Heterogeneous individual motility biases group composition in a model of aggregating cells

Aggregative life cycles are characterized by alternating phases of unicellular growth and multicellular development. Their multiple, independent evolutionary emergence suggests that they may have coopted pervasive properties of single-celled ancestors. Primitive multicellular aggregates, where coordination mechanisms were less efficient than in extant aggregative microbes, must have faced high levels of conflict between different co-aggregating populations. Such conflicts within a multicellular body manifest in the differential reproductive output of cells of different types. Here, we study how heterogeneity in cell motility affects the aggregation process and creates a mismatch between the composition of the population and that of self-organized groups of active adhesive particles. We model cells as self-propelled particles and describe aggregation in a plane starting from a dispersed configuration. Inspired by the life cycle of aggregative model organisms such as Dictyostelium discoideum or Myxococcus xanthus, whose cells interact for a fixed duration before the onset of chimeric multicellular development, we study finite-time configurations for identical particles and in binary mixes. We show that co-aggregation results in three different types of frequency-dependent biases, one of which is associated to evolutionarily stable coexistence of particles with different motility. We propose a heuristic explanation of such observations, based on the competition between delayed aggregation of slower particles and detachment of faster particles. Unexpectedly, despite the complexity and non-linearity of the system, biases can be largely predicted from the behavior of the two corresponding homogenous populations. This model points to differential motility as a possibly important factor in driving the evolutionary emergence of facultatively multicellular life-cycles

Les effets de la sélection naturelle et de la dérive génétique sur le polymorphisme neutre

The diversity of living organisms is essential for their capacity to evolve and adapt to environmental changes. Therefore, determining the factors responsible for the origin of diversity and for the maintenance of the genetic variance observed remains central and fundamental research objective. The aim of this thesis was to understand the evolutionary factors maintaining neutral polymorphism. Since the influence of evolutionary processes such as natural selection and genetic drift are complex, we developed complementary experimental and theoretical approaches in order to disentangle their contributions. Using a biological model consisting of the bacterium Escherichia coli and the social amoeba Dictyostelium discoideum enable us to study the natural variability of interactions between the two species. In the second part of this work, we studied the bacterial traits involved in this natural variability. We showed that bacteria carrying virulence genes were resistant to grazing by amoeba, a result which was in agreement with the coincidental evolution hypothesis of virulence factors. We then focus on population genetics aspects of our biological system. In coevolution experiments, we followed temporal allele frequency variations over 300 bacterial generations under four sets of environmental conditions: with or without biotic factor and with or without spatial structure. Our results did not differ from genetic drift predictions. The aim of theoretical model we developed was to address the demographic stochasticity effects on neutral allele fixation probability and time to fixation. We found that fixation probability and the time to fixation were affected by the demographic stochasticity compared with a model using a population of constant size (Moran model).La diversité des organismes est essentielle pour leur capacité à évoluer et s'adapter aux variations environnementales. De ce fait, déterminer les facteurs responsables de l'origine de cette diversité ainsi que de la maintenance de la variabilité génétique observée reste un objectif fondamental en recherche. L'objectif de cette thèse était de comprendre les facteurs évolutifs maintenant le polymorphisme neutre. L'influence des processus évolutifs tels que la sélection naturelle et la dérive génétique étant complexes, nous avons combiné des approches complémentaires expérimentale et théorique. Le système expérimental utilisé, la bactérie Escherichia coli et l'amibe sociale Dictyostelium discoideum nous a permis d'étudier dans un premier temps la variabilité naturelle des interactions existant entre les deux espèces. Dans une seconde partie, nous avons étudié les traits bactériens impliqués dans cette variabilité. Nous avons montré que les bactéries portant des facteurs de virulence sont plus résistantes à la digestion des amibes, ce qui est en accord avec l'hypothèse de coïncidence évolutive des facteurs de virulence. Le deuxième volet de cette thèse concerne les aspects de génétique des populations de ce système. La troisième partie de notre expérimentation était de suivre les variations temporelles des fréquences alléliques de populations bactériennes comportant un marqueur neutre, durant 300 générations et sous quatre conditions environnementales : avec ou sans structuration spatiale et avec ou sans facteur biotique. Nous avons observé que les variations des fréquences alléliques observées étaient compatibles avec la dérive génétique. L'objectif du modèle théorique a été dans un premier temps d'étudier les effets de la stochasticité démographique sur les probabilités de fixation d'un nouvel allèle neutre arrivant dans une population résidente ainsi que sur le temps de fixation. La probabilité de fixation ainsi que le temps de fixation sont modifiés par les effets stochastiques lorsque l'on compare nos modèles à taille de population fluctuantes à un modèle à taille de population constante tel que le modèle de Moran

