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The '''Drosophila connectome''' is the complete list of connections between [[neuron]]s (commonly referred to as the "[[connectome]]") in the ''[[Drosophila melanogaster]]'' (fruit fly) nervous system. The fly's nervous system consists of the brain plus the ventral nerve cord, and both are known to differ considerably between male and female. <ref>{{Cite journal |last=Kelley |first=Darcy B. |date=March 22, 2021 |title=Sexual dimorphism: Neural circuit switches in the Drosophila brain |url=https://pubmed.ncbi.nlm.nih.gov/33756143/ |journal=Current Biology |volume=31 |issue=6 |pages=R297-R298}}</ref> Dense connectomes have been completed for the female adult brain<ref name="Dorkenwald_2023">{{cite bioRxiv | vauthors = Dorkenwald S, Matsliah A, Sterling AR, Schlegel P, Yu SC, McKellar CE, Lin A, Costa M, Eichler K, Yin Y, Silversmith W, Schneider-Mizell C, Jordan CS, Brittain D, Halageri A, Kuehner K, Ogedengbe O, Morey R, Gager J, Kruk K, Perlman E, Yang R, Deutsch D, Bland D, Sorek M, Lu R, Macrina T, Lee K, Bae JA, Mu S, Nehoran B, Mitchell E, Popovych S, Wu J, Jia Z, Castro M, Kemnitz N, Ih D, Bates AS, Eckstein N, Funke J, Collman F, Bock DD, Jefferis GS, Seung HS, Murthy M | display-authors = 6 | title = Neuronal wiring diagram of an adult brain | date = June 2023 | biorxiv = 10.1101/2023.06.27.546656 }}</ref>{{Unreliable source?|sure=y|reason=Not peer reviewed|date=July 2023}}, the male nerve cord<ref>{{Cite bioRxiv | vauthors = Takemura SY, Hayworth KJ, Huang GB, Januszewski M, Lu Z, Marin EC, Preibisch S, Xu CS, Bogovic J, Champion AS, Cheong HS | date = June 2023 | display-authors = 6 |biorxiv = 10.1101/2023.06.05.543757 |title=A Connectome of the Male Drosophila Ventral Nerve Cord}}</ref>{{Unreliable source?|sure=y|reason=Not peer reviewed|date=July 2023}}, and the female larval stage.<ref name = "Winding_2023" /> The adult female fruit fly brain contains about 128,000 [[neuron]]s and about 50 million chemical synapses, the nerve cord about 23,000 neurons and 70 million synapses, and the female larval brain roughly 3,000 neurons and 548 thousand chemical synapses. All of these these numbers are known to vary between individuals. None of the available connectomes show electrical synapses, or gap junctions. ''Drosophila'' is the most complex creature with a connectome, which had only been previously obtained for three other simpler organisms, first ''[[Caenorhabditis elegans|C. elegans]]''. The connectomes have been obtained by the methods of [[neural circuit reconstruction]], which over the course of many years worked up through various subsets of the fly brain to the almost full connectomes that exist today. <!-- "(EM) brain images. In recent years, this approach has been applied to chunks of brains to reconstruct local connectivity maps that are highly informative, yet inadequate for understanding brain function more globally." A stack of EM images of an entire brain exist, suitable for sparse tracing of specific circuits. --> |
The '''Drosophila connectome''' is the complete list of connections between [[neuron]]s (commonly referred to as the "[[connectome]]") in the ''[[Drosophila melanogaster]]'' (fruit fly) nervous system. The fly's nervous system consists of the brain plus the ventral nerve cord, and both are known to differ considerably between male and female. <ref>{{Cite journal |last=Kelley |first=Darcy B. |last2=Bayer |first2=Emily A. |date=March 22, 2021 |title=Sexual dimorphism: Neural circuit switches in the Drosophila brain |url=https://pubmed.ncbi.nlm.nih.gov/33756143/ |journal=Current Biology |volume=31 |issue=6 |pages=R297-R298}}</ref> Dense connectomes have been completed for the female adult brain<ref name="Dorkenwald_2023">{{cite bioRxiv | vauthors = Dorkenwald S, Matsliah A, Sterling AR, Schlegel P, Yu SC, McKellar CE, Lin A, Costa M, Eichler K, Yin Y, Silversmith W, Schneider-Mizell C, Jordan CS, Brittain D, Halageri A, Kuehner K, Ogedengbe O, Morey R, Gager J, Kruk K, Perlman E, Yang R, Deutsch D, Bland D, Sorek M, Lu R, Macrina T, Lee K, Bae JA, Mu S, Nehoran B, Mitchell E, Popovych S, Wu J, Jia Z, Castro M, Kemnitz N, Ih D, Bates AS, Eckstein N, Funke J, Collman F, Bock DD, Jefferis GS, Seung HS, Murthy M | display-authors = 6 | title = Neuronal wiring diagram of an adult brain | date = June 2023 | biorxiv = 10.1101/2023.06.27.546656 }}</ref>{{Unreliable source?|sure=y|reason=Not peer reviewed|date=July 2023}}, the male nerve cord<ref>{{Cite bioRxiv | vauthors = Takemura SY, Hayworth KJ, Huang GB, Januszewski M, Lu Z, Marin EC, Preibisch S, Xu CS, Bogovic J, Champion AS, Cheong HS | date = June 2023 | display-authors = 6 |biorxiv = 10.1101/2023.06.05.543757 |title=A Connectome of the Male Drosophila Ventral Nerve Cord}}</ref>{{Unreliable source?|sure=y|reason=Not peer reviewed|date=July 2023}}, and the female larval stage.<ref name = "Winding_2023" /> The adult female fruit fly brain contains about 128,000 [[neuron]]s and about 50 million chemical synapses, the nerve cord about 23,000 neurons and 70 million synapses, and the female larval brain roughly 3,000 neurons and 548 thousand chemical synapses. All of these these numbers are known to vary between individuals. None of the available connectomes show electrical synapses, or gap junctions. ''Drosophila'' is the most complex creature with a connectome, which had only been previously obtained for three other simpler organisms, first ''[[Caenorhabditis elegans|C. elegans]]''. The connectomes have been obtained by the methods of [[neural circuit reconstruction]], which over the course of many years worked up through various subsets of the fly brain to the almost full connectomes that exist today. <!-- "(EM) brain images. In recent years, this approach has been applied to chunks of brains to reconstruct local connectivity maps that are highly informative, yet inadequate for understanding brain function more globally." A stack of EM images of an entire brain exist, suitable for sparse tracing of specific circuits. --> |
