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Summary
Microbiologists have long recognized the importance of horizontal gene transfer—the movement of genetic material between organisms—in adaptation. It is common for plasmids (rings of DNA that are separate from chromosomal DNA) and phage (viruses that infect bacteria) to move between bacterial species, and their movement is often accompanied by the transfer of traits, such as antibiotic resistance, that are of great adaptive importance. In contrast, in multicellular organisms, the mutational variation upon which natural selection acts is generated independently within each species. It has recently become clear, however, that horizontal transfer of traits can play a major role in arthropod evolution. Studies of insects have detailed cases in which traits under selection are encoded by a bacterial symbiont. These microbes are commonly maternally inherited within species and therefore provide variation that is accessible for the process of natural selection. Like plasmids, however, symbiotic bacteria also occasionally move to a different host species. On page 254 of this issue, Himler et al. (1) present just such a case study. It documents fast evolution after the introduction of a new symbiont into a population of the whitefly Bemisia tabaci. They detail the spread of a Rickettsia bacterial symbiont that, in just 6 years, dramatically increased the survival and fecundity of its host in the southwestern United States.
