Compte rendu de la réunion scientifique du 13-14 mars 2008 de Paris - ANR- Emerfundis

Cette réunion de travail avait pour but d'approfondir la réflexion sur les travaux des parties 'processus de dispersion' et 'changements évolutifs' (voir programme ci-dessous). Elle a été programmée lors de la réunion du REID de décembre au cours de laquelle nous avions fait un premier tour d'horizon des travaux qui seront réalisés dans toutes les parties du projet

Mate limitation, recurrent epidemic outbreaks, and the coexistence of sexual and asexual plant parasites

To date, relatively few studies have focused on the effects of sex on population dynamics. Previous models found that sexual reproduction may either dampen population fluctuations or accentuate population fluctuations, depending on mating system notably. Here we were interested in the effect of mate limitation in pathogens in which both sexual and asexual reproductions occur simultaneously within direct life cycles, a life history typical of many fungal plant pathogens for instance. We modelled the epidemiological dynamics of such species by considering a SIR model with two modes of transmission, one linear and the other bilinear in infected density. We found that the threshold for epidemic development is the same as expected under strictly asexual dynamics. We showed that sexual spore production promotes more complex dynamics (cycles) than strictly asexual reproduction. We further showed that sexual reproduction has important consequences for the coexistence of parasites infecting the same host: in parameter ranges where the parasite population dynamics fluctuate, the stable coexistence of parasites on the same host is possible, whereas it is not in the asexual case. We discuss the implications of these results for understanding complex epidemiological dynamics and point to some consequences for epidemics management. (Résumé d'auteur

Gender-role alternation in the simultaneously hermaphroditic freshwater snail Physa acuta: not with the same partner

In simultaneous hermaphrodites, gender conflicts that arise from two potential mates sharing the same gender preference may be solved through conditional reciprocity (or gamete trading). Conditional reciprocity had initially been considered widespread, but recent studies suggest that its real occurrence may have been overestimated, possibly because most mating observations have been performed on isolated pairs of individuals. Some resulting patterns (e.g., non-random alternation of sexual roles) were indeed compatible with conditional reciprocity but could also have stemmed from the two partners independently executing their own mating strategy and being experimentally enforced to do so with the same partner. Non-random alternation of gender roles was recently documented in the simultaneously hermaphroditic freshwater snail Physa acuta. To distinguish between conditional and unconditional gender alternations, we observed copulations of individually marked snails reared at three contrasted densities. We showed that density affected the overall frequency of copulations during the first 2days of the experiment with high-density boxes showing more copulations than low density boxes, but it did not affect gender alternation patterns. A change in gender role was observed more often than expected by chance over two successive copulations by the same individual, confirming previous studies. However, gender switches did not preferentially occur with the same partner. We conclude that gender alternation is not due to conditional reciprocity in P. acuta. It may rather stem from each individual having a preference for gender alternation. We finally discuss the mechanisms and the potential extent of this unconditional reciprocit

Experimental evidence of inbreeding avoidance in the hermaphroditic snail Physa acuta

Inbreeding depression should select for inbreeding avoidance behaviours. Here we test this hypothesis in two populations of the simultaneous hermaphroditic freshwater snail Physa acuta. We recorded the copulatory behaviour of 288 pairs of sib-mates, non-kin mates from the same population, or non-kin mates from two different populations. We find that kin discriminatory behaviours exist in this species, exclusively expressed by individuals playing the female role. We discuss the relevance of our finding in the context of the evolution of recognition systems and the consequences of such a behaviour in natural population

Evaluation of host partial resistance efficacy to a foliar disease using a simulation modeling approach : case of Mycosphaerella leaf spots diseases of banana

Black leaf streak disease (BLSD) due to the ascomycete Mycosphaerella fijiensis is considered as the most destructive foliar disease of bananas. It has just invaded the French Indies banana production area. The current control strategy requires frequent aerial fungicide applications on intensive production plots because of the cultivation of high-yield but BLSD susceptible varieties. The use of resistant varieties appears as the most durable and appropriate control. As none commercial resistant varieties are available for producers, CIRAD has set up a banana breeding program to create BLSD resistant varieties. As the evaluation of BLSD resistance efficacy of new hybrids created by CIRAD is both time-and space-consuming, a disease simulation model has been developed. It will allow to select resistant hybrids and to identify efficient resistance components. The model SiBatoKa is a mechanistic and discrete simulation model of Mycosphaerella leaf spot diseases. It describes, at a banana plant scale, the establishment and development of BLSD epidemics under optimal climatic conditions during several banana cycles (two years). It is built into two sub-models describing (i) the banana growth and (ii) the epidemics development. The banana-sub model simulating leaves growth is determinist. The disease-sub model simulates the complete infectious cycle including spore dispersal (at plant scale), the creation of lesions, their growth and the asexual and sexual sporulation stages. Available data on lesions (counts and surface) and sporulation collected under natural and artificial were used to calibrate the model. Parameters of the disease-sub-model are estimated with a Bayesian approach using the MCMC (Markov Chain of Monte Carlo) technics. Then a posteriori distributions of all parameters and residual variance are obtained. The inference of parameters and the results from the first simulations will be presented and discussed