaken genotypes in the presence and absence
of chemicals released by fish
-
Result: clones
from the period of heaviest fish stocking show the greatest shift in behaviour
across environments. They stay out of the light when they smell predators
147-161
Ch
5: Variation Among Individuals
-
Variation
among individuals is the raw material for evolution
-
The variation
must exhibit a particular property: it must be transmitted genetically from
parent to offspring
5.1
Three Kinds of Variation
-
Genetic
variation
-
Environmental
variation
-
Genotype-by-environment
interaction (briefly mentioned in reading above)
The Machinery
of Life
Proteins provide structure
and have different fxns
-
Hemoglobin
picks up oxygen when the cells pass thru the lungs and drops it off when they
pass thru other tissues
-
Mucigen mixes
w/ water to make mucin which lubricates the intestine
-
Rhodopsin is a
light-absorbing protein in rods
Proteins are chains of AAs
-
They’re all
made of combinations the 20 AAs
-
A protein’s 3D
shape is crucial to its ability to perform its life-sustaining fxns
-
For viruses,
RNA carries the instructions for organisms to know how to build proteins. For
others it could also be DNA that does this.
DNA structure
-
Shaped like
double helix
-
This section
talks about how it’s made of nucleotides A T G C and etc. I’m not making notes
on this because everyone taking this course should know this already lol
The physical location of a
gene on a chromosome is called a locus
-
The number of
chromosomes, sizes, and genes are similar across individuals in a species
-
Humans have 23
pairs of chromosomes containing roughly 22500 protein encoding genes
-
Chimps have
24, dogs 39, wine grapes have 19
Genetic
Variation
PTC – the bitter molecule
that some people can/can’t taste (Lec 07!)
-
Example of
variation across humans
-
TAS2R38 is
encoded on chromosome 7
o
Receptor
protein responsible for bitter flavours
o
2 most common
TAS2R38 alleles are AVI and PAV
o
PAVPAV are
most sensitive, AVIAVI are least, AVIPAV fall in between
o
Switching just
3 of the 333 AAs in TAS2K38 changes the protein’s shape and/or chemical
properties enough to alter either the protein’s ability to bind PTC, ability to
trigger a nerve impulse in response to binding, or both
-
PTC oes not
naturally occur in food
-
AVIAVI eats
more veggies than PAVPAV
-
If parents
were both heterozygous for the alleles then the offspring has a chance of
getting all 3 expressions
Genetic
Variation and Evolution
-
Genes are
passed from parents to offspring, genetic variants associated with higher
survival and reproductive success automatically become more common in
populations over time, while variants associated w/ untimely death and
reproductive failure disappear
-
Genetic
variation is the raw material for evolution
Environmental
Variation
Water flea and its
predator a larval insect (Lec 07)
-
Phantom midges
have been eating water fleas for 145 million years
-
Daphnia pulex suffers a lot of predation by the phantom midge larvae but only at
certain times and places (inducible defense)
o
Capable of
developing a morphology that is well defended against the midges
o
Can double the
strength and thickness of its carapace and grow ridges called neck teeth
-
They exposed
the feals to kairomone and compared them to unexposed individuals
o
The exposed
ones made more mRNA = more protein
Environmental
Variation and Evolution
Human athletes living at
low altitude but train at high altitudes produce more vascular endothelial
growth factor (VEGF)
-
VEGF
stimulates the growth of capillaries in the muscles
The diversity of colours
in E. coli is random variation in the interactions between the promoters and
the regulatory proteins that activate and deactivate them (promoters)
Environmental variation
supplies no raw material for evolution
-
Environmentally
induced changes in phenotype are not transmitted to future generations
Genotype-by-Environmental
Interaction
The Leopard gecko’s sex is
determined by the temperature at which it incubates while developing in the egg
-
Cool or hot
temps = female
-
Intermediate =
male
Rhen and colleages (2011)
wanted to know whether leopard geckos have genetic variation in the threshold
for temperatures for developing as a female vs. male
-
Analyzed data
from 13 generations
-
Compared sex
ratios among offspring that shared a father and hatched from eggs that incubated
at different temperatures
-
Analyzing
offspring with the same father but a variety of mothers allowed the researchers
to factor out variation due to non-genetic influences (maternal effects)
mothers might have had on their offspring via hormones, proteins, or mRNA they
might have placed on the eggs
The variation among
paternal families in the effect of temperature on sex ratio
-
For some
families incubation at 30 degrees vs. 32.5 degrees had little effect on sex
ratio
-
For others, it
had a dramatic effect
-
The pattern of
phenotypes an individual may develop upon exposure to different environments is
called its reaction norm
Genotype-by-Environmental
Interaction and Evolution
Humans: People with
genotype ll for the serotonin transporter gene, maltreatment in childhood has
little effect on the probability. People with ss genotype increases the
probability of depression a lot.
