Influence of population bottlenecks on the evolution of sociality in synthetic life-cycle

Abstract

Cooperation requires individuals to invest their resources for the benefit of others. Cooperative behaviours often involve the secretion of costly public goods, which can be exploited by spontaneous mutant cheater cells that do not contribute but can still benefit from the shared resources. Therefore, explaining the emergence and maintenance of cooperation is difficult, though not impossible. One of the mechanisms known to stabilise cooperation is the introduction of population bottlenecks - i.e., drastic and sudden decreases in the population size. Studies have repeatedly demonstrated that stringent population bottlenecks can effectively purge out cheaters from a population and increase high relatedness in the restored population. The beneficial effect of high-relatedness on the maintenance of cooperation has been discussed in kin selection theory and empirically demonstrated in Dictyostelium discoideum for fruiting body development and in Pseudomonas aeruginosa for biofilm formation. However, such empirical evidence is still relatively rare. Interestingly, many of these organisms exhibit more than one cooperative behavior. For example, the life cycle of Dictyostelium discoideum involves collective fruiting body formation and multicellular slug migration. Pseudomonas aeruginosa exhibits multicellular biofilm formation, quorum sensing behaviour and secretion of iron-chelating siderophores as part of its life cycle. Our model system, Myxococcus xanthus, also exhibits multiple cooperative traits, such as multi-spore-bearing fruiting bodies, group predation, social swarming, collective spore germination, and vegetative growth. In many empirical studies, only one or a minority of the cooperative traits were studied, whereas, as mentioned, the model system used can potentially exhibit more than one cooperative trait. In this thesis, we explored three important and interconnected questions. First, the influence of population bottlenecks on life cycle evolution where multiple cooperative traits are under selection. Second, the role of historical contingency on the future stability of cooperation. Third, the differential effect of the timing of placement of population bottleneck in a life cycle and its social evolution outcomes.

Using life cycle evolution and extending our study to natural isolates of M. xanthus, for the first time we showed the existence of trade-offs between cooperative traits. Within four cooperative traits under selection, we observed that the trade-off operated between predation and germination on one side and fruiting body development and vegetative growth on the other side. Overall, contrary to previous studies, we showed that a stringent bottleneck does not favour every cooperative trait, and the trade-off between cooperative traits constrained the life cycle evolution. Furthermore, we observed that, size of the bottleneck applied decided the direction of these trade-offs. In the second chapter, we demonstrated that the previous life history of the genotype, whether it evolved under high relatedness condition, or not, will matter in the future stability of the evolution of cooperation. We observed reduced evolvability in combination with the ancestral strain’s low cost of cooperation in development trait, influenced the persistence of the sporulation trait in stringent lines. In the third chapter, when we applied population bottlenecks at different stages of an identical life cycle, we observed different outcomes. These different lineages also evolved different inter-clonal interaction strategies depending on the timing of placement of the bottleneck. Together we demonstrate that population bottlenecks are an important ecological factor influencing the social evolution dynamics within evolving populations.