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A Drosophila glial cell atlas reveals a mismatch between transcriptional and morphological diversity [1]

['Inês Lago-Baldaia', 'Department Of Cell', 'Developmental Biology', 'University College London', 'London', 'United Kingdom', 'Maia Cooper', 'Austin Seroka', 'Institute Of Neuroscience', 'Howard Hughes Medical Institute']

Date: 2023-10

Morphology is a defining feature of neuronal identity. Like neurons, glia display diverse morphologies, both across and within glial classes, but are also known to be morphologically plastic. Here, we explored the relationship between glial morphology and transcriptional signature using the Drosophila central nervous system (CNS), where glia are categorised into 5 main classes (outer and inner surface glia, cortex glia, ensheathing glia, and astrocytes), which show within-class morphological diversity. We analysed and validated single-cell RNA sequencing data of Drosophila glia in 2 well-characterised tissues from distinct developmental stages, containing distinct circuit types: the embryonic ventral nerve cord (VNC) (motor) and the adult optic lobes (sensory). Our analysis identified a new morphologically and transcriptionally distinct surface glial population in the VNC. However, many glial morphological categories could not be distinguished transcriptionally, and indeed, embryonic and adult astrocytes were transcriptionally analogous despite differences in developmental stage and circuit type. While we did detect extensive within-class transcriptomic diversity for optic lobe glia, this could be explained entirely by glial residence in the most superficial neuropil (lamina) and an associated enrichment for immune-related gene expression. In summary, we generated a single-cell transcriptomic atlas of glia in Drosophila, and our extensive in vivo validation revealed that glia exhibit more diversity at the morphological level than was detectable at the transcriptional level. This atlas will serve as a resource for the community to probe glial diversity and function.

Data Availability: All raw and processed transcriptome data for the embryonic dataset are available from NCBI GEO (accession GSE208324; https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE208324 ). The scripts used to process the raw RNA-seq data and extract neuronal and glial clusters from the embryonic dataset are available at https://github.com/AustinSeroka/2022_stage17_glia . All other scripts, including midline glia annotation from the whole embryonic dataset, cleaned-up and annotated embryonic glial dataset, as well as the integrated, cleaned-up and annotated young adult optic lobe glial dataset are available at https://github.com/VilFernandesLab/2022_DrosophilaGlialAtlas . We used published single cell RNA sequencing datasets of the optic lobes: Özel et al. (2021) (NCBI GEO accession GSE142787; https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE142787 ) and Kurmangaliyev et al. (2020) (NCBI GEO accession GSE156455; https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE156455 ). Numerical values used to generate graphs are included in S5 Data file. The cleaned-up and annotated embryonic glial dataset and the integrated cleaned-up and annotated young adult optic lobe glial dataset are included here as source data files: S6 and S7 Data files, respectively. All other relevant data are within the paper and its Supporting information files.

Here, we leverage the simple and tractable Drosophila nervous system to explore the relationship between glial morphological diversity and transcriptional signatures. We generated and validated a single-cell atlas of all glial classes in 2 distinct Drosophila circuits—the embryonic VNC and the adult optic lobe. We chose these circuits as they are well characterised with many existing tools and reagents, they span 2 different circuit types (sensorimotor versus pure sensory), and they represent 2 distinct developmental stages. We identified several new glial morphological categories but found no clear correlation between glial morphological diversity and detectable transcriptional diversity. Moreover, we found that class-specific transcriptomes were conserved from embryo to adult, despite changes in circuit location and developmental stage. One exception was the glia of the optic lobe lamina, which accounted for the majority of glial diversity at a transcriptional level. The lamina and its associated glia lie in close proximity to an environmental interface, positioned immediately below photoreceptors of the compound eye; these glia were enriched for gene expression associated with immune-related functions. Our data suggest that within-class (e.g., astrocytes) glial morphological categories cannot be assumed to correspond to transcriptionally distinct subclasses. Instead, we propose that glia adopt different morphological and functional states in response to cues from their local environment.

