Epidemics by beneficial microorganisms drive evolution

Epidemics of beneficial microorganisms (beneficial to the host) generally go unobserved; a few recent advantageous epidemics have been reported, including an epidemic of corals by a Vibrio species that made them resistant to bleaching and epidemics of stinkbugs by Burkholderia spp. that conferred resistance of the insect to the insecticide fenitrothion. There are similarities and differences between epidemics caused by pathogenic and by beneficial microbes. Both contain genomic islands (GIs) involved in epidemics. GIs of pathogenic bacteria contain virulence genes, such as those encoding specific toxins that damage eukaryotic cells, in addition to fitness genes that assist colonization. GIs of beneficial bacteria lack virulence genes, but contain similar fitness genes. Pathogenic epidemics are detected almost immediately through symptoms and have a finite time, whereas beneficial epidemics mostly go undetected and can persist for long periods, possibly leading to genetic and thus evolutionary changes. The key to epidemics (beneficial or pathogenic) is rapid horizontal transmission of the causative agent from one host to another via contaminated surfaces, water, air, and by direct contact. Vertical transmission can also occur. Pathogenic bacteria elicit an immune response, while beneficial bacteria usually do not, since they are recognized by the host as "self". There are many examples of important evolutionary events that were driven by the acquisition of microbes by animals and plants. However, acquisition of a microbe, leading to evolution, can be considered an epidemic only if the infection spreads rapidly. We suggest that beneficial epidemics can help explain at least the initial steps of punctuated evolution.