"Modelling the evolution of ectosymbiosis in the context of eukaryogenesis"Krishnan, NandakishorThe symbiotic origin of mitochondria as an organelle and eventually eukaryogenesis from ancestral prokaryotes are considered major evolutionary transitions. The permanent (physically integrated) and obligate symbiosis between prokaryotes is assumed to be critical in the evolution of the new level of organism. However, the initial interactions and conditions that facilitated the evolution of the ancestral symbiotic merger or consortia are widely debated. Syntrophy, basically the metabolic exchange or utilization is widely accepted to be the phenomenon behind the establishment of such close microbial associations. Here, we demonstrate a possible evolutionary path toward an association between two unicellular species (host and symbiont) that only requires unidirectional syntrophy. We develop a theoretical model based on the hypothesis that an initial step in the evolution of such symbiosis could be the invasion of a mutant host capable of forming an ectosymbiotic association into a monomorphic resident system stabilized by syntrophy. Further, we analyze the ecological and evolutionary stability of the symbiotic association. The dynamics of the population densities of different phenotypes of all the species and metabolite concentrations are represented using a set of non-linear ordinary differential equations, and the species growth rates are represented using novel Malthusian functions based on a branching process. The species’ growth is bounded by metabolite-dependent growth inhibition. We observed that a mutant host capable of shielding it from toxic metabolite and highly metabolically active ectosymbionts could initiate obligate ectosymbiosis even if the mutant host incurs additional costs compared to the resident host. Our model of obligate ectosymbiosis could be a precursor to the evolution of endosymbiosis. |
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