Distributive conjugal transfer (DCT) in M. smegmatis is regulated by ESX-1 and ESX-4, which are loci encoding type VII secretion systems (T7SS). Mycobacteria encode between two and five chromosomal ESX loci, which play diverse roles in mycobacterial biology including iron acquisition, virulence, and lateral gene transfer. Conjugative plasmids that rely on a type IV secretion system and a T7SS were recently discovered in mycobacteria. As a follow up to my work on the signatures of DCT in mycobacterial genomes, I investigated the functional divergence of ESX loci in mycobacteria. Using loci from mycobacterial chromosomes and plasmids, I estimated the phylogeny of ESX loci in mycobacteria and found that a plasmid origin for T7SS is the most parsimonious based on ancestral reconstruction. I also found that episodic directional selection has occurred during the diversification of ESX loci on plasmids, their migration from plasmids to the mycobacterial chromosome, and the divergence of slow and rapid growing mycobacteria.
A manuscript describing this project is published in Genome Biology and Evolution (here)[https://academic.oup.com/gbe/article/9/3/398/3058803]!