Speakers
Description
Extrachromosomal genetic elements play a key role in the evolution of many organisms, particularly in adaptation to environmental stressors such as antibiotics. These elements include plasmids in bacteria, as well as mitochondrial and plastid genomes and extrachromosomal circular DNA (eccDNA) in eukaryotes. They frequently occur in their hosts in multiple copies, allowing increased mutational supply, intracellular genetic diversity, and gene dosage effects. Here we model evolutionary rescue of bacterial and yeast populations by genes of two types of extrachromosomal element—plasmids or eccDNA. Our findings show that bacterial adaptation via plasmids differs from chromosomal adaptation by modulating allele availability and establishment while slowing fixation. Both modelling and empirical validation indicate that segregation can hinder establishment of beneficial plasmid alleles. Furthermore, mutation rates and host population dynamics are influenced by conjugative plasmid transfer and plasmid instability. We further show that plasmid long-term persistence can be secured by a combination of partition systems and toxin-antitoxin systems, which likely play a crucial role in plasmid-mediated evolutionary rescue. For yeast, we develop stochastic models to describe rescue by gene amplification on eccDNA. We show that eccDNA abundance is driven both by intrinsic processes and by the replication of hosts; in particular, biased segregation of eccDNA increases the between-cell variation in copy number. This in turn affects the probability of successful rescue by eccDNA. Overall, we highlight the role of the unique genetic properties of extrachromosomal elements in scenarios of evolutionary rescue, offering insights into how genetic elements persist, adapt, and contribute to
population survival.