How fruit flies chase invisible ribbons of smell to get to their source

果蝇如何追踪隐形的嗅觉丝带以找到气味源头

A fruit fly hunting a piece of rotting fruit or a mate navigates by smell, following plumes of odor. Out in the wild, turbulent air swirls these plumes into a chaotic, broken landscape—dense chemical filaments laced through long stretches of clean air. A fly trying to find the source gets the smell in stutters, from constantly shifting directions, with no guarantee that a next whiff is coming at all. 果蝇在寻找腐烂水果或配偶时，依靠嗅觉进行导航，追踪气味羽流。在野外，湍流将这些羽流搅动成混乱、破碎的景观——浓密的化学细丝穿插在长段的洁净空气中。一只试图寻找源头的果蝇只能断断续续地闻到气味，且气味方向不断变化，甚至无法保证下一阵气味何时到来。

Scientist had little idea of how fruit flies manage this chaotic signal with a brain the size of a pinhead. For a long time, biologists stood by the “surge and cast” model, which posited that insects solved this with hardwired reflexes. The idea was that, when a fly registers the plume with olfactory neurons in its antennae, it simply flies upwind until it’s gone and then flies side to side attempting to catch it again. But now a team led by Vanessa Ruta, a neuroscientist at the Rockefeller University, has shown that fruit flies do something far more advanced. 科学家们此前几乎不知道果蝇是如何用针尖大小的大脑处理这种混乱信号的。长期以来，生物学家一直支持“激增与搜寻”（surge and cast）模型，该模型认为昆虫是通过硬连线的反射来解决这个问题的。其观点是，当果蝇通过触角上的嗅觉神经元感知到羽流时，它只需逆风飞行直到气味消失，然后左右飞行试图再次捕捉气味。但现在，由洛克菲勒大学神经科学家 Vanessa Ruta 领导的团队证明，果蝇的行为要复杂得多。

A treadmill for flies

果蝇的“跑步机”

The trouble with the traditional surge and cast model is that it struggles to explain how an insect tracks a meandering plume across long distances. Chemical cues floating in the air in the natural environment are often sparse and unreliable. But those same features make the mechanism behind olfactory navigation notoriously difficult to test. “Odors are invisible,” Ruta says, “and often they’re carried along by turbulent airflow.” We’ve got no way of knowing what the animal is smelling from one moment to the next. 传统的“激增与搜寻”模型的问题在于，它难以解释昆虫如何长距离追踪蜿蜒的羽流。自然环境中漂浮在空气中的化学线索往往稀疏且不可靠。但正是这些特征使得嗅觉导航背后的机制极难测试。“气味是看不见的，”Ruta 说，“而且它们通常随湍流移动。”我们无法得知动物在每一刻闻到了什么。

So, Ruta’s team designed an experiment to learn that. Scientists tethered a fly (Drosophila) in place over a small ball floating on a cushion of air in complete darkness. “It’s a little fly-sized treadmill,” Ruta explains. As the fly walked on the surface of the ball, its turns steered a nozzle that blew a steady stream of air at its antennae, so the insect always felt wind coming from a fixed direction, as if it were walking across an open field. 因此，Ruta 的团队设计了一个实验来探究这一点。科学家们将一只果蝇固定在完全黑暗中漂浮在气垫上的小球上方。“这是一个果蝇大小的跑步机，”Ruta 解释道。当果蝇在球面上行走时，它的转向会控制一个喷嘴，向其触角吹送稳定的气流，这样昆虫总是感觉到风来自固定的方向，就像在开阔的田野上行走一样。

Then the researchers piped apple cider vinegar into that airstream, switching it on and off depending on the fly’s position on this virtual field. This way, the researchers could precisely control the intensity and direction of the odor the fly was registering. “We could actually generate any kind of arbitrary chemical landscape,” Ruta says. The team could simulate a lifelike olfactory experience with turbulent plumes, straight-edged corridors, and gradients of smell running backwards. “It’s actually not a very complicated virtual reality system, but it is actually extremely powerful,” Ruta adds. For once, the experimenters knew the identity and quantity of every molecule the fly was receiving. The first thing they tested was the surge and cast model. And it did not hold up well. 随后，研究人员将苹果醋引入气流中，根据果蝇在这个虚拟场地上的位置开关气味。通过这种方式，研究人员可以精确控制果蝇感知到的气味强度和方向。“我们实际上可以生成任何任意的化学景观，”Ruta 说。该团队可以模拟逼真的嗅觉体验，包括湍流羽流、直边走廊以及反向运行的气味梯度。“这实际上不是一个非常复杂的虚拟现实系统，但它确实非常强大，”Ruta 补充道。实验人员第一次知道了果蝇接收到的每一个分子的身份和数量。他们测试的第一件事就是“激增与搜寻”模型，结果发现它并不成立。

