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Home , Kenyon cell, Mushroom bodies

Magazine R11

On the contrary. Wrangham’s book the hunting spider, Portia. Modern comes at a crucial time. Driven by Quick guide investigations, begun by Menzel, technical accomplishments, the Erber and Heisenberg, suggest that past years have seen huge interest the mushroom body is an important in identifying genes whose adaptive The mushroom center for and . changes underlie human evolution. For example, fruit flies learn to This enterprise may have, to some body associate a specific with an extent, shifted the focus away from electric shock, much like Pavlov’s environmental and in particular cultural Robert A.A. Campbell dogs learned that a bell signaled drivers of human evolution. Cooking and Glenn C. Turner* the arrival of dinner. Lesion studies, is a key cultural practice of humans including a reversible, temporally and it is Wrangham’s merit to identify precise block of mushroom body the clear-cut biological consequences What is a mushroom body? The output using the impressive genetic that may have followed in its wake. mushroom body is a prominent techniques in , showed But, it is important to note that, in his and striking structure in the brain that are essential view, clearly the cultural invention of several invertebrates, mainly for this learning. came before the biological adaptation. . It is found in , So, it was not a mutation in a gene scorpions, spiders, and even What are the inputs and outputs that primarily conferred a particular segmented worms. With its long of the mushroom body? In many advantage, and then was positively stalk crowned with a cap of cell insects, such as wasps and bees, selected for, but a learned cultural bodies, a GFP-labeled mushroom inputs to the mushroom body come technique that radically altered our body certainly lives up to its name from several different sensory interaction with the environment, in (Figure 1). The mushroom body pathways, including smell, taste, terms of the energy we take in. This is composed of small vision and hearing. Output regions changed the selective pressures and of known as Kenyon cells, named are less well defined, but one course numerous genetic changes — after Frederick Kenyon, who first intriguing feature is that the affecting our jaws, teeth, guts and not applied the Golgi staining technique of many mushroom body neurons least brains — will have ensued. to the brain. The bifurcate, sending one branch Moreover, Wrangham’s hypothesis brain, for instance, contains roughly towards the midline while the other points the way to empirical testing of 175,000 neurons per mushroom projects dorsally. Presumably these ideas. So much has been made body while the brain of the smaller this branching provides identical in recent years of looking for genetic fruit fly copies of mushroom body output changes underlying increases in only possesses about 2,500. Kenyon to different sets of downstream brain size or in cognitive capabilities, cells thus make up 20% and 2%, neurons. in particular language. And, while of respectively, of the total number course the appeal in finding such genes of neurons in each insect’s brain. How are mushroom bodies remains and is by no means obliterated bodies sit atop the made? One of the strangest things by the cooking-human hypothesis, they calyx, a tangled zone of about mushroom bodies is their are inherently difficult to test in a test representing the site of sensory development. They are derived tube or a mouse model. Wrangham’s input. Projecting away from the calyx from four neuroblasts that ideas, by contrast, pave the way for is the stalk comprised of Kenyon cell continue to divide throughout much much more straightforward tests. axons carrying information away to of the lifetime of the animal — Genes underlying changes in digestive the output lobes. in Drosophila from the larva until system development and function shortly before pupae hatch into that have been selected in humans, How did mushroom body adults. In some insect species, conceivably could be much more research start? In 1850, Felix mushroom body neurogenesis readily tested for physiological effects, Dujardin showed that the size of the even continues throughout simply because metabolism is so much mushroom body was correlated with adult life. During pupation, a easier to assess than cognitive skill. the complexity of social behavior in fraction of Kenyon cells lose their Despite its seemingly humble status different species of bees. Dujardin and one of their axonal among organ systems, the gut may suggested that mushroom bodies branches. Deprived of inputs, prove a gold mine for finding some of control aspects of insect behavior they subsequently re-grow a the key adaptive changes that ‘made that are not just simple reflexes and large dendritic tree, but remain us human’. Of course, ultimately, what even speculated that they might mono-axonal. The reason for this made humans cooks must be due play a role in ‘free will’. Although pruning is completely unknown. to cognitive capabilities and thus be few researchers have been bold Remarkably, despite the remodeling sought in our brains. It is, after all, this enough to study free will in insects, of roughly half the larval mushroom complex interplay between culture it is certainly true that arthropods body neurons, formed and biology that makes studying and display a wide array of sophisticated in the larval stage can persist into thinking about human evolution so behaviors: the bee waggle dance adulthood. uniquely fascinating. to communicate flower location, learning by observation in Are mushroom bodies analogous Florian Maderspacher is Current Biology’s Drosophila, and elaborate forward- to a particular area of the Senior Reviews Editor. planning and strategy formation by human brain? At different times Current Biology Vol 20 No 1 R12

