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Is behavior caused by genes or environment?
Is behavior caused by genes or environment?
Can the environment influence the function of
genes?
Gene sequence is inherited (you’re stuck with what you have)
atgtcagccgataactgactgatcgtaaattgagtttt
gene
However:
Gene expression is influenced by inheritance AND age,
tissue type, physical environment and social environment
protein
“gene expression” = mRNA abundance
messenger RNA
(mRNA)
atgtcagccgataactgactgatcgtaaattgagtttt
gene
Variation in gene sequence (DNA polymorphisms) can be
found by “gene mapping” and DNA sequencing
atgtcagccgataactgactgatcgtaaattgagtttt
atgtcagccgataactgactgatggtaaattgagtttt
DNA polymorphism
Variation in gene expression can be found using
“microarrays”
“gene expression” = mRNA abundance
protein
messenger RNA
(mRNA)
atgtcagccgataactgactgatcgtaaattgagtttt
gene
Microarrays
measure mRNA
abundance for
1000s of genes
simultaneously
Behavioral division of labor in the honey bee colony
Behavioral division of labor in the honey bee colony
Behavioral division of labor in the honey bee colony
% introduced bees that
initiated foraging by day 14
Age at onset of foraging is
socially regulated
50
40
30
20
10
10
20
30
40
% old foragers in colony
Behavioral division of labor in the honey bee colony
Age at onset of foraging is
socially regulated
Inhibitory social pheromone
Nurse
Forager
Behavioral division of labor in the honey bee colony
% of introduced bees
foraging at 15 days of age
Age at onset of foraging is
also genetically variable
60
40
20
0
Introduced bees
Host colony
m c l
m
m c
c
l
m c l
l
m, c and l are different
genetic strains
Honey bee brain microarray
Each pair of spots is a “detector” for
mRNA abundance for 1 bee gene.
We can analyze mRNA abundance for
~5500 different genes at the same time
(in a single dissected bee brain)
Comparing gene expression in a nurse brain vs. a forager
brain
Dissected
forager brain
labeling reaction
microarray hybridization
Forager mRNA level
Dissected
nurse brain
Nurse mRNA level
Do age-related changes in behavior involve changes in
gene expression in the brain?
“Typical” host colonies
(ca. 40,000 mixed-age bees)
Collect marked bees
day 7
~700 one-dayold bees
(marked)
Young
nurses
(n = 18)
day 30
Old
foragers
(n = 18)
YN
OF
Yellow = high mRNA
abundance
Blue = low mRNA
abundance
Genes
YN = young nurse
OF = old forager
About 1/3 of genes expressed in
the bee brain show differences in
expression!
Bee brains
But…
Are gene expression differences
really associated with behavior
differences or with age?
YN
OF
Yellow = high mRNA
abundance
Blue = low mRNA
abundance
Genes
YN = young nurse
OF = old forager
About 1/3 of genes expressed in
the bee brain show differences in
expression!
Bee brains
Changing age-demography in the colony (i.e., social
environment) allows us to collect “precocious” young
forgers and “over-aged” old nurses
Collect marked bees
~1500 one-day-old bees
(marked)
All bees are same age!!!
day 7
day 30
Young
nurses
(n = 6)
Old
nurses
(n = 6)
Young
foragers
(n = 6)
Old
foragers
(n = 6)
YN
OF
YN YF ON OF
Yellow = high mRNA
abundance
Blue = low mRNA
abundance
Genes
YN = young nurse
YF = young forager
ON = old nurse
OF = old forager
Bee brains
Gene expression was
primarily associated
with behavior, not
age!
YN
OF
YN YF ON OF
Yellow = high mRNA
abundance
Blue = low mRNA
abundance
Genes
YN = young nurse
YF = young forager
ON = old nurse
OF = old forager
Bee brains
Although 1/3 genes show differences, only ~50 genes are
needed to “predict” whether a bee is a nurse or a forager
Single-cohort
Typical colony (age-matched)
bees
bees
Y = young (7 ±2 days)
O = old (30 ±2 days)
N = Nurse
F = Forager
>2
1.5
1.25
1
0.8
0.67
<0.5
mRNA level
(Whitfield, Cziko & Robinson. Science 2003, 302: 296)
Is gene expression in the brain a cause or consequence
of behavior?
Brain gene
expression
• Upstream
regulation of
behavior
?
Behavior
-- or -?
Behavior
Brain gene
expression
• Consequence of
behavioral activity,
experience or
environmental
differences
Is gene expression in the brain a cause or consequence
of behavior?
Direct test of causation:
Manipulate
gene
expression in
brain
Effects on
behavior?
Is gene expression in the brain a cause or consequence
of behavior?
Direct test of causation:
Manipulate
gene
expression in
brain
Effects on
behavior?
Is gene expression in the brain a cause or consequence
of behavior?
Brain gene
expression
• Upstream
regulation of
behavior
?
Behavior
-- or -?
Behavior
Brain gene
expression
• Consequence of
behavioral activity,
experience or
environmental
differences
Brain gene expression in the absence of task-related
experience
Caged bees
(no task-related
experiences)
Brain gene expression in the absence of task-related
experience
Social cue or
treatment that
accelerates the onset
of foraging
Forager-like changes in
brain gene expression?
Caged bees
(no task-related
experiences)
Brain gene expression in the absence of task-related
experience
Social cue or
treatment that
accelerates the onset
of foraging
Forager-like changes in
brain gene expression?
Social cue or
treatment that delays
the onset of foraging
Nurse-like changes in
brain gene expression?
Caged bees
(no task-related
experiences)
Juvenile hormone analog (methoprene) accelerates the
onset of foraging and causes forager-like changes in
brain gene expression (in the absence of experience)
“marker” genes
Treat with
methoprene
(JH analog)
Caged bees
(no task-related
experiences)
High in
foragers
(32)
High in
nurses
(18)
Induced by
methoprene
(249)
13
0
Repressed by
methoprene
(222)
0
11
Whitfield CW et al. PNAS. 2006. Oct 31; 103(44):16068-75.
Queen mandibular pheromone (QMP) delays the onset of
foraging and causes nurse-like changes in brain gene
expression (in the absence of experience)
“marker” genes
Treat with QMP
Caged bees
(no task-related
experiences)
High in
foragers
(32)
High in
nurses
(18)
Induced by
QMP (602)
0
4
Repressed by
QMP (704)
18
2
Grozinger CM, Sharabash NM, Whitfield CW, Robinson GE. PNAS. 2003. Nov 25; 100:14519
Is behavior caused by genes or environment?
Can the environment influence the function of
genes?
Gene clusters
(expression level in brain)
Many behaviors… but how many involve gene expression
differences?
>2
1.5
1.25
1
0.8
0.67
< 0.5
Are gene expression changes robust at the level of
individual?
Can gene expression
profile in an individual
bee’s brain be used to
correctly predict
behavioral group?
“Leave-one-out” cross-validated class prediction
“Training set”
59 bees
(computer knows behavior)
“Test set”
1 bee
(computer
attempts to
classify)
nurse or
forager?
Method:
1. Computer identifies a set of
“predictor genes” for
behavior using expression
data from the 59 bees in the
“training set.”
2. Computer attempts to
classify the single “test” bee
based on its expression of
the predictor genes.
3. Repeat iteratively for all 60
individual bees.
Result:
Computer can correctly assign
behavior group for 57 out of
the 60 bees.