KeywordsSynaptic Contact Antennal Lobe Optic Lobe Reporter Line Drosophila Brain
The Drosophila connectome is a comprehensive description of all the connections between subunits comprising the central nervous system (CNS) of the fruit fly, Drosophila melanogaster.
A Drosophila connectome, within the brain, can be constructed at different levels of resolution. At low resolution, a connectome can be said to comprise neuropil compartments, with connections between them defined by neural pathways, as identified by light microscopy. In contrast, at high resolution, a connectome comprises individual neurons and all the chemical synapses between them. The components of such a cellular connectome are identified through electron microscopy (EM).
As of 2013, neuropil-level connectomes have been constructed for the entire Drosophila brain (Chiang et al. 2011), but only partial cellular connectomes have been reported. In particular, cellular connectomes have been reported for a cartridge of the lamina (Meinertzhagen and O’Neil 1991; Rivera-Alba et al. 2011) and a column of the medulla (Takemura et al. 2013), the first and second neuropil compartments, respectively, within the fly optic lobe (transmitting visual information). Despite their incomplete nature, these partial connectomes have already provided valuable insights into the computational functions of neuronal circuits.
Existing Neuropil-Level Connectomes
Large-scale, compartmental maps of the Drosophila brain have been obtained through the use of genetic reporters imaged by light microscopy (Chiang et al. 2011). Each reporter line fluorescently labels a sparse subset of neurons (Jenett et al. 2012; Ito et al. 2013). Aligning images from multiple lines to a standard Drosophila brain, the overlapping arbors of locally arborizing neurons are used to define individual compartments: local processing units (LPUs) (Chiang et al. 2011). The connections between these LPUs are found by identifying neurons projecting between them. In total, the resulting connectome contains 58 tracts between the 41 identified LPUs, a sparse subset (<10 %) of the 820 numerically possible LPU-LPU connections.
Cellular Connectomes and EM
Optic Lobe Cellular Connectomes
The optic lobe of flies has been the subject of experimental interest for over 50 years, driven by the advanced visual behaviors of flies (Heisenberg and wolf 1984). Despite the large size of the optic lobe, encompassing almost 50 % of the total CNS volume (Rein et al. 2002), the modular structure of its underlying neuropil compartments, corresponding to the ommatidia array of the compound eye, is highly amenable to connectomics.
The lamina receives most of its input directly from photoreceptors in the retina. It is thought to be mostly responsible for encoding contrast and for light adaptation. A cellular connectome of the repeating unit of the lamina, the lamina cartridge, has been reconstructed twice, in two different wild-type strains (Meinertzhagen and O’Neil 1991; Rivera-Alba et al. 2011). Both reconstructions are similar, with most differences apparently introduced by improvements in reconstruction quality and by differences in the interpretation of some postsynaptic elements. The consensus lamina connectome contains the terminals of six photoreceptors, along with 12 types of neurons (Rivera-Alba et al. 2011; Tuthill et al. 2013). In total, each cartridge contains ~480 presynaptic sites, contacting ~1,250 postsynaptic sites.
Directly downstream of the lamina, the medulla is the single largest neuropil of the fly’s brain and receives visual input both through the lamina and directly from a subset of photoreceptors. It is thought to be responsible for (1) the computation of local visual motion, by the comparison of inputs from different points in space, and (2) color vision, utilizing the direct inputs from spectrally tuned photoreceptors.
Characteristics of the Drosophila Connectome
Presynaptic terminals within the existing connectome reconstructions are typically of uniform size, and the number of synaptic contacts between different neuron pairs ranges over more than two orders of magnitude (1–150). Hence the synaptic weight for any pair is thought to be well approximated by the number of synapses in parallel. However, it is not known if it will be possible to generalize this quantitative assumption to the rest of the Drosophila connectome.
Synapse numbers for identified neurons are consistent across neurons of the same type in the lamina and medulla. However, as has been seen in both the antennal lobe (Chou et al. 2010) and the mushroom body (Murthy et al. 2008; Caron et al. 2013), this may not generalize to other parts of the brain. In general, learning associated or other forms of plasticity may decrease the stereotypy in the numbers of synaptic contacts, thereby increasing the difficulty of obtaining a single, defined connectome.
The Utility of Connectome Information: Functional Studies
One rationale for compiling a cellular connectome is to gain insight into the neuronal implementations of behavior. The existing optic lobe connectomes have already demonstrated such utility. They have been used to direct the attention of genetic experiments towards a specific neuron(s) (Gao et al. 2008) and to provide the neuronal circuit substrates for computational analyses (Takemura et al. 2013). For example, the comprehensive nature of the cellular connectome reconstruction in the medulla allowed the identification of candidate neurons comprising a circuit computing local motion, a computation that had been intensely studied for more than 50 years (Borst et al. 2010), yet with limited prior insight.
As of 2013, ongoing Drosophila reconstruction projects include the larval CNS connectome (Cardona et al.), and the connectome for seven medulla columns, utilizing a new imaging technique: focused ion beam milling scanning EM (FIB-SEM). FIB allows improved z-axis resolution, enabling more complete tracing of neurites thinner than the section thickness. This results in a more complete connectome, given that only 50 % of the postsynaptic sites could be traced to an identified neuron in the ssTEM dataset. Further, by including multiple columns, the resulting connectome should also allow an estimate of the variation between columns, as well as the exploration of color vision circuits (Franceschini et al. 1981).
We are obviously at the early stages of assembling the components of the Drosophila cellular connectome. However, from the current rapid pace of improvements in sample preparation, imaging, and software tools for reconstruction, it seems possible that the cellular connectome will become available for the entire fly’s CNS. This will require improvements in staining and sample preparation, increased reliability and speed of imaging, and improved automated image analysis algorithms. Additionally, information on the polarity of synaptic transmission, and the presence of electrical synapses, neither visible from EM, must be incorporated from other techniques (Meinertzhagen and Lee, 2011). However, even prior to the completion of the entire connectome, the demonstrated utility of the existing pieces of the Drosophila connectome suggests that each additional reconstruction should provide valuable biological insight.
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