Biomedical subjects

Jean-Baptiste Ferdy

Publications and source records attributed to Jean-Baptiste Ferdy.

6 recordsLinked to original sources

PubMed2006

Effect of bacterial symbionts Xenorhabdus on mortality of infective juveniles of two Steinernema species.

Steinernema species are entomopathogenic nematodes associated with Xenorhabdus bacteria. The life cycle of these associations is composed of two stages: (1) a free stage in the soil, where infective juveniles (IJs), which carry bacteria in their guts, search for new insect hosts; and (2) a parasitic stage, where the IJs infect insects, release their Xenorhabdus symbionts and reproduce in order to produce new IJs. Previous studies clearly showed benefits to the association for several Steinernema species during the parasitic stage. Nevertheless, no study has so far explored, during the free stage, the existence of costs or benefits to the association for different Steinernema. Here, we compared the survival of both symbiotic and aposymbiotic IJs in two nematode species: (1) Steinernema carpocapsae-exhibiting IJs that carry a high number of Xenorhabdus cells in their guts; and (2) its closely relative species, S. scapterisci-exhibiting IJs, that carry very few Xenorhabdus cells in their guts. We showed that the bacterial symbionts were costly for S. carpocapsae by increasing IJs' mortality but not for S. scapterisci. This difference in cost induced by bacteria to IJs during the free stage could be correlated with the difference in the numbers of bacteria carried by IJs of each nematode species.

Animals↗

PubMed2005

Diversification of transmission modes and the evolution of mutualism.

In order for mutualism to evolve, some force must align the interests of the two interacting partners. Vertical transmission can fill this role, but it is still unknown whether mutualism can be stable when vertically transmitted symbionts can evolve toward horizontal transmission. In this article, we investigate how symbionts' transmission mode and virulence should evolve, depending on the relationship between these two traits. We show that pathogens that reduce their host's fecundity can have more complex evolutionary dynamics than those that increase mortality. In some cases, runaway evolution of virulence can drive the host population extinct. In most cases, evolutionary branching results in the differentiation of avirulent, vertically transmitted symbionts from virulent, contagious pathogens. The population of symbionts then becomes polymorphic, and because the least virulent symbionts are the most frequent, the average virulence of symbionts is much lower than it would be in a monomorphic population. When the link between transmission and virulence results from correlated mutational changes and not from fixed constraints, vertically transmitted symbionts do not simply lose virulence; they evolve toward mutualism. We show that the force that stabilizes mutualism in such situations is the competition for transmission between symbionts.

Evolution, Molecular↗

PubMed2005

Extinction dynamics in experimental metapopulations.

Metapopulation theory provides a framework for understanding population persistence in fragmented landscapes and as such has been widely used in conservation biology to inform management of fragmented populations. However, classical metapopulation theory [Levins, R. (1970) Lect. Notes Math. 2, 75-107] ignores metapopulation structure and local population dynamics, both of which may affect extinction dynamics. Here, we investigate metapopulation dynamics in populations that are subject to different migration rates by using experimental metapopulations of the annual plant Cardamine pensylvanica. As predicted by classical metapopulation theory, connected populations persisted longer than did isolated populations, but the relationship between migration and persistence time was nonlinear. Extinction risk sharply increased as the distance between local populations increased above a threshold value that was consistent with stochastic simulations and calculation of metapopulation capacity [Hanski, I. & Ovaskainen, O. (2000) Nature 404, 755-758]. In addition, the most connected metapopulations did not have the highest persistence levels. Stochastic simulations indicated an increase in extinction risk with the highest migration rates. Moreover, calculation of population coherence [Earn, D. J. D., Levin, S. A. & Rohani, P. (2000) Science 290, 1360-1364], a metric that predicts synchronous cycles, indicated that continuous populations should cycle in phase, resulting in an increased extinction risk. Determining empirically the optimal migration level to improve survival chances will be challenging for any natural population. Migration rates that would not increase migration above the threshold value would be ineffectual, but migration rates that would homogenize local densities could increase the risk of coherent oscillations and enhance extinction risk.

Ecology↗

PubMed2004

Extinction times and moment closure in the stochastic logistic process.

We investigate the statistics of extinction times for an isolated population, with an initially modest number M of individuals, whose dynamics are controlled by a stochastic logistic process (SLP). The coefficient of variation in the extinction time V is found to have a maximum value when the death and birth rates are close in value. For large habitat size K we find that Vmax is of order K1/4 / M1/2, which is much larger than unity so long as M is small compared to K1/2. We also present a study of the SLP using the moment closure approximation (MCA), and discuss the successes and failures of this method. Regarding the former, the MCA yields a steady-state distribution for the population when the death rate is low. Although not correct for the SLP model, the first three moments of this distribution coincide with those calculated exactly for an adjusted SLP in which extinction is forbidden. These exact calculations also pinpoint the breakdown of the MCA as the death rate is increased.

Animals↗

PubMed2003

Within-host parasite dynamics, emerging trade-off, and evolution of virulence with immune system.

Virulence is an evolutionary paradox because parasites never benefit from their host's death. The adaptive explanation of virulence is classically based upon the existence of physiological constraints that create a trade-off between parasites' epidemiological traits (virulence, transmissibility, and clearance). Here we develop an epidemiological model where infections are dynamic processes and we demonstrate how these dynamics generate a trade-off between emerging epidemiological parameters. We then study how host's immune strength modifies this trade-off and hence influences virulence evolution. We found that in acute infections, where parasites are engaged in a race with immune cells, immunity restrains more the duration of the infection than its intensity. As a consequence parasites evolve to provoke more virulent but shorter infections in strongly immunized hosts.

Biological Evolution↗

PubMed2002

Evolution of mutualism between globeflowers and their pollinating flies.

Plant/seed-eater pollinators mutualisms involve a plant pollinated by an insect whose larvae develop by eating a fraction of host-plant seeds. The outcome of the interaction therefore depends on the number of ovules fertilized by adult visits and the number of seeds destroyed by larvae. Among the very few cases of such mutualisms reported so far is the globeflower-globeflower flies mutualism, which is unique in that it involves several congeneric fly species (Chiastocheta genus) coexisting within a single host-plant species, Trollius europaeus. These species exhibit contrasted oviposition behaviors resulting in a more or less beneficial outcome for the plant. We designed an adaptive dynamics model to investigate how morphological traits of globeflower could affect the evolution of oviposition in its pollinating flies. Three fly traits (flower age at oviposition, clutch size and the level of avoidance of already parasitized flowers) and one plant trait (closed or open corolla) were examined. Whatever the shape of the flower, evolutionary branching occurs between early and late ovipositing flies, driven by strong competition among larvae within a fruit. Once this branching occurred, the closed shape of the corolla is likely to offer a better protection to eggs of early but not of late ovipositing flies. The difference in egg survival results in higher competition among early larvae and thus selects for decreased clutch size in early flies. This can be seen as a first step in the evolution of a mutualistic behavior. The prediction of our model fits field observations of fly behavior, giving theoretical support to the hypothesis of fly sympatric speciation within its host plant. Moreover, flower closed globe shape can be positively selected in globeflowers as it results in a reduction of parasitism strength. This last evolution therefore leads to a stable mutualism between globeflowers and globeflower flies.

Animals↗