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Perturbations environnementales
Quels impacts sur la diversité et la spécificité dans un
système hôte-pathogène ?
Timothée Poisot & Michael E HochbergUniversité Montpellier 2
Institut des Sciences de l’[email protected]
Gamme d’activités vs. performance
Biotique et abiotique
Jack of all trades is master of none ?
LA SPÉCIFICITÉ (IN A NUTSHELL)
Les interactions sont complexes
Faire des prédictions est difficile !
Spécificité
Compétition
RichesseSpéciation
Dynamique démographique
Modèle génétique
Coûts
Fitness
SPECIFICITY OF PHAGE-BACTERIA COEVOLUTION
Figure 4. Host range at the individual level. Frequency of phageisolates sampled from one coevolved and one evolved popula-tion, with a given infectivity range (number of coevolved bacterialcolonies that a phage isolate can infect) obtained from streakingassay. Only probabilities of infection above or equal to 66% areconsidered.
assays, phage isolates from a single population of the coevolutiontreatment at transfer 16 were not able to infect all the coevolvedbacterial colonies, confirming the existence of genetic specificitythrough resistance in the bacteria and/or counter-defenses in thephage. Coevolved phage isolates had a significantly broader hostrange than those from the evolved population (Fig. 4; mean ±SE: 4.46 ± 0.58 vs. 0.54 ± 0.16, Mann–Whitney W = 832,P < 0.001), confirming the results obtained at the populationlevel (Fig. 2). In addition, coevolved phage had a more diversehost range ranking (from 0 to 10) than did evolved phage (0 to 2)(Fig. 4; Kolmogorov–Smirnoff D = 0.67, P < 0.0001). This sug-gests that the coevolved population is composed of both general-
Table 1. Effects of replicate, phage isolate, bacterial colony, and their interaction on phage-imposed reduction in bacterial growth (RBGij)for coevolved and evolved phage (GLM on ranked data with replicate, phage isolate, bacterial colony as random factors).
Treatment Source of variation df SS MS F P
Coevolution Replicate 2 215,211 107,606 0.67 0.517Phage isolate 19 2,159,438 113,655 1.45 0.144Bacterial colony 14 23,934,138 1,709,581 11.88 <0.001Phage!Bacteria 266 12,293,285 46,215 1.52 <0.001Replicate!Phage 38 2,380,895 62,655 2.06 <0.001Replicate!Bacteria 28 3,587,198 128,114 4.21 <0.001Error 532 16,179,669 30,413Total 899 60,749,835
Evolution Replicate 2 745,514 372,757 3.16 0.058Phage isolate 19 1,599,062 84,161 1.23 0.284Bacterial colony 14 13,370,925 955,066 8.78 <0.001Phage!Bacteria 266 14,241,285 53,539 1.12 0.146Replicate!Phage 38 2,389,018 62,869 1.31 0.105Replicate!Bacteria 28 2,888,544 103,162 2.15 0.001Error 532 25,515,459 47,961Total 899 60,749,807
ists and specialists.Using the interaction matrix of infectivity, we found that
coevolved phage had a significantly nested pattern of infection(Fig. 1A, C; Monte Carlo test N = 188, P = 0.0017). This patternis consistent with a multilocus GFG model of interaction (Sasaki2000; Morgan et al. 2005). By contrast, evolved (i.e., control)phage showed a random pattern of infection (Fig. 2B, D; MonteCarlo test N = 11, P = 0.39).
For RBG assays, the effect of phage isolates on bacterialgrowth was not significant within both the evolved and the co-evolved populations (Table 1). Rather, a higher amount of thevariation was due to the effect of the bacterial colony and thereplicate. Nevertheless, we found a highly significant interactioneffect between phage isolate and bacterial colony in the coe-volved treatment (Table 1; GLM F266,899 = 1.52, P < 0.001),but not in the evolved one (GLM F266,899 = 1.12, P = 0.146). Themean variance in RBGij was significantly higher in the coevolvedthan in the evolved population (Fig. 5; Mann–Whitney W = 566,P < 0.001). This means that within-population diversity and as-sociated specificity in RBG emerged only when antagonistic co-evolution was permitted.
