The Accessory Genome as a Cradle for Adaptive Evolution in Pathogens

Pathogens offer many of the most fascinating and well-studied examples of evolution because their speed of adaptation allows observation of evolutionary change within human lifetimes. The rapid evolution observed in pathogenic viruses, bacteria, and fungi affect human, animal, and plant health globally. For example, influenza viruses regularly recombine genes affecting host range, infection pathways, and virulence prior to the emergence of deadly outbreaks [1]. The emergence of antibiotic resistance in bacteria, including highly resistant Staphylococcus aureus strains and the reemergence of resistant tuberculosis, is well documented [2,3]. The recent outbreak in Africa of wheat stem rust caused by the Ug99 pathotype of the fungus Puccinia graminis tritici surprised many plant pathologists, who thought this ancient pathogen was defeated during the green revolution [4]. It is clear that pathogens engage host species in a constant race to evolve new defense mechanisms. By extension, we are constantly challenged to find new treatments and better containment measures to protect human health and essential crops.