Phenotypic plasticity:
organism that develops diff phenotypes in different environments
-
When
populations have genetic variation for environmental sensitivity, populations
can evolve greater or lesser plasticity
Tobacco hornworms
-
They are
normally green
-
Black ones are
black until they are exposed to high temperatures shortly before molting. After
heat shock, they may emerge with green coloration (almost as green as the
normal ones)
-
This variation
in sensitivity is an example of genotype-by-environment interaction
-
They took the
most sensitive caterpillars and used them as founders of a high-plasticity line
-
They look the
least sensitive and used them as founders of a low-plasticity line
-
Picked
caterpillars at random and used them as founders of an unselected line
-
They
maintained the 3 lines for 13 generations
o
Each
generation they gave the caterpillars a heat shock, then selected breeders
according to the same criteria they used for the founders
o
The artificial
selection program yielded dramatic evolution in both selected lines
o
The
low-plasticity line remained black regardless and the high-plasticity line
became extremely sensitive with many of them being green like normal tobacco
hornworms
o
The unselected
line had relatively the same sensitivity over time
Genotype-by-environment
interaction can serve as raw material for the evolution of different reaction
norms
5.2
Where New Alleles Come From
New alleles arise from
alterations to existing alleles (mutations)
-
The enzymes
that copy DNA in cells are called DNA polymerases
-
Mutations
creating new alleles can arise as a result of alterations to DNA that escape
repair before or during replication, or because of errors that occur during
replication itself and escape repair afterward
Example of mutation due to
DNA alteration
-
Cytosine that
has already been chemically modified by the addition of a methyl group will
sometimes react with water, lose an amine group, and thereby transform to
thymine
-
If the
mismatch is not corrected before replication, one of the resulting DNA
molecules will have a T-A pair in substitution for the ancestral C-G pair
Example of mutation due to
copying error
-
DNA strands
can become misaligned, resulting in insertion or deletion of nucleotides
Premutations are DNA alterations still susceptible to repair
-
Genetically
engineered mice whose DNA polymerases lacked the ability to proofread and
correct newly synthesized DNA (lec 08)
-
Result was
high cancer rates and short life spans
Deficient DNA maintenance
and repair also appears to be an underlying defect in a variety of human
cancers
How Mutations
Alter Protein Fxn
Mutations to new alleles
can influence phenotypes if they alter the expression and/or the function of
proteins
-
Protein
synthesis follows a 2 step processs: transcription and translation
o
Transcription:
DNA to mRNA
o
Translation:
mRNA to protein
Point mutation:
substitution of 1 base for another
-
Transition:
substitution of a purine for a purine or a pyrimidine for a pyrimidine
-
Transversion:
substitution of a purine for a pyrimidine or vice versa
Silent substitution: when
a mutation doesn’t change the codon for an amino acid, not changing the protein
synthesized
-
Ex. A to T
transversion changing the DNA codon CAA to CAT, which makes the complementary
change from GUU to GUA. They still both specify valine
Replacement/nonsynonymous
substitution
-
Opposite of
silent substitution. So if the codon changes but instead becomes a different AA
this changes the protein fxn
Nonsense mutation:
introduces a premature stop codon
Introns: must be spliced
out before translation
-
Mutations in
splice sites can prevent introns from being removed resulting in production of
abnormal proteins
Indels: collective term
for point mutations, insertions, and deletions
163
169-174
5.5
Rates and Fitness Effects of Mutations
-
Rates and
fitness effects of mutations have been hard to study because mutations are rare
and their consequences are often subtle
Mutation rates
Mutation Accumulation
Experiment (Lec 08)
Sequenced genomes of 5
lineages of thale cress derived from an already-sequenced common ancestor
-
Each of the 5
lineages had been grown under optimal conditions for 30 gens and each
propagated each gen from a single randomly chosen seed
-
Lineages had
been allowed to accumulate mutations that were not culled by natural selection
-
When comparing
genomes, they found 99 base substitutions and 17 insertions and deletions
-
Different kinds
of organisms have different rates
Fruit fly: 1 indel for
every 3 base substitution
Humans: 1 indel occurs for
every 17 base substitutions
C. elegans: more indels
than base substitions
-
A given gene
in the C. elegans genome is more likely to be duplicated from one generation to
the next than a given nucleotide within the gene is to experience a
substitution
-
Data indicates
that mutation rates are low in cellular organisms
-
Genomes are
large
-
Every human
inherits about 3 dozen point mutations via each of the gametes
Fitness
Effects of Mutations
-
Most mutations
altering fitness are deleterious -> cause harm and damage
-
Site directed
mutagenesis -> technique used by biologists to see how mutations affect
fitness
o
Introduce
random point mutations into genomes of viruses and then into the cells of the
viruses’ hosts
o
Then compare
the fitness of the mutant viral strains to that of the strain they were
prepared from
-
Example ->
effects of 100 random point mutations on the fitness of bacteriophage f1, a
single stranded DNA virus of E. coli
o
Quarter of the
mutations are lethal
o
Around 40% are
neutral
o
Of the rest,
most are deleterious, while only two mutations were beneficial
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