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The authors offer insights on a study which focused on clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR-Cas9) gene editing. Topics discussed include the reported evolution of Cas9 enzyme from IscB transposon proteins, the RNA-guided programming of transposon-encoded nucleases, and the possible contribution of transposons to the evolution of immune systems.

The article discusses a study within the issue by Kiers and colleagues regarding evolution research in which scientists offer insight into the mechanisms that stabilize the symbiotic relationship between plants and soil fungi. Topics include an overview of research investigating the plant-fungal symbiosis between the soil fungi Glomus mosseae and the plant Allium porrum (leek), the discrimination of fungi by plants depending on the cooperativeness of their fungal partners, and the ability of plants to switch fungal partners when it is uncooperative.

Integrons are adaptive bacterial devices that rearrange promoter-less gene cassettes into variable ordered arrays under stress conditions, thereby sampling combinatorial phenotypic diversity. Chromosomal integrons often carry hundreds of silent gene cassettes, with integrase-mediated recombination leading to rampant DNA excision and integration, posing a potential threat to genome integrity. How this activity is regulated and controlled, particularly through selective pressures, to maintain such large cassette arrays is unknown. Here, we show a key role of promoter-containing toxin-antitoxin (TA) cassettes as systems that kill the cell when the overall cassette excision rate is too high. These results highlight the importance of TA cassettes regulating the cassette recombination dynamics and provide insight into the evolution and success of integrons in bacterial genomes.