(A) Schematic of the cross-section of the adult optic lobe and its 4 neuropils: lamina, medulla, lobula, and lobula plate with glial classes indicated. Dashed lines indicate the layers of the specific neuropil. Cortex (neuronal cell bodies) indicated in grey. (B) Maximum projection showing MCFO clones of perineurial glial cells covering the medulla, lobula, and lobula plate. Cells were oblong-shaped and tiled together. (C) A cross-sectional view of the lamina showing a chalice glial cell (a type of lamina perineurial glia at the rim of the lamina cortex and neuropil). (D) A cross-sectional view of the lamina showing fenestrated glial cells (a type of lamina perineurial glia), separating the compound eye from the lamina. (E) Maximum projection showing MCFO clones of subperineurial glia that covered the medulla, lobula, and lobula plate as large squamous cells that tiled together. (F) A cross-sectional view of the lamina showing a single carpet glia (a type of lamina subperineurial glia), along the rim of the lamina cortex and neuropil. (G) A cross-sectional view of the lamina showing pseudocartridge glia (a type of lamina subperineurial glia), as irregularly shaped cells above the lamina cortex. (H) A cross-sectional view of the medulla, lobula, and lobula plate showing a single cortex glial cell in the lobula cortex with typical membranous and honeycomb-like morphology. (I) A cross-sectional view of the lamina showing distal satellite glia (a type of lamina cortex glia), excluded from the most proximal region of the lamina cortex. Note that inner chiasm glia were also labelled by this driver (bottom). (J) A cross-sectional view of the lamina showing proximal satellite glia in the most proximal region of the lamina. Inset shows a single proximal satellite glia labelled by repo-Gal4. (K) A cross-sectional view of the medulla, lobula, and lobula plate showing MCFO clones labelling ensheathing glia and a subset of neurons (asterisk). (L) Examples of ensheathing glia in the distal medulla sending primary projections from the surface of the neuropil to layer M7. (M) An example of an ensheathing glial cell in the lateral medulla sending a primary from the outer neuropil surface inwards along layer M7. (N) An example of an ensheathing glial cell in the lobula plate neuropil sending several short processes with minimal secondary branches into the neuropil. (O) A cross-sectional view of the lamina showing marginal glia (a type of lamina-specific ensheathing glia), which project partway into the lamina neuropil from the distal neuropil surface. (P) A cross-sectional view of the optic lobe showing 2 morphologically distinct tract ensheathing glia (called chiasm glia) located in the outer chiasm between the lamina and medulla, or the inner chiasm between the medulla, lobula, and lobula plate. Outer chiasm glia did not send projections into the neuropils, but the inner chiasm glia projected into all 3 neuropils. The dashed line separates images in the same brain on different z-stacks. (Q) A cross-sectional view of the medulla, lobula, and lobula plate showing astrocytes. (R) Examples of distal medulla astrocytes with short and long morphologies. The short distal medulla astrocyte (asterisk) projected to layer M6, while the long distal medulla astrocyte (arrowhead) projected to layer M8. (S) An example of a lateral medulla astrocyte projecting into the neuropil laterally along the M7/8 layers. (T) An example of a proximal medulla astrocyte (also called chandelier glia) projecting from the proximal surface of the medulla, up to layers M9 and M10. (U, V) Examples of astrocytes in the lobula and lobula plate, which (U) projected into the lobula neuropil from the cortex-neuropil border or (V) projected into both the lobula and pobula plate neuropils. (W) Example of an epithelial glial cell (right; the lamina astrocyte population), which projected across the entire distal-proximal neuropil length. Cyan, yellow, magenta, and purple mark the MCFO clones, while white labels HRP (labels the neuropils) in the main panels. Insets show the MCFO clones in greyscale. Dashed lines are used to outline neuropil borders or separate an inset. All scale bars represent 20 μm. HRP, horseradish peroxidase; MCFO, multicolour flip-out.

