A mosquito variant found in London subway tunnels, basements and other underground urban spaces has now caused a surprise. Until now, this “molestus” subspecies of the common mosquito Culex pipiens, which is adapted to sucking blood in humans, was considered a prime example of the accelerated evolution of animals in urban environments. This variant was believed to have developed in London’s underground spaces around 200 years ago. But new analyzes now reveal that this mosquito subspecies did not originate in the modern subsoil, but rather in the Mediterranean region more than a thousand years earlier. Researchers report that this mosquito was probably already changing from a bird-biting variant to a variant adapted to human blood.
The common mosquito Culex pipiens, also known as the northern house mosquito, is one of the most common mosquito species in Europe. When we have a mosquito bite, she is usually behind it. However, it has long been known that this species of mosquito occurs in two subspecies: Culex pipiens pipiens only bites birds and is considered the original variant of these mosquitoes. Culex pipiens molestus, on the other hand, is specialized for humans and other mammals. Both forms cannot be distinguished from one another externally, but they can be distinguished genetically and in their behavior. The bird-stinging variant goes into a dormant state in winter and is dependent on a previous blood meal to lay its eggs. However, the human-stinging form molestus, which occurs primarily in cities and their underground habitats, which are warm even in winter, remains active all year round. In an emergency, she can also lay her eggs without a blood meal.
Prime example of rapid evolution?
But how did this human-piercing form of Culex pipiens come about? “This enigmatic mosquito became famous in London during World War II and seemed so perfectly adapted to life underground that it was thought it must have evolved there,” explains senior author Carolyn McBride of Princeton University. “It became a prime example of rapid evolution in modern cities.” According to popular belief, Culex pipiens molestus evolved from the bird-piercing form in subways and basements in Northern Europe – and only in the last 200 years or so. It was therefore considered a prime example of how animals can adapt to human environments and urbanization. But what’s wrong with this scenario? McBride, first author Yuki Haba from Princeton University and their colleagues have now investigated this. “For the first time, we evaluated a large set of population genetic data to clarify when, where and in what ecological context Culex pipiens molestus arose,” say the researchers.
For their study, the researchers cooperated with 150 organizations to examine around 12,000 Culex pipiens specimens from a wide variety of geographical and genetic contexts. The samples also included mosquitoes from what is now London, as well as museum specimens that had been collected and preserved in London’s underground since 1940. The team carried out high-resolution genomic analyzes on almost 360 specimens from 77 different locations in the northern hemisphere. The analyzes showed that the molestus and pipiens forms in more northern latitudes today actually differ significantly genetically and form two distinguishable populations. However, in more southern latitudes such as North Africa and the Mediterranean, this separation is less clear. Nevertheless, these populations show no signs of recent mixing, as Haba and his colleagues report. They have therefore investigated a suspicion that has so far been little researched: “The southern pipiens forms could be closer to the molestus variant because molestus arose from them there,” the researchers postulate. They checked whether this was the case through further comparative genome analyses.
Originated thousands of years ago in the Middle East
These analyzes confirmed the alternative scenario. Accordingly, the human-stabbing molestus variant did not originate in the underground of the cities of Northern Europe, but in the Mediterranean region. “The earliest molestus lineages are genetically consistent with above-ground mosquitoes from the eastern Mediterranean,” report Haba and his colleagues. According to their genome-based dating, the human-biting mosquito variant arose not just 200 years ago, but 1,300 to 12,000 years ago. “Our genomic data confirms that the notorious London underground mosquito evolved a long time ago – long before subways existed,” says co-author Mara Lawniczak from the UK’s Wellcome Sanger Institute. Accordingly, the mosquito variant Culex pipiens molestus developed at the time when the first people settled in the Middle East. In adaptation to this, the mosquitoes, which originally only bit birds, began to discover humans as hosts and to adapt to them and their environment. “Urbanization is therefore not responsible for the emergence of the molestus variant, but it made it easier for these mosquitoes to expand their range,” explain the researchers. Only our cities, which are warm even in winter, and their underground probably made it possible for the molestus mosquitoes to spread so far north.
The prime example of urban evolution and adaptation accelerated by our urban environment is therefore not one, at least not with regard to the emergence of the human-biting molestus mosquito variant. Nevertheless, these mosquitoes do show recent adaptations to city life. This also includes a higher rate of crosses with the bird-piercing pipiens form, as Haba and his colleagues determined. “Gene flow between molestus and pipiens shows a positive correlation with human population density,” they report. “The more people who lived within a radius of one to ten kilometers from our sample specimens, the higher the probability of finding genetic evidence of interbreeding.”
Also relevant for human health
This hybridization of both mosquito variants is also of considerable importance for human health. The mosquitoes resulting from these crosses bite both birds and people and can therefore transmit pathogens between the two. “This ability is considered to be the driving force behind the increasing infections with the West Nile virus in the USA and southern Europe,” explain the researchers. This virus originally only occurs in birds, but is now spreading to humans too. Such an infection usually causes no or only mild flu-like symptoms with fever, but in rare cases it can trigger meningitis. “Our work now opens up the possibility of specifically investigating the potential connection between urbanization, hybridization and the spillover of the virus from birds to humans,” says Haba.
Source: Yuki Haba (Princeton University) et al., Science, doi: 10.1126/science.ady4515