Perhaps a more illuminating way to What else does the mushroom view the mushroom body is from body do? There is a well-established the perspective of how sensory connection between sleep and information is represented there. memory. For example, sleep can Although the neurons that provide prevent a newly established memory olfactory input to the mushroom from degrading. By the standard body respond broadly to many definitions — altered brain activity, , mushroom body neurons are increased arousal threshold, less very odor-selective and responses movement — insects do sleep. appear to be relatively sparse. Blocking mushroom body output Sparse representations are a alters a fly’s sleep patterns, affecting hallmark of learning and memory the duration of sleep intervals. centers — if neurons respond very Perhaps the mushroom body can act selectively to particular stimuli, then as a gate, uncoupling sensory input memories can also be accurately from behavioral output during sleep. formed and recalled. From this Another role for the mushroom body is perspective, the mushroom body during decision-making: like the rest is typical of memory centers in the of us, flies confront choices. Normal brain in general. flies abruptly and consistently switch between two different choices What do mushroom bodies do? when one is more salient. However, Perhaps because it is so prominent one study indicated that blocking and accessible to lesion studies mushroom body output causes this and physiology, sometimes it seems sharp switch to become a smooth that mushroom bodies are involved transition instead. It is unclear what in all interesting insect behaviors. this means but perhaps it is another One example is olfactory learning instance where the mushroom body and memory: a great deal of work acts as a gate or switch, this time has focused on understanding enabling the fly to crisply decide how sparse representations arise which stimulus to follow. All things Figure 1. Mushroom bodies of Drosophila. in the mushroom body, and how considered, maybe Dujardin wasn’t Top panel: anterior view of a fly head dissected learning modifies those activity too far off when he considered that to reveal GFP-labeled mushroom bodies. Lower panel: close-up of the mushroom body patterns. Surprisingly, these studies the mushroom body is what elevates color-coded for depth. Kenyon cell bodies at revealed a connection between insect behavior above the reflexive. the top send out a bulging array of dendrites sparseness and oscillatory neuronal Its role in associative learning, sleep immediately below, while the axons bundle to- activity, such as one sees in the and decision-making suggests that it gether to create a stalk before bifurcating to brain waves of human EEGs. The may serve as a gateway, selectively form two output lobes. selectivity of Kenyon cell spiking coupling input from different senses arises from the timing of the inputs. to appropriate behavioral output. Synaptic excitation and inhibition It certainly seems fair to ask: is the by different people, the mushroom arrive in rapidly alternating waves. mushroom body where the fly makes body has been considered loosely Only during the brief peaks of up its mind? analogous to three different these oscillations can sensory regions in the mammalian brain: inputs lead to the generation of Where can I find out more? first, the , because action potentials in Kenyon cells. Joiner, W.J., Crocker, A., White, B.H., and Sehgal, A. (2006). Sleep in Drosophila is of its involvement in learning and Thus, like the waves on an EEG, regulated by adult mushroom bodies. Nature memory; lesioning the mushroom insect neural circuits also exhibit 441, 757–760. Perez-Orive, J., Mazor, O., Turner, G.C., body in impairs their oscillatory activity. In this case, the Cassenaer, S., Wilson, R.I., and Laurent, G. memory for spatial locations, much oscillations create highly stimulus- (2002). Oscillations and sparsening of odor like hippocampal lesions do in selective neural responses, a role representations in the mushroom body. Science 297, 359–365. rodents. Second, the cerebellum, they are also likely to play in the Strausfeld, N.J., Hansen, L., Li, Y., Gomez, R.S., again because of its involvement human brain. Imaging experiments and Ito, K. (1998). Evolution, discovery, and interpretations of mushroom in learning, particularly of precisely that track neuronal levels bodies. Learn. Mem. 5, 11–37. timed motor movements; both suggest that learning modifies Yu, D., Akalal, D.B., and Davis, R.L. (2006). the mushroom body and the mushroom body response Drosophila alpha/beta mushroom body neurons form a branch-specific, long-term cerebellum are composed of properties. Specifically, output cellular memory trace after spaced olfactory densely packed tracts of axons from a particular region of the conditioning. 52, 845–855. Zhang, K., Guo, J.Z., Peng, Y., Xi, W., and Guo, that make contact with dendrites mushroom body is augmented after A. (2007). -mushroom body circuit of large neurons that insert into the learning. Understanding how these regulates saliency-based decision-making in tracks like a comb. Third, piriform observations of global activity relate Drosophila. Science 316, 1901–1904. cortex, because both piriform to changes of individual mushroom and mushroom body are only two body neurons is the next step in Cold Spring Harbor Laboratory, Cold synapses away from the sensory understanding how this circuit Spring Harbor, NY 11724, USA. layer of the olfactory system. creates associative memories. *E-mail: [email protected]