The yellow fever mosquito or dengue mosquito (Aedes aegypti) Photo: Muhammad Mahdi Karim.
Ancient slave trade routes and modern insecticides have shaped the evolution of the dengue mosquito Aedes aegypti and the malaria mosquito Anopheles funestus.
The evolution of mosquitoes that spread malaria and dengue turns out to be closely intertwined with human history. Two new papers in Science show how ancient trade routes and modern pesticides determined the genetic trajectory of Aedes aegypti and Anopheles funestus. For Aedes aegypti, which spreads dengue and Zika among other diseases, researchers have managed to unravel the origins of a globally distributed, human-associated subspecies. Analysis of 1,206 genomes from 73 populations reveals that this subspecies first arose in the Americas, not in Africa. During the transatlantic slave trade – around 1700 – ancestral populations from West Africa arrived in Argentina. There, the human-associated subspecies evolved, and a century later it spread across the globe. In the 20th century, these yellow fever mosquitoes returned to Africa, where they are now mixing with native populations. In doing so, they also brought resistance genes that undermine commonly used insecticides. Genetic profiles previously known only from Latin America are now being found in West African cities.
The malaria mosquito Anopheles funestus is a major vector of malaria in Africa. Photo: James Gathany / Frank Collins (University of Notre Dame, USCDCP)
Anopheles funestus, the principal vector in sub-Saharan Africa, shows a different pattern. Genomic analysis of 701 individuals – including specimens from museum collections dating from the 1930s and 1960s – reveals that the species is genetically far more diverse than previously thought. Some populations remain genetically connected across thousands of kilometres, whilst others, such as those in northern Ghana and southern Benin, are strictly isolated. Resistance genes are entirely absent from the historical museum specimens; today they dominate in many regions. Notably, such variants have often arisen independently and subsequently spread rapidly.
Slave trade
A concrete example from the study concerns Burkina Faso, where the researchers identified a recent surge in resistance variants in Anopheles funestus. On 18 August, the government also abruptly halted the international Target Malaria project, following a disinformation campaign and growing political mistrust (Science, 3 September). That project used genetically modified malaria mosquitoes to combat the disease. The story is therefore twofold: the transatlantic slave trade laid the foundation for the global spread of dengue vectors, whilst modern pesticides sent the evolution of malaria mosquitoes into overdrive.
Control
Together, the studies make clear that uniform control strategies are untenable, and that effective approaches can only work when tailored to local genetic realities. ‘The findings highlight how profoundly human interactions, both ancient and modern, have shaped the genetic diversity of vectors,’ writes Tamar Carter, a mosquito researcher at Baylor University in Texas, in a perspective in Science. ‘This calls for fine-grained, locally adapted interventions, rather than a one-size-fits-all solution.’
A recent distribution map of the dengue or yellow fever mosquito Aedes aegypti in Europe, showing that two introduced populations are also present in the Netherlands. One of these introductions has been traced to the transport of second-hand car tyres from Miami in the United States (Emerging Infectious Diseases, 2011). Map source: European Centre for Disease Prevention and Control.