(A) Schematics of the embryonic CNS along different axes with the 5 glial classes indicated. Cortex (neuronal cell bodies) indicated in grey. (B) Dorsal surface view of the VNC showing surface-only and channel-associated perineurial and subperineurial glial cells on the surface with characteristic fibrous membranes and loose tiling. (C) Cross-sectional view of the VNC showing the same cells as in (B) now showing the tapered projection from a (dorsal) channel-associated perineurial glial cell that tiles with neighbouring surface-only perineurial glial cells. (D) Lateral view of the VNC showing channel-associated perineurial glia on the ventral and dorsal surfaces, each sending a single projection with ventral channel-associated perineurial glia sending longer processes than their dorsal counterparts. (E) Ventral surface view of the VNC showing surface-only and channel-associated perineurial and subperineurial glial that tile with each other loosely. (F) Cross-sectional view of the VNC showing the same cells as in (C) now showing the tapered projection from a (dorsal) channel-associated perineurial glial cell that tiles with a neighbouring surface-only perineurial glial cell. (G) Surface view of the VNC showing polygonal-shaped surface-only subperineurial glia on the surface. (H) Surface view of the VNC showing polygonal-shaped surface-only subperineurial glia (cyan) on the surface and an underlying channel-only subperineurial glial cell (magenta). (I) Cross-sectional view of the VNC showing the same cells as in (H). (J, K) Surface view (J) and oblique view (K) of the VNC showing 2 ventral surface- and channel-associated subperineurial glia (cyan and magenta) with extensions towards the neuropil. (L) Lateral view of the VNC showing subperineurial glia on the ventral and dorsal surfaces, sending projections along the channels forming tube-like structures. (M) Cross-sectional view of the VNC showing a single cortex glial cell in the cortical region forming a membranous, honeycomb-like structure. (N, O) Lateral view (N) and cross-sectional view (O) of the VNC showing ensheathing glial cells at the border of the cortex and neuropil. Tract ensheathing cells extend longer or shorter processes through axon tracts entering the neuropil, while neuropil ensheathing cells only extend processes along the neuropil border. (P) Cross-sectional view of the VNC showing a type 1 astrocyte and a type 2 astrocyte sending processes into the neuropil. Type 1 astrocyte processes were highly ramified, whereas type 2 astrocyte processes were less ramified. Co-expression of Pros and Repo indicated astrocyte identity (insets). (Q–S) Cross-sectional views of the VNC showing single astrocytes in greyscale, from dorsal, lateral, and ventral nucleus positions, with corresponding morphological type 1 or 2 indicated. Green marks Repo, and cyan and magenta mark MCFO clones in all panels, except (P) with Pros in magenta. Dashed lines outline the neuropil and full lines outline the VNC. Clones represent samples at 0 h after larval hatching. All scale bars represent 10 μm. CNS, central nervous system; MCFO, multicolour flip-out; VNC, ventral nerve cord.

Drosophila glia share several key morphological and functional attributes with their vertebrate counterparts, including maintaining neurotransmitter and ionic homeostasis, providing trophic support for neurons, acting as immune cells, and modifying neural circuit function [ 1 , 18 – 20 ]. In the Drosophila CNS, neuropils contain synaptic connections, while neuronal cell bodies are located at the cortex, around the periphery of neuropils; axon tracts connect different neuropils to each other. Drosophila glia can be categorised based on morphology and by their association with these anatomical structures as either outer or inner surface glia, cortex glia, ensheathing glia, or astrocytes (Figs 1 and 2 ). Surface glia comprise 2 sheet-like glia called the perineurial and subperineurial glia [ 20 , 21 ]. Together, these form a double-layered surface that spans the nervous system, which acts as a blood (or hemolymph)–brain barrier (BBB) [ 20 , 21 ]. Cortex glia envelop neuronal cell bodies in cortical regions of the CNS, whereas ensheathing glia can wrap axonal tracts between neuropils (aka tract ensheathing glia) or wrap neuropil borders [ 20 , 22 ]. Astrocytes also inhabit neuropil regions with ensheathing glia, but extend many fine projections into the neuropil to associate with neuronal synapses, akin to vertebrate astrocytes [ 20 , 23 ]. Although Drosophila has a simplified nervous system with reduced numbers of glia relative to mammals, striking morphological diversity exists between and within glial cell classes during development and in the adult [ 22 , 24 – 26 ]. For example, in the highly ordered visual system, astrocytes of distinct morphologies can be found across neuropils and within the same neuropil [ 24 , 27 ]. Whether these morphological categories correspond to distinct subclasses with unique transcriptional profiles and functions is not known.

