Population genetics of beneficial heritable symbionts

Most species of insects are infected with heritable, facultative symbionts. Such symbionts first appear within a host lineage as a result of lateral transfer from other host species. Although some facultative symbionts are reproductive parasites and thus adversely affect the transmission of host nuclear genes, there is growing evidence that many are beneficial to their hosts by, for example, conferring protection from natural enemies. The origin, spread, and maintenance of such symbionts bears many similarities to, as well as important differences from, the process of adaptive evolution by beneficial nuclear mutations. The time is ripe for the development of a coherent theory of the ‘population genetics’ of beneficial heritable symbionts.