Caught in the act: Rapid, symbiont-driven evolution

Facultative bacterial endosymbionts can transfer horizontally among lineages of their arthropod hosts, providing the recipient with a suite of traits that can lead to rapid evolutionary response, as has been recently demonstrated. But how common is symbiont-driven evolution? Evidence suggests that successful symbiont transfers are most likely within a species or among closely related species, although more distant transfers have occurred over evolutionary history. Symbiont-driven evolution need not be a function of a recent horizontal transfer, however. Many endosymbionts infect only a small proportion of a host population, but could quickly increase in frequency under favorable selection regimes. Some host species appear to accumulate a diversity of facultative endosymbionts, and it is among these species that symbiont-driven evolution should be most prevalent. It remains to be determined how frequently symbionts enable rapid evolutionary response by their hosts, but substantial ecological effects are a likely consequence whenever it does occur.