Morphological diversity among glia has been documented alongside that of neurons for over a century [ 3 ]. This morphological heterogeneity exists not only between broad glial classes (i.e., astrocytes, oligodendrocytes, Schwann cells, and microglia) but also within classes [ 4 – 7 ]. It has long been appreciated that mammalian astrocytes from different brain regions vary in morphology [ 3 , 8 , 9 ], and recent advances in RNA-sequencing (RNA-seq) technologies have revealed regionalized molecular diversity in astrocytes and other central nervous system (CNS) glial cell classes (e.g., oligodendrocyte progenitor cells and microglia) [ 10 – 16 ]. Confoundingly, astrocytes are known to be highly plastic cells. Most notably, in response to injury, astrocytes undergo a process called astrogliosis, become “reactive,” and alter their morphology dramatically [ 17 ]. It is clear that astrocyte reactivity represents a change in cell state due to underlying differences in environment. Thus, in healthy conditions, it is difficult to distinguish whether the morphological diversity of astrocytes is a consequence of cell-fate diversity and/or cell-state. In other words, what is the relationship between glial morphology and transcriptional profile?

Nervous systems contain more distinct cell types than any other organ. This cellular diversity underlies the complexity and multifunctionality of circuits and processing networks in the brain and, thus, defines the breadth of an animal’s behavioural repertoire. Not surprisingly, categorising neural cell types has long been, and continues to be, a major endeavour in the field. Although much emphasis has been placed on categorising neuronal diversity, we know much less about the extent of glial diversity. Given their pivotal roles in every aspect of nervous system development and function [ 1 , 2 ], understanding glial diversity is also imperative.

Results

Morphological diversity of embryonic Drosophila glia To determine the relationship between glial morphology and transcriptional signature, we began by characterising glial morphology across distinct Drosophila brain regions and developmental stages. We focused on glia in the VNC, akin to the vertebrate spinal cord, in the late embryonic (stage 17) CNS. The developing VNC contains 5 major glial classes: astrocytes, ensheathing glia, cortex glia, and 2 types of surface glia, which are all neuroectodermal in origin and express the marker reversed polarity (repo) [20]. We used enhancer trap Gal4 drivers expressed in each of these glial cell classes to sparsely label cells using the multicolour flip-out (MCFO) cassette. We then visualised single glial cell morphology at 0 hours after larval hatching (0h ALH) to assess morphological diversity both within and between classes (Fig 1). Note that in addition to the 5 major glial classes described above, the VNC contains a distinct class called the midline glia, which are a transient population found only during embryonic and larval stages [28–31]. Although midline glia express wrapper, otherwise known as a cortex glia marker [32–34], they do not resemble cortex glia in form or function but instead ensheath commissural axons and play critical roles in axon guidance and VNC morphogenesis [35]. Moreover, unlike the other major glial classes described above, midline glia are mesectodermal in origin [36,37] and do not express the pan-glial marker repo or the broad glial marker glial cells missing (gcm) [35]. Midline glia have been characterised extensively by several groups [35,38–42]; therefore, given their distinct origin and the ambiguity surrounding their functional classification, we instead focused our analyses on repo+ glia.