Social Conflicts in Dictyostelium discoideum: A Matter of Scales

International audienceThe 'social amoeba' Dictyostelium discoideum, where aggregation of genetically heterogeneous cells produces functional collective structures, epitomizes social conflicts associated with multicellular organization. 'Cheater' populations that have a higher chance-quantified by a positive spore bias-of surviving to the next generation when mixed with cooperators bear a selective advantage. Their spread is thus expected to undermine collective functions over evolutionary times. In this review, we discuss the two main approaches adopted to conceptualize social conflicts in Dictyostelium discoideum: describing social interactions as a property of cell populations (strains), or as a result of individual cell choices during the developmental process. These two points of view are often held equivalent and used interchangeably. While the population-level view grants more direct evolutionary inference, however, the cell-level interpretation reveals that such evolutionary predictions may be modified if mechanisms such as dependence on the environment, development and intrinsic unpredictability of cell fate choices are taken into account. We conclude by proposing a set of open questions that in our opinion lie at the core of a multi-scale description of aggregative life cycles, where the formulation of predictive evolutionary models would include cell-level mechanisms responsible for spore bias alongside population-level descriptors of multicellular organization

Single-cell phenotypic plasticity modulates social behaviour in Dictyostelium discoideum

Abstract In Dictyostelium chimeric aggregates, strains social behaviour is defined based on their relative representation in the spores, the reproductive cells resulting from development. Some strains, called ‘cheaters’, that produce more than their fair share of spores are considered a threat to the evolutionary stability of multicellular organization. The selective advantage gained by cheaters is indeed predicted to lead to the progressive loss of collective functions whenever social behaviours are genetically determined and in the absence of mechanisms guaranteeing strong assortment. However, genotypes are not the only determinant of spore bias, and the relative role of genetic and plastic phenotypic differences in strains evolutionary success is unclear. Here, we control phenotypic heterogeneity by harvesting cells in different growth phases, and study the effects of plastic variation on spore bias in chimeras composed of isogenic or genetically different populations. Our results indicate that variation in cell mechanical properties induced by differences in populations growth phase affects their probability to aggregate, hence their chance to differentiate into spores during multicellular development. The contribution of this non-genetic source of variation to spore bias is non-negligible, and sometimes even overrides genetic differences between co-aggregating strains. These results support the idea that unavoidable variation in the timing of aggregation, a parameter that is not under the control of any single cell or genotype, can compete with selection on genetically determined traits. Our observations suggest that heterogeneity in mechanical properties during aggregation may contribute to limit the spread of ‘cheaters’, thus to the evolutionary stability of aggregative multicellularity

From grazing resistance to pathogenesis: the coincidental evolution of virulence factors.

To many pathogenic bacteria, human hosts are an evolutionary dead end. This begs the question what evolutionary forces have shaped their virulence traits. Why are these bacteria so virulent? The coincidental evolution hypothesis suggests that such virulence factors result from adaptation to other ecological niches. In particular, virulence traits in bacteria might result from selective pressure exerted by protozoan predator. Thus, grazing resistance may be an evolutionarily exaptation for bacterial pathogenicity. This hypothesis was tested by subjecting a well characterized collection of 31 Escherichia coli strains (human commensal or extra-intestinal pathogenic) to grazing by the social haploid amoeba Dictyostelium discoideum. We then assessed how resistance to grazing correlates with some bacterial traits, such as the presence of virulence genes. Whatever the relative population size (bacteria/amoeba) for a non-pathogenic bacteria strain, D. discoideum was able to phagocytise, digest and grow. In contrast, a pathogenic bacterium strain killed D. discoideum above a certain bacteria/amoeba population size. A plating assay was then carried out using the E. coli collection faced to the grazing of D. discoideum. E. coli strains carrying virulence genes such as iroN, irp2, fyuA involved in iron uptake, belonging to the B2 phylogenetic group and being virulent in a mouse model of septicaemia were resistant to the grazing from D. discoideum. Experimental proof of the key role of the irp gene in the grazing resistance was evidenced with a mutant strain lacking this gene. Such determinant of virulence may well be originally selected and (or) further maintained for their role in natural habitat: resistance to digestion by free-living protozoa, rather than for virulence per se

