Dengue mosquito evolves rapidly, researchers show

Group studied 20 generations of a single population of Aedes aegypti, observing high rates of genetic variability that enable the mosquito to adapt to adverse factors.

By Karina Toledo

Agência FAPESP – Probably no other mosquito species has adapted so successfully to the hostile environment of the big city as Aedes aegypti, the white-striped vector for dengue fever and several other arboviral diseases that afflict humans, such as yellow fever, chikungunya and Zika.

A. aegypti has acquired resistance to some insecticides, as well as the ability to reproduce in steadily dwindling amounts of water, which does not even need to be as clean as in the past. Worse, it used to bite only during the day, but now it also bites at night, provided there is sufficient artificial lighting to make its victim visible.

A study published recently in the journal PLoS One by a group of researchers affiliated with the Butantan Institute, in São Paulo, Brazil, may help to explain why A. aegypti’s adaptive potential so far exceeds that of other mosquito species.

The researchers monitored the population of these insects in Butantã, a densely urbanized neighborhood of Sao Paulo City, for 14 months, covering five seasons (fall of one year through fall of the next). Eggs, pupae and larvae collected each month were divided into five groups, one for each season. Some 20 generations of mosquitoes from the same population were studied.

Using two different methods, the researchers investigated genetic variability among the five groups and the various generations, comparing the changes in DNA alleles over time.

“From the first sampling to the last, and in all comparisons we made month by month, we found statistically significant differences, as if we were comparing individuals from different populations, i.e., collected at different sites. This high genetic variability suggests that the species is capable of evolving rapidly and may mean it adapts quickly to adversity,” said Lincoln Suesdek, principal investigator for the study, which was supported by FAPESP.

The study was conducted during Caroline Louise’s Scientific Initiation scholarship, supervised by Suesdek, and followed a line of work that commenced during Paloma Oliveira Vidal’s PhD research, also supported by a scholarship from FAPESP.

“During my doctoral research, I collected samples of the mosquito from several cities of São Paulo State at a single time of year,” Vidal said. “I compared genetic variability between the different populations in a single generation. The results were similar to the findings of the project that compared individuals of the same population across several generations.”

According to Suesdek, two different techniques were used in both projects to measure genetic variability. One of them, traditionally used in kinship analysis, focused on microsatellites, also called simple sequence repeats; these are short segments of DNA with a repeated sequence of base pairs that indicate the most recent evolutionary variations. The technique is similar to that used in paternity testing.

The other method, never used before in this type of study, involved measuring differences in wing morphology. The researchers selected points on the mosquito’s wing as anatomical markers and used software to assess positional variations between these points in the different groups.

“Microsatellites are highly informative markers, but this method is costly and labor-intensive,” Suesdek said. “We wanted to test a simpler, less costly marker. Previous studies showed that insect wing shape, like the human face, is heritable. This feature is hard to assess with the naked eye, however.”

The group’s aim was to find out whether these two markers displayed the same pattern. The results showed a correlation, although it was not 100%.

“We succeeded in predicting part of their genetic variability by studying wing shape, but the method doesn’t replace DNA analysis. It can be a preliminary technique for use when nothing is known about the population and a quick test is required to understand microevolution,” Suesdek said.

Year-long control

On comparing the results of the analysis performed during her project with data from the scientific literature, Louise concluded that A. aegypti is evolving faster in São Paulo than in other cities for which similar data exist.

“The expectation was that variability would be lower in the winter because the insect reproduces more slowly at lower temperatures,” Louise said. “We indeed found a lower rate of reproduction during winter months, but genetic variability remained high in all the months covered by the samples. This finding reinforces the importance of combating the mosquito throughout the year, and not just in summer.”

According to Louise, the mosquito is best controlled by a combination of measures, from elimination of breeding grounds to rotation of insecticides to avoid selection of resistant individuals.

Suesdek also stressed the need to invest in research geared toward the development of new chemical and biological control methods, such as new insecticides and genetically modified mosquitos.

“The outlook gives cause for concern. Both the government and local communities must do their part,” the team said.