A female Culex pipiens mosquito. Photo: Lawrence Reeves (University of Florida)
The human-biting subspecies of the common house mosquito did not originate in the tunnels of London, but in human settlements along the Nile.
The so-called London Underground mosquito turns out to be thousands of years older than the London Underground itself. A new study debunks a persistent myth about the origins of the common house mosquito Culex pipiens: the human-biting subspecies molestus did not arise in twentieth-century London, but over a thousand years ago in the eastern Mediterranean, probably in Egypt, where the mosquito had already adapted to human settlements (Science, 24 October).
Confined spaces
Although the two subspecies of the common house mosquito – which occur in temperate regions worldwide – look identical, they differ markedly in behaviour and ecology. The above-ground Culex pipiens pipiens feeds primarily on birds, mates in the open air, and overwinters as an adult female in a state of diapause. The underground C. p. molestus, by contrast, remains active throughout the year, mates in confined spaces, and does not require a blood meal for egg maturation.
These traits made molestus notorious in London during the Second World War, when civilians sheltering in Underground stations were bitten in large numbers. The aggressive mosquito acquired the nickname London Underground mosquito and was long held up as a prime example of rapid urban evolution. The idea that the subspecies had actually originated in London, however, lived mainly in the popular press; genetic evidence for it was lacking.
Settlements
An international team of researchers tested this hypothesis by analysing the DNA of 357 mosquitoes from 77 populations across Europe, North Africa, and West Asia. The results show that molestus diverged from the bird-biting pipiens between approximately 1,300 and 12,500 years ago, most likely in the eastern Mediterranean. The oldest and genetically most diverse populations were found in Egypt, where densely populated settlements along the Nile were already emerging at that time. The study thereby confirms earlier suspicions of a Mediterranean origin.
Hybridisation between the bird-biting pipiens and the human-biting molestus is rare
The key behavioural traits of molestus – mating in enclosed spaces, overwintering without diapause, and laying eggs without a blood meal – are also found in present-day, above-ground populations from Egypt. This suggests that these traits first arose above ground, in close association with human environments, and later served as an evolutionary pre-adaptation for success in the modern city.
Hybridisation between the bird-biting pipiens and the human-biting molestus is rare, but occurs more frequently in densely populated areas, reaching around 5 per cent in Paris. Such crosses may play a role in the transmission of West Nile virus, because the hybrids bite both birds and humans, the researchers conclude.