From Grazing Resistance to Pathogenesis: The Coincidental Evolution of Virulence Factors

Abstract To many pathogenic bacteria, human hosts are an evolutionary dead end. This begs the question what evolutionary forces have shaped their virulence traits. Why are these bacteria so virulent? The coincidental evolution hypothesis suggests that such virulence factors result from adaptation to other ecological niches. In particular, virulence traits in bacteria might result from selective pressure exerted by protozoan predator. Thus, grazing resistance may be an evolutionarily exaptation for bacterial pathogenicity. This hypothesis was tested by subjecting a well characterized collection of 31 Escherichia coli strains (human commensal or extra-intestinal pathogenic) to grazing by the social haploid amoeba Dictyostelium discoideum. We then assessed how resistance to grazing correlates with some bacterial traits, such as the presence of virulence genes. Whatever the relative population size (bacteria/amoeba) for a non-pathogenic bacteria strain, D. discoideum was able to phagocytise, digest and grow. In contrast, a pathogenic bacterium strain killed D. discoideum above a certain bacteria/ amoeba population size. A plating assay was then carried out using the E. coli collection faced to the grazing of D. discoideum. E. coli strains carrying virulence genes such as iroN, irp2, fyuA involved in iron uptake, belonging to the B2 phylogenetic group and being virulent in a mouse model of septicaemia were resistant to the grazing from D. discoideum. Experimental proof of the key role of the irp gene in the grazing resistance was evidenced with a mutant strain lacking this gene. Such determinant of virulence may well be originally selected and (or) further maintained for their role in natural habitat: resistance to digestion by free-living protozoa, rather than for virulence per se

Phenotypic and genotypic characteristics of the <i>E. coli</i> strains studied.

a<p>Phylogenetic groups determined as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0011882#pone.0011882-Clermont1" target="_blank">[74]</a>.</p>b<p>Virulence genes determined as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0011882#pone.0011882-Johnson3" target="_blank">[73]</a>, (+) presence, (−) absence. All these data are from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0011882#pone.0011882-Diard1" target="_blank">[72]</a> except the amoeba model data that have been generated in this work.</p>c<p>Phenotypes tested as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0011882#pone.0011882-Diard1" target="_blank">[72]</a>, abbreviations used are: RSer: serum resistance, RBil: bile resistance, RLys: lysozym/lactoferrin resistance, Mot: motility, GenTim: generation time. Binarization of data in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0011882#pone.0011882-Diard1" target="_blank">[72]</a>.</p>d<p>Biological model tested: mouse <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0011882#pone.0011882-Johnson3" target="_blank">[73]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0011882#pone.0011882-Diard1" target="_blank">[72]</a>, K: mouse killer (9 or 10 mice killed over the 10 inoculated), NK: mouse non-killer (no mouse lethality or 1–5 mice over the 10 inoculated), <i>D.discoideum</i>, G: grazing, GR: grazing resistance.</p

Bacterial lysis plaque occurrence.

<p>(A) A non-virulent <i>E. coli</i> strain (B REL606; 10⁸ cells) was plated with <i>D. discoideum</i> (10² cells). Bacterial lysis plaques were observed, characteristic of the grazing phenotype. (B) A virulent <i>E. coli</i> strain (536; 10⁸ cells) was plated with <i>D. discoideum</i> (10² cells). No bacterial lysis plaques were observed, characteristic of the grazing resistance phenotype.</p