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Mosquitoes transmit diseases to approximately one hundred million people each year and their bites have conditioned the history of mankind. There are approximately 3 500 species of mosquitoes throughout the world. The vast majority are general practitioners who bite any vertebrate that are to your step.

The human diseases transmitted by mosquitoes cause only a half-dozen species of three genera (Aedes, Anopheles, and Culex), which have evolved to seleccionarnos specifically thanks to the carbon dioxide that we emit, and our auras of the body.

why some mosquitoes have specialized in biting the human?

most researchers think that specialize in the people would not have of course no particular advantage for mosquitoes prior to the development of the cultures of sedentary makes approximately 10 000 years ago. Once settled, the human population could have provided an easy-to-use, secure and always available, unlike the other groups of migratory animals that will only guarantee blood seasonally.

genomic data are consistent with the hypothesis that the two groups of mosquitoes specialized in humans evolved within that period of time cultural. However, the question is, what tradeoffs physiological, anatomical, morphological, and behavioral prompting that some mosquitoes chose to bite humans and to not do this in the pets that have accompanied us since long before we become sedentary.

The mosquito specialized in human are not limited only to sting people, but they also tend to breed in habitats modified by man. They lay their eggs in the water and humans are the only animals that we manipulate the water to remove it, to channel it and collect it for domestic consumption. For this reason, there have been speculations about the dependence of reproductive of mosquitoes to the human sources of water, particularly in arid regions, could also have played a key role in the specialization of these diptera.

why some mosquitoes find us irresistible, while others do not pay attention?

to answer that question, a team of researchers from the University of Princeton (EE. UU.) developed in sub-saharan Africa, a project based on the collection of eggs of Aedes aegypti. It was a good choice because, besides being one of the mosquito specialized in human more fearsome as responsible zika, yellow fever and dengue fever, its populations are divided into two subspecies.

The subspecies aegypti thrives in urban habitats of tropical America and Asia, where he specialized in biting humans up to the point that 95 % of the food of the females, who are strongly attracted to our body odor comes from human blood.

In contrast, females of the populations general of the subspecies formosus tend to prefer the odor of other non-human vertebrate of the blood is nourished. It is thought that the specialist anthropic evolved from ancestors generalist africans between 5 000 and 10 000 years ago, possibly in the north of Senegal or Angola.

Like all mosquitoes, both species lay their eggs in the water, so the researchers began placing thousands of ovitrampas, a small cup full of water and leaves dirty that simulate the pools of water small stream that constitute the ideal habitat for spawning.

To get samples significant of the different environments in which mosquitoes breed, the ovitrampas were placed near large population centres (in cities of up to more than 2 000 people per km2) and agricultural areas covered with natural vegetation in which mosquitoes rarely encounter people. Also covered a wide range of climates, from habitats to semi-arid with seasonal rainfall to forest ecosystems with rainfall throughout the year.

In total, were collected eggs of mosquitoes in 27 different locations. Once dry, the eggs behave as seeds can remain dormant for six months or a year prior to hatching. This led to his transfer to Princeton with the goal of raising new populations in laboratory conditions.

Diagram of the olfactometer used in the research.

Obtained these, the researchers tested the insects with smells from human and guinea pig. The experiment consisted of building what we would call an olfactometer: a large plastic box full of mosquitoes, with two tubes removable. While in one was placed a guinea pig, one of the researchers introduced their arm for several hours in the other. The one and the other bait odour were protected from the bites direct for a few filters preventing the passage of insects.

a Few minutes after placing the tubes with their respective baits, mosquitoes were entering by one or other tube. After a time, they withdrew the tubes to count how many had chosen one or the other. The results revealed that the mosquito from populated areas they liked most auras human. The result more telling was related to the weather: the mosquitoes that came from places that had a rainy season followed by a dry season, long and warm, they preferred the human.

(A) The preference for the human is increased significantly with the increase of the density of population and in habitats with rainfall highly seasonal (B), while decreases in habitats with more rain in the warmer season of the year (C). The climatic variables in (B) and (C) can be combined into an index of overall intensity of the dry season (D). The color and the size of the point correspond to the seasonality of precipitation (scale in (C) and the density of the human population (proportional to the scale in (A), respectively. Modified and translated from the original publication.

The why of that response may be related to the life cycle of the mosquito. Aedes aegypti lays its eggs just above the water surface in holes of trees, cavities and fissures of rocks or in man-made containers. If the eggs are kept moist, they can hatch immediately. However, eggs deposited at the end of the rains in wilderness areas have to go into latency to survive during the dry season until the return of rain, an especially difficult challenge when the drought is prolonged and warm.

The standing water, the critical factor for larvae, it is difficult to find in these environments are extremely barren, but abounds around the human populations that collect water to survive, which throughout the year gives mosquitoes an incubator water for the development of their larvae. That suggests two things.

on the one hand, although the dry seasons long and hot were the selective factor key, the mosquito populations of arid regions have evolved towards specialisation in humans as a byproduct to take advantage of the situation of dependence of the water that is stored for the set. In the second place, when the larvae pass into the state hematófago adult blood available is the closest of the humans, with the added advantage that the new victims lack the skins tough and difficult to penetrate those of other vertebrates, including the domestic.

genomic analyses also revealed that the mosquito specialists in people differ genetically from the generalists, and that the preference for humans was developed in a single place unknown to later expand throughout Africa as the dry weather expanded across the continent. Then, in the era of slavery, trade, slave spread diseases like malaria and other tropical areas.

Although the research published in Current Biology, focused on the origin and evolutionary history of mosquitoes, if they are correlated with the climate data from the IPCC and UN population the results suggest that, as a result of climate change and the increasingly intense pressure of development, in the near future there will be more mosquitoes, transmitters of human diseases in all over the world.

Manuel Peinado Lorca is a Professor of University. Department of Life Sciences and Researcher at the Instituto Franklin for north American Studies, University of Alcalá

This article was originally published on The Conversation.

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