VNC surface glia The Drosophila CNS is bathed in circulating hemolymph, which contacts the VNC at its main surface and along dorsoventral channels that perforate the VNC along the midline between pairs of longitudinal connectives and neighbouring neuromeres [43,44]. The surface glia comprise 2 glial classes, perineurial on the outer surface of the VNC and subperineurial glia positioned below perineurial glia [20,21]. Early glial studies did not distinguish between these 2 classes and referred to both collectively as subperineurial glia for their position below the perineurium [44,45]. An early characterisation of gene expression in glia of the embryo reported CG5080 and moody expression in surface glia along the main VNC surface and associated with the dorsoventral channels (“channel glia”), a result since confirmed by bulk RNA profiling of surface glia [45,46]. It is important to note that these studies did not evaluate glial morphology, whether CG5080 and moody were expressed by both surface glial classes, nor whether they were co-expressed in the same cells [45,46]. Indeed, Moody has since been shown to be expressed exclusively in subperineurial glia [47,48]. However, CG5080 expression has not been characterised further, thus implying 3 possibilities: (i) it may be expressed in both perineurial and subperineurial glia; (ii) it may be expressed in perineurial glia exclusively; or (iii) it may be expressed in subperineurial glia exclusively. To analyse surface glia morphology in the VNC, we used the CG5080-Gal4 and moody-Gal4 drivers to generate MCFO clones. CG5080-Gal4-labelled cells with fibrous membranes and which tiled loosely with each other to cover the main surface of the VNC, typical characteristics of perineurial glia described by others [38] (Fig 1A–1F). Therefore, it is likely that CG5080-Gal4-labelled cells that belong to the perineurial glial class. Interestingly, CG5080-Gal4-labelled cells located along the midline of the dorsal and ventral surfaces of the VNC each sent a single tapered projection inward along the dorsoventral channels (Figs 1C, 1D, 1F, and S1A–S1C; N = 51 clones from N = 13 brains). When viewed at the surface, these cells tiled with and were indistinguishable from the other CG5080-Gal4-labelled surface-only cells (Fig 1B–1F; N = 140 clones from N = 13 brains). Ventral midline cells projected further than their dorsal counterparts and were present along the entire anterior-posterior axis of the VNC, whereas cells that projected from the dorsal surface were observed with lower frequency towards more posterior positions of the VNC (Figs 1D and S1A). To further confirm that the channel-associated cells belong to the perineurial glial class, we used CG5080-Gal4 to label individual cells while co-labelling all glia with repo-LexA>LexAop-myr::tdTomato. We observed that the channel-associated cells occupied the outermost glial surface of the VNC (S1B and S1C Fig). Taken together with the fact that these cells tiled with surface-only perineurial glia, these data argue that they are a subclass of perineurial glia. Hereafter, we refer to them as “channel-associated perineurial glia.” Since moody is a known marker of subperineurial glia [47,48], we next generated MCFO clones labelled with moody-Gal4. Although we recovered cells at the surface of the VNC and associated with the dorsoventral channels, these did not resemble clones labelled by CG5080-Gal4 (Fig 1A–1F). Instead, consistent with previous reports of subperineurial glia [38], moody-Gal4 MCFO clones at the surface of the VNC were polygonal in shape, tiled tightly with each other, were larger than CG5080-Gal4-labelled cells, and displayed a honeycomb-like pattern within their cell boundaries, as a result of their membranes cupping neuronal cell bodies at the outer surface of the cortex [25] (Figs 1A, 1G–1L, and S1D). While most cells labelled by moody-Gal4 were only associated with the main surface of the VNC (69.3% of total clones were associated with the main surface only; N = 101 clones from N = 12 brains), some of the cells, present along the VNC midline sent short (non-tapering) projections along the dorsoventral channels (21.8% of total clones). In addition, moody-Gal4 also labelled cells that exclusively lined the dorsoventral channels (8.9% of total clones; Fig 1H and 1I). Thus, we observed a continuum of glial morphologies between the 2 extremes of surface-only and channel-only subperineurial glia (Fig 1J–1L). Importantly, glia with intermediate morphologies (i.e., which associated with both the surface and the channels) tiled tightly with neighbouring surface-only associated cells (Fig 1H and 1I). As aforementioned, CG5080 and moody were reported to be expressed in surface glia though co-expression and expression across both surface glial classes were not assessed [45,46]. Here, we used CG5080-Gal4 and moody-Gal4-labelled MCFO clones to evaluate cell morphology, tiling properties, size and position, and clarify whether these labelled the same or different surface glial classes. Consistent with previous reports [38,47,48], our data argue that moody-Gal4 labels the subperineurial glial class, which can be further subdivided into surface-only, surface- and channel-associated, and channel-only morphological categories. By contrast, our data suggest that CG5080-Gal4 labels the perineurial glial class, which can be further subdivided into surface-only and channel-associated morphological categories. CG5080-Gal4-labelled cells did not resemble cells labelled by moody-Gal4. Instead, they covered smaller domains at the outermost surface of the brain, possessed fibrous membranes and tiled loosely with each other, all features that are associated with perineurial glia [38]. Thus, we confirm moody as a marker of subperineurial glia and identify CG5080 as a new marker of perineurial glia. Moreover, we show that surface glia associated with the dorsoventral channels (referred to as “channel glia” in early reports) [44,45] also consist of perineurial and subperineurial glia, which had not been appreciated previously.