COST OF GENERALISM
Phage isolates issued from the same single population in the co-evolution treatment at transfer 16 show a cost of adaptation withincreasing host range (Fig. 6; linear regression R2 = 0.236, F1,17 =4.94, P = 0.041). The higher the host range, the lower the re-duction of the bacteria growth rate by the phage isolate. Thiscost at the genotypic level is consistent with a GFG model ofinteraction.
EVOLUTION JANUARY 2008 7
Poullain et al. 2008, Coberly et al. 2009
Coexistence!
UN TRAIT EN (CO)ÉVOLUTION
Wilson & Yoshimura, 1994
ET UN ENVIRONNEMENT QUI CHANGE
PerturbationMortalité Ressources
Intensité
Fréquence
Type
Dynamique
UNE PERTURBATION, C’EST…
A CHAQUE PERTURBATION
Populations Ressources
Perturbationmortalité de masse
retour des ressources
Différents tauxde mortalité
0 50 100 150 200 250 300
01
23
4
Temps
Condition
0.0 0.2 0.4 0.6 0.8 1.00.0
0.2
0.4
0.6
0.8
1.0
Hétérogénéité temporelle Hétérogénéité spatiale
DEUX TYPES D’HÉTÉROGÉNÉITÉ
SpécificitéCommunautés simples Complexité naturelle
Type de stratégies CoexistenceInvestissement
NOTRE QUESTION
Trois aspects considérés dans ce travail
Notre modèle couplela dynamique des
populationset
l’architecture génétique
L’hôte et le pathogène mutent et s’éteignent
Epistasie antagoniste (compromis)
Hôtes
Pathogènes
Ressources
Progénie
Entrée deressources
Sortie deressources
Co
mp
éti
tio
ne
tM
ort
ali
té d
e m
as
se
Spec. Adapt. Spec. Adapt.
Poisot & Hochberg (in prep.)
(en éliminant l’adaptation)MESURER LA SPÉCIFICITÉ
Un nouvel indicateur de spécificité basé sur la différence de performance entre hôtes successifs
Plus de survie = plus d’infectivité et de
résistance
Des perturbations trop fréquentes empêchent
l’évolution de l’infectivité0.05 0.10 0.20 0.50 1.000
.00.2
0.4
0.6
0.8
1.0
Fréquence de perturbation
Investissem
ent
Mortalité
99 %
90 %
50 %
None
La spécificité maximale est atteinte pour des
perturbations moins fréquentes quand la mortalité augmente
0.05 0.10 0.20 0.50 1.000.0
0.2
0.4
0.6
0.8
1.0
Fréquence de perturbation
Spécific
ité
La diversité est maximale pour des perturbations
fréquentes quand la mortalité est faible
Une plus grande mortalité décale ce patron vers les
fréquences faibles0.05 0.10 0.20 0.50 1.000
.00.1
0.2
0.3
0.4
Fréquence de perturbation
Coexis
tence
Mortalité
99 %90 %
50 %None
Res
sour
ce 1
Res
sour
ce 2
Phag
e
Res
sour
ce 1
Res
sour
ce 2
Phag
e
Res
sour
ce 1
Res
sour
ce 2
ET MAINTENANT?
Niche fondamentale vs. niche réalisée : le rôle du pathogène
La spécificité dépend de la fréquence et de l’intensité des perturbations
L’investissement dans l’infectivité est déterminé par l’environnement
La coévolution rend possible la
coexistence avec des variations
temporelles
TAKE HOME MESSAGES
Merci de votre attention
Michael E Hochberg
Financé par le contratANR «COMUTE»
Timothée Poisot
Financé par une boursedoctorale CNRS/Région