Modes of fitness increase during the aggregative multicellular development program of Myxococcus xanthus

Abstract

In nature, environmental conditions are in constant flux. Organisms must dynamically respond and adapt to the presence of other biological entities as well as abiotic factors in order to survive and reproduce. Both cooperative and competitive interactions between organisms are common. Cooperation involves the expression of a trait that enhances the fitness of other individuals, and usually is costly to express. Conflict arises when the evolutionary interests of interacting individuals do not align, such as when some individuals benefit from a cooperative action without contributing to its expression. The fitness advantage of such social defectors can allow them to increase in frequency, and if their frequency increase is not controlled they can be disruptive to cooperation. Conflict can therefore have devastating effects on cooperative systems and there is much interest in understanding how cooperation is maintained despite the threat of its exploitation. Abiotic pressures such as fluctuations in resource availability or temperature can also affect the evolutionary trajectory of a population. Many organisms, including microbes, can respond to fluctuating environments by evolving bet-hedging strategies. These can either be conservative, where an organism expresses a ‘safe’ trait value, or diversifying, where multiple phenotypes are produced as a risk-spreading strategy. Both forms of bet-hedging ensure survival in an unpredictable environment, which can make them appear maladaptive when observed in isolation or over short time scales and without environmental fluctuations. Myxococcus xanthus is a model system for studying the genetics of bacterial development. Its aggregative multicellular lifestyle and cooperative behaviors also make it an attractive system to study the evolution of social behavior. One of the most prominent features of the M. xanthus lifecycle is the cooperative formation of multicellular fruiting-body structures, a development program initiated in response to starvation. Conflict between interacting individuals of different genotypes, evidenced by (sometimes extreme) fitness asymmetries during chimeric fruiting-body formation in M. xanthus, is not uncommon. The mechanisms responsible for increased fitness during multicellular development, however, are not well understood. The aim of this thesis is to further explore how fitness can be increased in response to biotic and abiotic selective pressures during M. xanthus social behaviors, particularly during cooperative fruiting-body formation. In the first chapter I investigate how biotic interactions, namely the presence of competitors, influences the ways that fitness increases can be gained by evolving a nearlysaturated mutant library under conditions that select for genotypes with high spore production in chimera, i.e. in developmental groups composed of multiple genotypes. I then characterize the range of different strategies that M. xanthus can use to gain a relative fitness advantage during chimeric development. The results show that evolved populations can be composed of multiple distinct competitors and that several qualitatively distinct strategies result in fitness gains during chimeric development. Conflict during social interactions can be dampened by preferentially interacting with highly related individuals. One mechanism that promotes high relatedness is kin discrimination. In the second chapter, I investigate the role of TraA/B-mediated outer membrane exchange (OME) as a potential driver of colony merger incompatibilities (CMIs) between swarming groups of M. xanthus. CMIs are a form of microbial kin discrimination and are known to reduce the level of chimeric fruiting-body formation at the interface of two swarms in M. xanthus. I test whether disrupting TraA functionality influences CMI phenotypes or strong antagonisms during chimeric development between natural strains. The results show that kin discriminatory behaviors in M. xanthus are complex and likely multifactorial, which is in line with what is known about kin discrimination in other bacterial species. Two differentiated cell types produced during M. xanthus development, stressresistant spores and viable rod cells (peripheral rods), have the potential to survive and resume growth after nutrients become replete. In the final chapter of this thesis, I investigate how selection in environments that vary in the length of starvation can influence cell-type partitioning during multicellular development. Specifically, I test for a potential contribution of peripheral rod cells to group-level fitness during starvation-induced development. I show that the relative benefit of producing different developmental cell types (spores versus viable rods) changes in response to how quickly nutrients are replenished after starvation initiation. Therefore, another path to increased fitness during multicellular development could involve altered investment in developmental cell types.