Optic lobe glia display morphological diversity within glial classes In addition to the relatively simple VNC, we also focused on the adult optic lobe, which is more structurally complex than the VNC with 4 distinct neuropils called the lamina, medulla, lobula, and lobula plate (Fig 2A). Several other groups have characterised the morphology of each of the 5 major glial classes (perineurial, subperineurial, cortex, ensheathing, and astrocyte) present in the optic lobe in detail [24,25,27]. Therefore, we used previously characterised Gal4 lines [24,25,54] (S3 Fig) to generate MCFO clones to visualise glial morphologies and validate morphological diversity within each class (summarised in Fig 2; see Materials and methods for more details).

Perineurial glia morphologies were transcriptionally distinct Our previous clonal analysis unveiled 2 perineurial glia morphologies: surface-only perineurial glia and channel-associated perineurial glia (Figs 1A–1F and S1). Validating cluster marker expression in vivo revealed that surface-only and channel-associated perineurial glia were transcriptionally distinct, with cluster #4 corresponding to surface-only perineurial glia and cluster #10 corresponding to channel-associated perineurial glia. Clusters #4 and #10 were located adjacent to each other on the UMAP. PRL-1 and pippin were enriched in cluster #4 (Fig 4B) and Gal4 drivers for these markers predominantly labelled clones with surface-only perineurial glial morphology (84.6% of 16 brains contained exclusively surface-only perineurial glia clones labelled by PRL-1-Gal4; 87.5% of 13 brains contained exclusively surface-only perineurial glia clones labelled by pippin-Gal4; Figs 4B, 4C, S11C, S11D, and S11J). By contrast, cluster #10 expressed high levels of CG6126 and CG5080, but low expression of PRL-1 and pippin (Fig 4B). Consistent with these expression patterns, 100% of brains (N = 11 brains) contained both surface-only and channel-associated perineurial glia clones labelled by CG6126-Gal4, and 92.9% of brains (N = 13 brains) contained both surface-only and channel-associated perineurial glia clones labelled by CG5080-Gal4 (Figs 1B–1F, 4D, and S11J). Thus, the 2 perineurial glia morphologies are transcriptionally distinct.

Transcriptional profiles for subperineurial glia, ensheathing glia, and astrocytes did not distinguish morphological categories Our prior clonal analyses revealed morphological heterogeneity in the subperineurial glia population, along a continuum from surface-only to channel-only (Fig 1H–1L), as well as 2 morphological categories within ensheathing glia: neuropil-only and tract-associated (Fig 1N and 1O), and 2 morphological categories within astrocytes: type 1 and type 2 (Fig 1P–1S). Interestingly, while our annotations revealed separate transcriptional clusters corresponding to surface-only perineurial glia and channel-associated perineurial glia (Figs 4A–4D and S11), we could resolve just 1 transcriptional cluster for subperineurial glia based on strong expression of CG10702, Ntan1, and moody (cluster# 9; Fig 4B and 4E); 1 transcriptional cluster for ensheathing glia based on strong expression of Eaat2 (cluster #7; Figs 1N, 1O, 4B, and 4G); and 1 transcriptional cluster for astrocytes based on strong expression of alrm (cluster #3; Figs 1P–1S and 4B). We questioned whether any of the markers enriched in our astrocyte cluster might distinguish type 1 from type 2 astrocytes. To this end, we generated MCFO clones under the control of enhancers for genes expressed in the astrocyte cluster. All drivers gave rise to clones containing both type 1 and type 2 astrocytes at the expected proportions based on our alrm MCFO study (approximately 38% type 1 and approximately 62% type 2), with the exception of pum, which gave a slightly higher proportion of type 1 astrocytes (52%), but still contained both clones (Figs 4A, 4B, 4H, S12, and S13). Similarly, we failed to identify any markers that distinguished the subperineurial glial morphologies or the ensheathing glial morphologies (Figs 4E, 4G, S11, and S13). Thus, although distinct perineurial glia morphologies corresponded to distinct transcriptional signatures, the same was not true for subperineurial, ensheathing, or astrocyte morphologies, with the caveat that scRNA-seq may fail to detect genes that are expressed at low levels. Overall, these data suggest that morphological diversity cannot be equated with transcriptional diversity, at least at present levels of detection. As the developing Drosophila VNC contains a single, simple, neuropil, which may not accurately represent the diversity of more complex brain regions, we next turned to the more complex adult Drosophila optic lobe to assess how accurately cellular identity can be gauged by glial morphology.

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