Jean-Baptiste Ferdy

Extinction and Introgression in a Community of Partially Cross‐Fertile Plant Species

We develop a model to study the demography and genetics of an encounter between two partially cross‐fertile plant species. We assume prezygotic reproductive isolation between the species, a common situation when the species differ by their phenology or floral traits that cause assortative mating. Three outcomes are possible: coexistence of both species with minimal introgression; domination by one species, with the other becoming extinct or surviving only through recurrent migration; or domination of the community by hybrid derivatives, with both species surviving but with a rather high level of introgression between them. The first situation is reached when interfertility is low, while the third requires high interfertility to develop. Occurrence of the second situation is observed with intermediate values of interfertility. Gene flow from nearby monospecific populations can prevent both introgression and the domination of the community by one species. Conversely, increasing the number of loci that determine the reproductive isolation between species or decreasing the degree of nonadditive interactions (epistasis and/or dominance) between alleles and loci makes introgression more likely. We found that hybridization can create positive frequency dependence and make extinction possible, even when hybrid individuals have no intrinsic fitness advantage.

Pollinator-induced density dependence in deceptive species

Many animal-pollinated species experience low visitation rates and, in some cases, stand on the brink of extinction because they are poorly fertilized. Among these plants, some are deceptive species (flowering plants that do not offer any reward to their pollinators). A learning process that pollinators undergo determines visitation rate in those food frauds that do not mimic rewarding models. Pollinators that visit cheating species avoid them after having experienced the absence of reward a few times and then visit rewarding plants. We modeled this learning process, using classical optimal foraging and game theory tools, and applied our model to survey how visitation rate can be adjusted in deceptive species in a density-dependent way and how it can influence the population dynamics of those species. We found pollinator behavior to induce positive density dependence at low density (Allee effect) and therefore to create a threshold density under which population survival is not possible. Moreover, negative density dependence occurs at high density so that in most cases pollination limitation creates a stable demographic equilibrium. Stochastic simulations were performed to investigate the stability of populations at these equilibria and estimate their mean time to extinction. Because some parameters such as pollinator density or habitat fragmentation were explicitly taken into account, we tried to describe environmental conditions conducive to a deceptive plants survival.

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.