Experimental evolution toward extinction in a molecular host–parasite system

Theoretical studies have proposed that coevolution with parasitic replicators plays a critical role in the evolution of primitive life; however, experimental verification of the potential outcomes of such coevolutionary dynamics remains limited. We previously conducted a coevolutionary experiment using an RNA-protein replication system that resulted in the spontaneous diversification of host and parasitic RNAs into five distinct lineages with robust coreplication. Here, we report contrasting evolutionary outcomes from a second long-term coevolutionary experiment. Using a droplet flow reactor (FR) system with increased dilution frequency over 5,000 h (1,600 generations), we observed reduced diversity and frequent extinctions in later experimental stages. Coreplication assays of RNA clones revealed that the primary cause of this diversity loss was the shortened reaction time resulting from frequent dilution. Further analysis of RNA clones that emerged during evolution suggested that the frequent extinctions resulted from the appearance of highly competitive parasite species and the dominance of host species that exhibited reduced replication ability. These findings demonstrate that coevolution between host and parasitic replicators can result in diversity loss and frequent extinctions depending on dilution conditions, highlighting the critical role of environmental parameters, such as dilution ratio and frequency, in enabling primitive replicators to evolve sustainably toward the emergence of life.