Edge tracking

边缘追踪

In the experiment with a straight corridor of vinegar about 50 millimeters wide, flowing with the wind, the flies did not do the obvious thing and march up the middle. Instead, they hugged one edge, riding it through a repeating two-step process. The moment a fly crossed into the odor, it would whip around and move back out, loop through the clean air outside, then make a beeline back to the boundary. The team called it edge tracking. 在实验中，面对一条约 50 毫米宽、随风流动的醋味直走廊，果蝇并没有像预想的那样从中间穿过。相反，它们紧贴着边缘，通过一个重复的两步过程进行移动。当果蝇进入气味区时，它会迅速转身移出，在外部的洁净空气中绕行，然后径直飞回边界。研究团队将其称为“边缘追踪”。

The flies also spent far more time loitering outside the plume than inside it, even though nearly all their forward progress toward the source happened during those brief dips into the scent. “Flies will track meters along the edge of a plume and never cross over,” Ruta says. The first explanation the researchers came up with was that the flies were climbing a rising gradient of odor toward the source. But they didn’t do that either. When the team reversed the gradient, so the vinegar grew fainter as the fly advanced, the flies tracked the edge just as well. 果蝇在羽流外徘徊的时间也远多于羽流内，尽管它们向源头前进的几乎所有进展都发生在那短暂进入气味的瞬间。“果蝇会沿着羽流边缘追踪数米，而从不越过，”Ruta 说。研究人员最初的解释是，果蝇是在沿着气味浓度升高的梯度向源头爬升。但事实并非如此。当团队反转梯度，使醋味随着果蝇的前进而变淡时，果蝇依然能很好地追踪边缘。

The team also worked with flies genetically engineered so light could switch their olfactory neurons on directly, in both antennae at once. When scientists swapped real odor for a beam of 660-nanometer red light, the insects tracked the edge of this light plume flawlessly. The real surprise, though, came when the team took a close look at what the flies were doing when there was no smell at all to guide them. 该团队还使用了基因工程改造的果蝇，使其嗅觉神经元可以通过光线直接激活（双侧触角同时激活）。当科学家用 660 纳米的红光束代替真实气味时，昆虫完美地追踪了这条“光羽流”的边缘。然而，真正的惊喜出现在团队仔细观察果蝇在没有任何气味引导时的行为时。

Olfactory memories

嗅觉记忆

When the researchers made a plume vanish while a fly was outside it, the insect kept heading back to where the edge should have been. Rather than searching at random, they were heading to where they knew the odorant should be. “It looked as if they had a stored memory that was still steering them in that direction,” Ruta says. So, the team started looking for these stored olfactory memories. 当研究人员在果蝇位于羽流外时让羽流消失，昆虫仍会持续飞向羽流边缘本应存在的位置。它们没有随机搜索，而是飞向它们认为气味应该存在的地方。“看起来它们似乎有一种存储的记忆，仍在引导它们朝那个方向前进，”Ruta 说。于是，团队开始寻找这些存储的嗅觉记忆。

The first candidate was a brain structure called the central complex, which is like a navigation hub in the insect brain. Using fluorescent imaging to watch neurons fire in real time, Ruta and her colleagues pinpointed neurons that acted as the fly’s compass. Throughout edge tracking, that compass stayed locked to the wind, indifferent to whether the fly was in odor or out. When the team silenced these neurons, the flies got lost entirely, wandering upwind with no idea how to get back. 第一个候选对象是一个名为“中央复合体”（central complex）的大脑结构，它就像昆虫大脑中的导航中心。利用荧光成像实时观察神经元放电，Ruta 和她的同事确定了充当果蝇“指南针”的神经元。在整个边缘追踪过程中，该指南针始终锁定风向，无论果蝇是否处于气味中。当团队抑制这些神经元时，果蝇完全迷失了方向，逆风徘徊，不知道如何返回。

The neuronal compass, though, was just one piece of the puzzle. To find the smell that got lost in the wind, the fly must have a goal—a memory of what to search for. The team went searching for that memory. The team located olfactory memories in another group of neurons found in something called the fan-shaped body, already known to encode where a fly wants to go. Like in a compass, the activity of these neurons acts like a pointer, but in this case, the one that marks the fly’s intended destination. Out in the clean air, this pointer swung away from the compass to aim at the plume’s edge, several seconds before the fly physically turned to head back. Just as with the compass neurons, silencing the neurons encoding the odor’s edge position made it impossible. 然而，神经指南针只是拼图的一部分。为了找到在风中丢失的气味，果蝇必须有一个目标——即关于搜索什么的记忆。团队开始寻找这种记忆。他们将嗅觉记忆定位在另一组神经元中，这些神经元位于被称为“扇形体”（fan-shaped body）的结构中，已知该结构负责编码果蝇想要去的地方。就像指南针一样，这些神经元的活动就像一个指针，但在这种情况下，它标记的是果蝇预期的目的地。在洁净的空气中，这个指针会偏离指南针，指向羽流的边缘，这发生在果蝇实际转向返回之前的几秒钟。正如指南针神经元一样，抑制编码气味边缘位置的神经元使得追踪变得不可能。