Experimental Manipulation of Mode
pilated
host
phage with
extra gene
infected
ampR host
Jim Bull
Ian Molineux
Bill Rice
Vertical Treatment
• Bull et al. (1991) used E. coli
and filamentous phage f1
Horizontal Treatment
amp
• A gene conferring ampicillin
resistance was inserted into the
phage genome
• In a Horizontal transmission
treatment, phage was forced to
pass from infected to uninfected
hosts.
growth
with amp
dilution
•In a Vertical transmission
treatment, phage was forced to
pass from mother to daughter cell.
growth
with amp
growth
with amp
introduce
fresh hosts
heat
kill
hosts
• Bull et al. found that phage
evolved under vertical transmission
were more benevolent to their host–
the growth period of infected host
was significantly shorter.
• The phage in the vertical
transmission treatment reduced its
genome drastically.
infected host growth period (min)
Evolution à la Mode
90
80
70
60
50
40
30
20
10
0
Vertical
• Further study revealed that
changes in both the host and the
phage were responsible for the
benevolent coexistence under
vertical transmission.
t0
t1
P
P
H
H
Horizontal
P
P
H
H
H
P
H
Take 3 minutes to talk to your neighbor about the following:
How might you experimentally test how a “vertically evolved” phage competes against a
“horizontally evolved” phage? What competition conditions would be interesting to vary?
(Hint: think of conditions that would favor benevolence or virulence– e.g., host density, host
susceptibility, etc.)
The Evolution of Virulence
Lecture Outline
• Introduction to virulence theory
• Transmission mode experiment
• Transmission timing experiment
• Metapopulation experiment
• Summary
The Case of the Gypsy Moth
• Etienne Leopold Trouvelot, an amateur
entomologist, brought eggs of the
Gypsy moth back from Europe in the
1860’s.
• Trouvelot soon noticed that some of
the larvae from his backyard had
escaped into nearby woods.
Gypsy moth
Trouvelot’s house in Medford
Trouvelot
• Within two decades, the first local
outbreak of gypsy moth occurred.
• Preventative measures (such as
removal of egg masses and burning
infested vegetation) were attempted and
eventually abandoned.
• Currently, the gypsy moth has spread
over a good portion of the northeast
region of the US.
burning vegetation
removing eggs
Gypsy moth range (circa 2007)
A Weapon against the Gypsy Moth
• A viral agent, nuclear polyhedrosis
virus, has been used to control high
density outbreaks of the gypsy moth–
earning the virus the nickname
“Gypcheck”
• The virus enters the catepillar
healthy catepillar
through the gut (when it consumes
infected vegetation) and a few weeks
later ruptures the catepillar’s cells,
releasing the virus into the
environment.
• Here we have a system that is
extremely important from a
management perspective; however,
we also have the opportunity to
explore evolutionary phenomena (e.g.,
evolution of virulence) within this
insect-virus community.
aspen killed by gyspy moth
infected catepillar
host
pathogen
Experimental Manipulation of Timing
Late Treatment
inclusion
body
Paul Ewald
• Cooper et al. (2003)
performed an experiment in
which the timing of
transmission of NPV between
catepillars was manipulated.
• In an Early treatment, the
virus was transferred from
infected living larvae to
uninfected larvae after 5 days.
• In a Late treatment, the virus
was transferred from infected
living larvae to uninfected
larvae after 9 days.
uninfected
larvae
infection
infection
incubation (5 days)
separation
initiate next cycle
Vaughn Cooper
incubation (9 days)
separation
initiate next cycle
Early Treatment
The Importance of Timing (Scenario 1)
• Cooper et al. found that
virus transmitted early
evolved relatively higher
virulence (higher mortality of
the larval host) than virus
transmitted late.
Late Treatment
Early Treatment
• Let us assume that more
virulent virus mutants can
out-compete less virulent
viral strains within the host:
-Scenario 1: Virulent mutants
that kill their host between days 5
and 9 are selected against in the
Late treatment
more virulent virus is
available for next transfer…
only less virulent virus is
available for next transfer…
The Importance of Timing (Scenario 2)
• Cooper et al. found that
virus transmitted early
evolved relatively higher
virulence (higher mortality of
the larval host) than virus
transmitted late.
Early Treatment
Late Treatment
• Let us assume that more
virulent virus mutants can
out-compete less virulent
viral strains within the host:
-Scenario 1: Virulent mutants
that kill their host between days 5
and 9 are selected against in the
Late treatment
-Scenario 2: More
benevolent mutants (e.g., that
exhibit restraint in use of host
resources) experience a founder
event (due to the viral bottleneck)
and then inhabit a host that is
more likely to live 9 days in the
Late treatment.
more benevolent virus
could be picked…
more benevolent virus is
more likely to be transferred…
The Importance of Timing
-Scenario 1: Virulent mutants that kill their
host between days 5 and 9 are selected against
in the Late treatment
-Scenario 2: More benevolent mutants (e.g.,
that exhibit restraint in use of host resources)
experience a founder event (due to the viral
bottleneck) and then inhabit a host that is more
likely to live 9 days in the Late treatment.
• After several cycles of selection, virus
from the Early treatment was significantly
more virulent than virus from the Late
treatment, but not significantly different
from the ancestor.
• This suggests that Scenario 2 played a
part (note that the scenarios are not
mutually exclusive).
Early
Late
Ancestor
Resource Management by a Pathogen
• Cooper et al. found that while the
(more virulent) Early virus was more
productive after 5 days, the (less
virulent) Late virus was more
productive after 9 days.
• This suggests a tradeoff between
short-term and long-term productivity.
more virulent
ancestral
more benevolent
Early
Late
• More virulent virus might compete
better and produce more in the shortterm, but (due to quick consumption
of its host) its long-term prospects are
not so pleasant.
• Less virulent virus employs a
prudent strategy with regards to its
host’s resources– this may favor
better long-term exploitation (in terms
of host survival and viral yield).
• If the more virulent type is a better
competitor, then the virus is playing a
“tragedy of the commons”
dead
alive