Transmission modes and the evolution of feminizing symbionts.

Vertically transmitted symbionts can distort their host's reproduction to increase their own transmission. In Wolbachia and some other symbionts, a particular distortion of this sort is feminization, whereby genetic males, which cannot transmit symbionts, are converted during development into functional females, which do transmit symbionts when they reproduce. In this work, we propose a model to study how feminization intensity (i.e. penetrance) can evolve under different ecological constraints in WZ/ZZ hosts. More specifically, our model incorporates both imperfect vertical and horizontal transmission modes. The model shows that for most parameter values feminizing symbionts drive genetic females to extinction, which in turn favours the evolution of maximum feminization penetrance. Once genetic females are extinct, the actual value of feminization penetrance never depends on the efficiency of vertical transmission. Instead, the model shows that in conditions where the reproductive rate is high at demographic equilibrium, higher feminization levels are favoured. One consequence of this can be, for example, that evolutionarily stable feminization penetrance increases with the host's natural death rate, just as the virulence is predicted to do with the host's natural death rate in classic epidemiological models. Finally, we found that horizontal transmission had no impact on how feminization penetrance evolved when genetic females were extinct. However, horizontal transmission can permit genetic females to coexist with symbionts and, in this case, we demonstrate that the presence of genetic females selects symbionts for lower feminization penetrance.

Patterns of floral colour neighbourhood and their effects on female reproductive success in an Antirrhinum hybrid zone.

The maintenance of genetic integrity of parental populations is often explained by selection against hybrids. However, the selection agents are usually unknown. The role of environmental biotic interactions is often suspected but has rarely been demonstrated. In plants for instance, mutualism with pollinators may be involved. After verification that pollen deposition is a limiting factor for fruit set, we used an individual-based study and a representation of pollinator colour perception to test the effects of local plant density and floral colour neighbourhood on female reproductive success in an Antirrhinum hybrid zone. In addition to flower colour and density effects, the composition of the floral neighbourhood was found to influence fruit set, suggesting that most plants were usually better fertilized when similar to their neighbours. However, the plants of one particular type were sometimes favoured when very different from their neighbours. The implications for hybrid zone dynamics are discussed.

Manifold aspects of specificity in a nematode-bacterium mutualism.

Coevolution in mutualistic symbiosis can yield, because the interacting partners share common interests, to coadaptation: hosts perform better when associated with symbionts of their own locality than with others coming from more distant places. However, as the two partners of a symbiosis might also experience conflicts over part of their life cycle, coadaptation might not occur for all life-history traits. We investigated this issue in symbiotic systems where nematodes (Steinernema) and bacteria (Xenorhabdus) reproduce in insects they have both contributed to kill. Newborn infective juveniles (IJs) that carry bacteria in their intestine then disperse from the insect cadaver in search of a new host to infect. We ran experiments where nematodes coinfect insects with bacteria that differ from their native symbiont. In both Steinernema carpocapsae/Xenorhabdus nematophila and Steinernema feltiae/Xenorhabdus bovienii symbioses, we detected an overall specificity which favours the hypothesis of a fine-tuned co-adaptation process. However, we also found that the life-history traits involved in specificity strongly differ between the two model systems: when associated with strains that differ too much from their native symbionts, S. carpocapsae has low parasitic success, whereas S. feltiae has low survival in dispersal stage.

When mutualists are pathogens: an experimental study of the symbioses between Steinernema (entomopathogenic nematodes) and Xenorhabdus (bacteria).

In this paper, we investigate the level of specialization of the symbiotic association between an entomopathogenic nematode (Steinernema carpocapsae) and its mutualistic native bacterium (Xenorhabdus nematophila). We made experimental combinations on an insect host where nematodes were associated with non-native symbionts belonging to the same species as the native symbiont, to the same genus or even to a different genus of bacteria. All non-native strains are mutualistically associated with congeneric entomopathogenic nematode species in nature. We show that some of the non-native bacterial strains are pathogenic for S. carpocapsae. When the phylogenetic relationships between the bacterial strains was evaluated, we found a clear negative correlation between the effect a bacterium has on nematode fitness and its phylogenetic distance to the native bacteria of this nematode. Moreover, only symbionts that were phylogenetically closely related to the native bacterial strain were transmitted. These results suggest that co-evolution between the partners has led to a high level of specialization in this mutualism, which effectively prevents horizontal transmission. The pathogenicity of some non-native bacterial strains against S. carpocapsae could result from the incapacity of the nematode to resist specific virulence factors produced by these bacteria.

Pollinator behavior and deceptive pollination: learning process and floral evolution.

Some species of flowering plants engage in nonmodel deceptive pollination, attracting pollinators by large nonmimetic floral displays and providing no reward. Pollinators can learn to avoid deceptive plants and to favor nectariferous species. The reproductive success of these species is expected to be density dependent for two opposite reasons: the commoner cheating flowers are, the easier they are to avoid and the lower the quality of the patch, making it more difficult to recognize that unrewarding flowers are not profitable. When a deceptive species is made up of multiple floral variants, pollinators' learning could decrease the reproductive success of any particularly common floral variant. Within a population of deceptive plants, mean reproductive success could, therefore, vary with the number of floral variants. We investigate these three hypotheses by modeling the behavior of pollinators foraging in communities of deceptive and rewarding flowers. Simulations revealed that the reproductive success of deceptive flowers varies in a density-dependent manner and that floral variants can be submitted to negative frequency-dependent selection. We compare density dependence in nonmodel deceptive species to what is expected in Batesian mimics and discuss possible selection of morphological variants. Finally, we survey how pollinators' learning capacities can make mean reproductive success depend on morphological variability within a population.