Aedes mosquitoes are some of the deadliest animals on the planet. Two species in particular, Aedes aegypti (yellow fever mosquito) and Aedes albopictus (Asian tiger mosquito), have established invasions all over the world, and it is these two species that confer the world’s burden of dengue and various other arboviral diseases. Attempts to eradicate both species with insecticides has led to the evolution of insecticide resistance, and the same resistance mutations appear across global populations of both species. 

This leads us to an interesting set of questions:

1. Why do we see the same resistance mutations across the world? Have these evolved in multiple populations independently, or spread by accidental transport of mosquitoes in ships, trucks, and planes?
2. By observing genetic patterns at a specific, well-studied resistance gene, can we identify other genes with similar resistance effects?

Our new paper considers these questions, and with each step unveils a range of insights into the recent evolutionary history of Aedes aegypti and Aedes albopictus.

To begin with, consider the following maps.

The top map is of Aedes aegypti, the bottom is Aedes albopictus. The maps show locations we collected mosquitoes from, but more specifically show the geographical distributions of two DNA mutations (yellow and teal hexagons) found in the voltage sensitive sodium channel gene that confer strong resistance to insecticides in these mosquitoes.

A few things stand out here. Firstly, the yellow mutation was observed in both species – a sign of parallel evolution in response to the same natural selection pressure (insecticide use). But these mutations also have a wide distribution within each species. This could possibly reflect parallel evolution within each species, where the same mutation evolves multiple times independently in each population. But it could also reflect gene flow between populations, in which resistant mosquitoes secretly stow away on ships and other human transport, and spread resistance into new populations once they disembark. We hypothesised that if mosquitoes had spread resistance by stowing away, the source and recipient populations should be genetically similar at the resistance gene, while if they had evolved independently they should be very distinct.

We conducted a series of genomic analyses that revealed a single evolutionary origin of the yellow mutation in Aedes albopictus and the teal mutation in Aedes aegypti, but two independent origins of the yellow mutation in Aedes aegypti. Aedes aegypti with the yellow mutation in Saudi Arabia, Malaysia, New Caledonia, Brazil, and USA all received it from the same ancestral source and had the same genetic background, while the other genetic background was spread throughout Southeast Asia, East Africa, and the West Pacific. These distances are very impressive and point to a huge amount of unrecorded mosquito movement going on.

One way to conceptualise this movement is through the following image, which shows a single year of international ship travel. Most coastal cities are highly connected to this shipping network, with darker lines showing higher shipping activity. So even with long distances to travel, Aedes mosquitoes have many opportunities to relocate.

So having investigated resistance at the sodium channel gene, we then looked for other genes that showed similar genetic patterns. This search uncovered a second set of genes, known as glutathione S-transferases, or GSTs, that had also spread across global populations of Aedes aegypti.

Interestingly, while there are 15 regions in the in Aedes aegypti genome that contain GSTs, it was this one specific set of GSTs, the epsilon class, where resistance had evolved at least three times and spread across populations. The below map indicates where this spread has occurred, with the three different symbols showing the three independent evolutionary backgrounds that have spread across populations.

At present, we don’t know exactly what resistance benefits the GSTs confer – this will require additional experiments on live mosquitoes collected from the regions of interest. But we have enough information to infer quite a few interesting things.

First, this is evidence that this specific set of genes, the GST epsilon class, is very important in insecticide resistance. We can tell this because three different resistance backgrounds have independently evolved at this same set of genes, and confer a sufficient advantage to have spread across different continents.

Second, it seems that resistance at these GST genes, like with the sodium channel gene, has a cost to the mosquito when insecticide use is low. We can infer this from the fact that none of the resistance backgrounds has reached 100% frequency in any of the populations. These are known as partial selective sweeps. We saw a similar pattern in the sodium channel gene, where the only locations with 100% frequency of a specific background were Vanuatu and Bali, Indonesia. As resistance can increase in frequency very rapidly under the right conditions, the intermediate frequencies we observe suggest that sometimes individuals without resistance are at an advantage.

Finally, there are some local patterns that demand further investigation. One is in Australia, where we observed a partial sweep of GSTs throughout the range, despite insecticidal control of mosquitoes being rare in this country. Another is in New Mexico, USA, where the local population appears to have a larger number of GST epsilon genes that other locations, which may add to resistance. And for both Aedes aegypti and Aedes albopictus, we found sets of genes all over the genome that were genetically linked to the sodium channel and GST backgrounds, with different genes linked to different backgrounds, suggesting that the sodium channel and GST genes are interacting with a wide range of other genes to produce local resistance phenotypes.

Many of the details of how natural selection operates on these specific genes remain to be determined. Nevertheless, our genomic data indicate how different movement patterns and evolutionary dynamics underpin Aedes distributions locally and across the world, highlighting future challenges in how to control these disease vectors.