Although it often doesn't seem like it on a warm summer night, insects that feed on human blood are a rarity. There may be as many as 10 million species of insects; only about 100 of them specialize on humans. But rare doesn't mean unimportant. A single species of mosquito, Aedes aegypti, can spread yellow and dengue fevers, making it a major player in public health.

Aedes aegypti is also unusual in that, while it specializes in humans, we've found a population of what researchers have termed "forest" mosquitos that prefer to go after other animals. Now, researchers have used the forest mosquitos to help us understand how their peers have ended up specialized in feeding on humans. The answer, in part, is that the mosquitos have evolved a receptor that makes us smell good.

The key for this new paper was a forest mosquito population in Kenya, last observed in the 1970s. The research involved in the new work returned to the site in 2009, and the team found that mosquitos with two different color patterns still existed in the region. Returning to the lab with them, they found that the forest mosquitos tended to prefer the scent of guinea pigs, while mosquitos isolated in or near houses preferred human smells.

The two populations, however, were closely enough related that they could interbreed, which allowed the authors to create hybrid populations and determine whether they preferred humans or animals.

The hybrids that resulted had a mixed affinity, showing various degrees of preference for guinea pigs and people. The authors selected those hybrids with the strongest preferences and looked at what genes are expressed in their olfactory organs, as mosquitos use scents to home in on prey. They identified 14 genes that were different between the two groups of animals, two of which were olfactory receptors, meaning they signaled whenever they latched on to a specific scent molecule.

The authors focused on one of these receptors in this paper (chances are they're planning a separate paper for the other). They found it responded to a molecule called sulcatone, which is a component of human body odor; guinea pigs produce much less of it, as do cows, chickens, and sheep. The receptor itself is responsible for the affinity for this human scent; the same receptor from forest mosquitos had a greatly reduced affinity. The change in odorant affinity came about through a large number of changes in the DNA that encodes the protein.

It would be easy to take these results as an indication that mosquitos simply evolved an enhanced ability to sense sulcatone, and that was that. But the situation is quite a bit more complex than this superficial view. Simply adding sulcatone to the scent of guinea pigs wasn't enough to convince human-favoring mosquitos to suddenly go after a guinea pig, so there must be other sensory cues involved. And it's not simply enough to have a receptor that latches on to sulcatone. In mosquitos that favor humans, this specific receptor is highly expressed, suggesting that changes in gene regulation have also occurred.

It's clear that sensing sulcatone is only part of the story when it comes to attracting these mosquitos to humans, but sensing sulcatone is also only part of the story of the mosquitos' adaptation to human hosts. Rather than laying their eggs in natural water sources, which are higher in nutrients, these mosquitos prefer water storage containers in or near buildings. And the mosquito larvae can also survive the periods of starvation they face there.

These hybrid mosquitos bred by the authors may tell us a lot more about the ways in which Aedes aegypti has adapted to humans. That's likely to be important. Even if we develop a sulcatone-scent-blocking chemical, the mosquitos will almost certainly evolve a way to find humans without relying on it. We're likely to need multiple avenues of attack to make progress against the threat posed by mosquitos. But by examining more of the factors that have allowed Aedes aegypti to specialize in feeding on humans, we might open up some other avenues.

Nature, 2012. DOI: 10.1038/nature13964 (About DOIs).