Instant evolution

In just six years, symbiotic bacteria have dramatically altered a population of sweet potato whiteflies in the southwestern US, accelerating the development and boosting the reproductive fitness of the crop-damaging pest.

Sweet potato whiteflies
Credit: Stephen Ausmus

The discovery, published this week in linkurl:Science,;https://www.sciencemag.org/content/current is a surprisingly rapid example of evolution that could have significant impacts on ecology and agriculture."It's like instant evolution," said linkurl:Molly Hunter,;https://ag.arizona.edu/ento/faculty/hunter.htm senior author and an entomologist at the University of Arizona. "The whole population has been transformed over a very short period of time.""It's quite unexpected," added linkurl:Frank Jiggins,;https://www.gen.cam.ac.uk/research/Jiggins/index.html an evolutionary geneticist at the University of Cambridge, who was not involved in the research. "It's now clear there is a lot of important adaptation in insect populations that should actually be attributed to bacterial symbionts."Hunter and her colleagues analyzed whitefly samples collected from 2000 to 2006 in the southwestern US -- saved and frozen by a colleague studying insecticide resistance -- and gathered additional samples in 2008 and 2009 from the field. They were surprised to find a dramatic increase in the frequency of infection by the bacteria Rickettsia bellii. While only 1 percent of the whiteflies collected had Rickettsia in 2000, the microbes had spread through half the population by 2003, and today, the infection rate is nearly 100 percent.The Rickettsia actually confer fitness benefits to their hosts, the team learned: Infected whiteflies develop faster, produce more offspring, and are more likely to survive to adulthood. Infected insects also produced a higher proportion of daughters, an evolutionary advantage for the maternally inherited bacteria, but a potential long-term disadvantage for the whitefly. As the populations' sex ratio becomes skewed towards females, daughters will have more competition for mates than sons. Thus, Rickettsia acts simultaneously a beneficial and a parasitic partner of its host species. "It's very interesting to see both traits combined together in a single bacterium," said Jiggins. "It's like Jekyll and Hyde."Additional studies suggest rapid invasions may be commonplace in insect populations. A strain of Wolbachia bacteria swept through a fly population in California in just 3 years in the 1990s, for example, resulting in the fly population's increased resistance to viral infection.While the team does not know the specific mechanism underlying the whitefly's new traits, the changes are likely encoded by an "accessory genome," a package of bacterial genes that are transferred to their hosts during infection, noted Jiggins in an accompanying perspective in Science. The findings can help the agricultural industry prepare for changes in the whitefly populations, said Hunter, as well as help groups interested in using symbionts to control pest populations. A team in Australia, for example, is experimenting with linkurl:spreading Wolbachia through a mosquito population,;https://www.eliminatedengue.com/Portals/58/moreira%20et%20al%202009.pdf which appears to prime the insect's innate immune system and inhibit the ability of dengue to infect the species. "Understanding all the different things a symbiont can do to its host, both good and bad, is valuable," said Hunter. Himler, A.G, et al., "Rapid Spread of a Bacterial Symbiont in an Invasive Whitefly Is Driven by Fitness Benefits and Female Bias," Science, 332:254-6, 2011.
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[23rd May 2006]