PilD mutant is a new cheater in Pseudomonas aeruginosa quorum-sensing evolution

Pathogen virulence is largely driven by the evolution and adaptation of the genome. LasR is a master regulator of quorum-sensing (QS) system. The QS-inactive LasR-null mutant regains QS activation in a laboratory evolution experiment through obtaining mutations in mexT, a gene encoding transcriptional regulator. How the QS-active LasR-MexT mutant continuously evolves is still unknown. Here we passaged the LasR-MexT mutant in casein broth and monitored its evolutionary trajectory. By examining the proteolytic phenotype of isolated single colonies, we found a protease-negative subpopulation arising from the parental population. The whole genome sequencing analysis revealed that these protease-negativ colonies bear mutations in pilD gene. In accordance with its role as a prepilin peptidase of type IV system, PilD mutants are defective in twitching motility. In conclusion, we identified a new PilD mutant cheater that evolved from the parental LasR-MexT mutant population. Our work demonstrates that population evolution is highly dynamics, with arising of distinct subpopulations containing different social behaviors.