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== Why ''Drosophila'' == |
== Why ''Drosophila'' == |
The Drosophila connectome is the complete list of connections between neurons (commonly referred to as the "connectome") in the Drosophila melanogaster (fruit fly) nervous system. The fly's nervous system consists of the brain plus the ventral nerve cord, and both are known to differ considerably between male and female. [1] Dense connectomes have been completed for the female adult brain[2][unreliable source], the male nerve cord[3][unreliable source], and the female larval stage.[4] The adult female fruit fly brain contains about 128,000 neurons and about 50 million chemical synapses, the nerve cord about 23,000 neurons and 70 million synapses, and the female larval brain roughly 3,000 neurons and 548 thousand chemical synapses. All of these these numbers are known to vary between individuals. None of the available connectomes show electrical synapses, or gap junctions. Drosophila is the most complex creature with a connectome, which had only been previously obtained for three other simpler organisms, first C. elegans. The connectomes have been obtained by the methods of neural circuit reconstruction, which over the course of many years worked up through various subsets of the fly brain to the almost full connectomes that exist today.
Connectome research (connectomics) has a number of competing objectives. On the one hand, investigators prefer an organism small enough that the connectome can be obtained in a reasonable amount of time. This argues for a small creature. On the other hand, one of the main uses of a connectome is to relate structure and behavior, so an animal with a large behavioral repertoire is desirable. It's also very helpful to use an animal with a large existing community of experimentalists, and many available genetic tools. Drosophila looks very good on these counts:
In 2023, the full connectome (for a female) was published: "we present the first neuronal wiring diagram of a whole adult brain, containing 5x10^7 chemical synapses between ~130,000 neurons reconstructed from a female Drosophila melanogaster. [..] The technologies and open ecosystem of the FlyWire Consortium set the stage for future large-scale connectome projects in other species."[2]Aprojectome, a map of projections between regions, can be derived from the connectome.
Before in 2011, a high-level connectome, at the level of brain compartments and interconnecting tracts of neurons, for the full fly brain was published.[7] A version of this is available online.[8]
Detailed circuit-level connectomes exist for the lamina[9][10] and a medulla[11] column, both in the visual system of the fruit fly, and the alpha lobe of the mushroom body.[12]
In May of 2017 a paper published in bioRxiv presented an electron microscopy image stack of the whole adult female brain at synaptic resolution. The volume is available for sparse tracing of selected circuits.[13][14]
In 2020, a dense connectome of half the central brain of Drosophila was released,[15] along with a web site that allows queries and exploration of this data.[16] The methods used in reconstruction and initial analysis of the connectome followed.[17]
In 2022, a group of scientists mapped the motor control circuits of the ventral nerve cord of a female fruit fly using electron microscopy.[18] In 2023, a dense reconstruction of the male fly ventral nerve chord was released[19].
In 2023, Michael Winding et al. published a complete larval brain connectome.[20][4] This connectome was mapped by annotating the previously collected electron microscopy volume.[21] They found that the larval brain was composed of 3,016 neurons and 548,000 synapses. 93% of brain neurons had a homolog in the opposite hemisphere. Of the synapses, 66.6% were axo-dendritic, 25.8% were axo-axonic, 5.8% were dendro-dendritic, and 1.8% were dendro-axonic.
To study the connectome, they treated it as a directed graph with the neurons forming nodes and the synapses forming the edges. Using this representation, Winding et al found that the larval brain neurons could be clustered into 93 different types, based on connectivity alone. These types aligned with the known neural groups including sensory neurons (visual, olfactory, gustatory, thermal, etc), descending neurons, and ascending neurons.
The authors ordered these neuron types based on proximity to brain inputs vs brain outputs. Using this ordering, they could quantify the proportion of recurrent connections, as the set of connections going from neurons closer to outputs towards inputs. They found that 41% of all brain neurons formed a recurrent connection. The neuron types with the most recurrent connections were the dopaminergic neurons (57%), mushroom body feedback neurons (51%), mushroom body output neurons (45%), and convergence neurons (42%) (receiving input from mushroom body and lateral horn regions). These neurons, implicated in learning, memory, and action-selection, form a set of recurrent loops.
A natural question is whether the connectome will allow simulation of the fly's behavior. However, the connectome alone is not sufficient. Additional information needed includes gap junction varieties and locations, identities of neurotransmitters, receptor types and locations, neuromodulators and hormones (with sources and receptors), the role of glial cells, time evolution rules for synapses, and more.[22][23]
The fruit fly connectome has been used to identify an area of the fruit fly brain that is involved in odor detection and tracking. Flies choose a direction in turbulent conditions by combining information about the direction of air flow and the movement of odor packets. Based on the fly connectome, processing must occur in the “fan-shaped body” where wind-sensing neurons and olfactory direction-sensing neurons cross.[24][25]
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