The coevolution of hosts and symbionts based on virulence and mode of transmission is a complex and diverse biological phenomenon. We introduce a conceptual model to study the stable coexistence of an obligate symbiont (mutualist or parasite) with mixed-mode transmission and its host. The existence of evolutionarily and ecologically stable coexistence is analyzed in the framework of coevolutionary dynamics. Using an age-structured Leslie model for the host, we demonstrate how the obligate symbiont can modify the host's life history parameters (survival and fecundity) and the long-term growth rate of the infected lineage. The evolutionary success of the symbionts is given by the long-term growth rate of the infected population (multi-level selection). When the symbiont is vertically transmitted, we find that the host and its symbiont can maximize the long-term growth rate of the infected lineage. Moreover, we provide conditions for the ecological and evolutionary stability of the resident host-symbiont pair in the coevolutionary model, which does not allow invasion by any rare mutants (each mutant dies out by ecological selection). We observed that ecological competition, clearing of infection, and density-dependent interactions could play a role in determining the criteria for evolutionary stability.
The origin of eukaryotes and organellogenesis have been recognized as a major evolutionary transition and subject to in-depth studies. Acknowledging the fact that the initial interactions and conditions of cooperative behaviour between free-living single-celled organisms are widely debated, we narrow our scope to a single mechanism that could possibly have set-off multi-species associations. We hypothesize that the very first step in the evolution of such cooperative behaviour could be a single mutation in an ancestral symbiont genome that results in the formation of an ecto-commensalism with its obligate ancestral host. We investigate the ecological and evolutionary stability of inter-species microbial interactions with vertical transmissions as an association based on syntrophy (cross-feeding). To the best of our knowledge, this is the first time that a commensalistic model based on the syntrophy hypothesis is considered in the framework of coevolutionary dynamics and invadability by mutant phenotype into a monomorphic resident system.
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