Early eukaryogenesis by host-initiated obligate ectosymbiosis and metabolic inhibition

The evolution of cellular-level symbiosis in microbes is a significant phenomenon to understand the origin of ancestral eukaryotic cells from prokaryotes. Though syntrophy is considered to be the initial interaction mechanism between the involved individuals, the process of internalization (endosymbiosis) and integration to form a symbiotic consortium requires better clarity. This work deals with a theoretical model based on the hypothesis that an intermediate ectosymbiosis might have evolved before complete endosymbiosis. Assuming an already syntrophic environment of free-living hosts and symbionts, we investigate how obligate ectosymbiosis can evolve between the individuals of different species. An asymmetric multilevel selection approach is considered to mechanistically analyze the driving factors in the formation and fixation of the symbiotic consortia. A metabolite-based growth rate and growth inhibition are utilized to effectively model the ecological selection dynamics between the host and the symbiont species. We observed that reduced metabolic growth inhibition due to the effective reduction in exposure to toxic metabolites by the common surface area of contact might be an incentive to stabilize the ectosymbiotic consortia. Furthermore, robust syntrophy between the host and their ectosymbionts might have facilitated obligacy in their association. The model gives insights into the critical elements in the evolution of endosymbiotic features.