This document studies how different frequencies and intensities of ecosystem perturbations affect the evolution of specificity in a host-pathogen system. It finds that: 1) Pathogens with higher survival invest more in infectious loci, allowing higher infectivity, but frequent perturbations impede this evolution. 2) Maximal specificity occurs at lower perturbation frequencies as mortality increases. 3) Strategy diversity is greatest at higher perturbation frequencies for low mortalities, shifting to lower frequencies as mortality increases above 50%.

1. Impact of envIronmental perturbatIons on the evolutIon of specIfIcIty
In a coevolvIng host-pathogen system
Timothée Poisot & Michael E HocHberg
(1) Université Montpellier 2
(2) CNRS, Institut des Sciences de l’Evolution, CC 065, Place Eugène Bataillon, 34095 Montpellier CEDEX 05, France
Corresponding author : timothee.poisot@univ-montp2.fr
We investigated how different frequencies and intensities of an ecosystem-wide Resource in ow Resource output
perturbation (mass mortality and resource renewal) affect the evolution of specifi-
city in a model host-pathogen system. Resources
Modified GfG matrix
PATHOGEN We study a model (adapted from [1]) of the ecological dynamics Metabolic rate
Av V of resources (R), hosts (H) and pathogens (P), and assume a
S a 1
HOST
gene for gene (GfG [2]) relationship in infectivity / resistance. Host
R 0 b Both hosts and pathogens diversify through mutation, and each
where both a and b genotype can go extinct. Infectivity Lack of resource
are lowered infectivi-
ty rates, with 0<a,b<1
Parasite
We assume (1) that infection requires the parasite have infectious alleles
at all interactive loci [3,4], and (2) epistasis, such that higher resistance Burst size Competition Washout
decreases metabolic capacity, more infectivity decreases burst size.
P
athogens with higher survival Measuring specificity
1.0
Mortality
99 % 50 % We developed an indicator of specificity, S, based on the ran-
90 % None have higher infectivity (invest-
king of performance on succesives hosts.
0.9
ment in infectious loci); frequent
Specificity is corrected by abundance
0.8
perturbation events impede infecti- NP Pi
Investment
i=1 Si P
S=
0.7
vity evolution. NP
And represented as
0.6
S − Smin
SR =
0.5
SM ax − Smin
0.4
1 2 5 10 20
This indicator is both sensitive and robust to incompleteness
of the sample : suitable for both theoretical and empirical stu-
Time between perturbations
dies (Poisot & Hochberg, in prep. b)
M
1.0
aximal specificity occurs at
0.8
lower perturbation frequen- Futures directions for this work
cies as mortality increases. Evolu- How does patho-
0.6
gen resistance
Specificity
tion towards specificity depends on interact with the
0.4
exploitation of se-
the costs associated with being a veral resources?
generalist, as well as with environ-
0.2
This question will
mental conditions. be explored by
0.0
using a simple
1 2 5 10 20
Time between perturbations
model with three typical situations, in order to disentangle the va-
F
0.4
Mortality
inally, strategy coexistence rious effects.
99 % 50 %
90 % None
(diversity) is maximal at higher (1) Pathogen targets an independant locus
0.3
(2) Pathogen targets a locus involved in uptake of one of two re-
perturbation frequencies for low sources
Range of strategies
mortalities, and the maximum shifts (3) Pathogen targets both loci involved in resource uptake
0.2
to lower perturbation frequencies The impact of different resource dynamics is currently being in-
vestigated.
0.1
as mortality increases above 50 %.
[1] Weitz et al. (2005) Proc Natl Acad Sci USA 102(27)
0.0
[2] Forde et al. (2008) Nature 455(7210)
1 2 5 10 20
Time between perturbations
[3] Frank (1993) Evolution 47(6)
Poisot & Hochberg (in prep. a)
[4] Frank (1996) Quart Rev Biol 71(1)
Poster presented at the 12th conference of the European So- We thank N. Mouquet, O. Kaltz, S. Morand and S. Gandon for
ciety for Evolutionary Biology, Torino, Aug. 2009, as part of the comments, T. Petzoldt for help with the simecol package. Work
symposium «Integrating ecology with parasite evolution» funded by ANR COMUTE to MEH and CNRS PhD grant to TP