Projet de recherche doctoral numero :2612


Date depot: 1 janvier 1900
Titre: Emergence et évolution des agrégats (micro)biologiques : rôle de la taille des groupes et du processus d'agrégation
Directrice de thèse: Edith PERRIER (UMMISCO)
Domaine scientifique: Sciences et technologies de l'information et de la communication
Thématique CNRS : Non defini

Resumé: Keywords: group size, aggregation, agent-based models, emergence of cooperation, collective behavior --- {{{Introduction}}} Questions. Ensembles of cells displaying coordinated behaviours and an internal homogeneity are the fundamental brick constituting multicellular organisms. Although not as organised as tissues in multicellular life forms, microbial aggregates often stand as individual recognizable units, to the point that the border between unicellularity and multicelluarity is sometimes blurred. For instance, bacterial cells communicate through quorum sensing mechanisms [Waters] and are often found in multispecific 'supercolonies' called biofilms. Eukaryotes are also known to be able to estimate population density [Wuster], a particularly striking example being the complex life cycle of Dictyostelium species, consisting of an alternation of unicellular and multicellular, differentiated, phases [Jang]. In order to be recognized as 'individuals', cellular aggregates need to display a certain regularity in their size and shape, which is often conserved within the same species and the outcome of a strict regulation. The macroscopic features of aggregates depend on the one side on individual properties such as the interaction strength and the propensity to sociality, on the other it is subjected to selection from the environment. Therefore, the question of aggregate size or density evolution involves two different biological and temporal scales, one dealing with the mechanistic processes of aggregation, the other one involving the function and long-term survival of the population. Aggregate size is primarily regulated by the interaction rules among different individuals, determined for instance by physical constraints on adhesion, signalling molecules, intracellular dynamics [Bonabeau, Gomer], and such rules are themselves subjected to mutations introducing new strategies in the population. The main question addressed in this thesis is: How is the distribution of aggregate size determined, both in terms of mechanistic processes and in terms of the evolutionary history? Particular issues of this general problem are: -# How do interaction rules affects aggregation and the aggregates' sizes? -# What is the spectrum of possibly achievable aggregate sizes given some interaction rules? -# How evolutionary pressures for group cohesion affect back size distributions? {{{State of the art}}} The emergence of multicellular organisms is a fundamental step of the complexification of living forms in the course evolution. The evolutionary stability of multicellular assemblages is however questioned on the basis on the internal conflicts that may arise among the components of a whole. Such diverging interests are expected to be present at the level of cells composing a multicellular organism, as well as that of individuals composing societies. The evolution of multicellularity therefore stands together with the evolution of cooperation in the formal framework of game theory, whose particularly relevant instances are n-players games (where n is the players group size). Within this framework, explicit conditions have been evidenced for the evolutionary stability of cooperative assemblages in terms of costs and benefits for the individual units composing the aggregate [Nowak 2006]. Several hypothesis have been discussed on the necessity of additional mechanisms to promote cooperation, and notably those related to the knowledge and communication of the individual strategic choices: 'green beard' [Jensen 2006], 'reputation' [Suzuki 2005], 'punishment' [Boyd 2003]. Other hypothesis, based on the existence of alternative states than 'cooperators' and 'defectors', lead to nonequilibrium dynamics reminiscent of co-evolutionary red-queen oscillations [Hauert 2002]. Although the role of group size has been long recognised as an essential determinant of the stability of cooperative acts, small groups being more prone to cooperation than big ones, only few authors have addressed the evolutionary origin and determinants of group size [Aviles 1999, 2002]. These general game-theoretical models have often focused on the economic interaction among organisms, such as humans, with developed cognitive capabilities, and their application to the microbial world is not straightforward. Models that explain the evolution of coordinated behaviour in microbs rely on specific hypothesis on the genetic uniformity of aggregates [Brown], or on the 'discovery' of new solutions, such as somatic and germ line differentiation, that solve conflicts arising among selfish cells. Moreover, these models typically neglect the way in which groups form. Dictyostelium is a perfect model organism for this study, since it has been the subject of a thorough genetic and molecular characterization, including the recent development of single-cell measure techniques [Gregor], its cell-cell interaction dynamics has been largely addr

Doctorant.e: Garcia Thomas