> Research over the last 20 years has shown that a large fraction of insect species has intimate associations with symbiotic bacteria that often have major effects on their biology. Some symbionts are mutualists increasing their host's fitness, whereas in other cases the host obtains no benefit or even suffers from carrying the micro-organism.
> Over the last few years a series of fascinating examples of how some insect endosymbionts provide benefits to their host have been discovered, and in many cases the benefits are realized only when the host faces an abiotic or biotic challenge. It is likely that we are still some way from fully understanding all of the complexities of the mutualistic interaction.
> One of the best-studied endosymbiont interactions in the field involves a strain of Wolbachia attacking Drosophila simulans in California. [It has been] recently shown that over the 20 years that it has been studied, it has evolved to be less costly to its host.
Heredity (2010) 104, 237–238