Transcriptomic and functional characterization indicate sexual dimorphism of discrete circadian neuron subtypes

This study reveals that sexual dimorphism in the *Drosophila* circadian network arises from sex-specific gene expression in distinct clock neuron subtypes, which utilize differential cell adhesion molecules (dpr9 in males and dpr3 in females) to establish unique synaptic connections with downstream regulators of dimorphic behaviors.

The Gist

The Big Picture: Why Do Males and Females Act Differently at Different Times?

Imagine the brain as a massive, bustling city. Inside this city, there is a Master Clock Tower (the circadian system) that keeps time for everyone. This tower tells the city when to wake up, when to sleep, and when to be active.

For a long time, scientists thought this Clock Tower worked exactly the same way in both male and female flies. They knew that males and females act differently (for example, males are very active in courting at specific times, while females are busy with egg-laying), but they didn't know how the clock told them to do these different things.

This paper is like a detective story. The researchers went inside the Clock Tower, looked at the individual workers (neurons), and discovered that the workers themselves are different depending on whether they are in a male or female city. They found specific "gendered" workers who act as special messengers, connecting the time-telling clock to the parts of the brain that control male and female behaviors.

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The Investigation: Taking a "Selfie" of Every Worker

To solve the mystery, the researchers used a high-tech camera called single-cell RNA sequencing. Think of this as taking a high-resolution "selfie" of every single neuron in the clock tower to see what kind of "uniform" (genes) they are wearing.

They took pictures of thousands of neurons from both male and female flies. When they compared the photos, they found something surprising:

1. Most workers look the same: The majority of the clock neurons in males and females are identical twins. They wear the same uniform and do the same job.
2. But some workers are different: They found four specific groups of workers (subtypes of neurons) that were totally different between the sexes.
   • In males, these workers wore a "Male Uniform."
   • In females, these workers wore a "Female Uniform."

These specific groups were mostly found in the Lateral Neurons (LNds), which are like the "Evening Shift" workers of the clock.

The Uniforms: What Makes Them Different?

What was on these different uniforms? The researchers found that the most important difference was in the tools the workers carried.

Specifically, they carried different Cell Adhesion Molecules (CAMs).

• The Analogy: Imagine the neurons are like construction workers. To build a bridge to the next building, they need specific tools.
  • Male workers carried a tool called dpr9.
  • Female workers carried a tool called dpr3.

These tools are like "molecular glue" or "connectors." They determine which other buildings (neurons) the worker can connect to. Because the males and females had different tools, they built different bridges to different parts of the brain.

The Connection: Building the Bridge to Behavior

The researchers wanted to know: Where do these different workers go?

They discovered that these specific "Evening Shift" workers (the LNds) act as a bridge. They connect the Clock Tower to the Behavior Control Center.

• In the Male City, the workers use their dpr9 tool to build a strong bridge to a specific control center (called pC1 and pCd neurons) that drives male courtship behavior.
• In the Female City, the workers use their dpr3 tool to build a bridge to similar control centers that drive female receptivity.

The "Plug" Analogy:
Think of the Clock Tower as a power outlet. The behavior centers (like mating or egg-laying) are appliances. The neurons are the cords.

• In males, the cord has a Male Plug (dpr9) that fits perfectly into the Male Appliance.
• In females, the cord has a Female Plug (dpr3) that fits perfectly into the Female Appliance.
  If you try to plug a Male cord into a Female socket, it doesn't work. The researchers proved this by "breaking" the tools. When they removed the dpr9 tool from males, the bridge collapsed, and the signal from the clock couldn't reach the behavior center. The same happened when they removed dpr3 from females.

The Conclusion: Two Different Cities, One Clock

The main takeaway is that the circadian clock doesn't just tell time; it also knows the sex of the organism.

1. Specialized Messengers: There are specific neurons in the clock that are "sexually dimorphic" (different by sex).
2. Molecular Keys: These neurons use sex-specific molecules (like dpr9 and dpr3) as keys to unlock the correct behavioral pathways.
3. The Result: This ensures that when the clock says "It's 6:00 PM," the male brain hears "Time to court," and the female brain hears "Time to be receptive," because the wiring is physically different.

In summary: The paper reveals that nature doesn't just tweak the volume of the clock for males and females; it rewires the circuits entirely. It uses specific molecular "glue" to ensure the clock talks to the right parts of the brain for the right sex, creating a seamless link between the time of day and the right behavior.

Technical Summary

1. Problem Statement

While many sexually dimorphic behaviors in Drosophila (e.g., mating drive, receptivity, oviposition) exhibit distinct time-of-day preferences, the molecular and circuit mechanisms linking the circadian clock to these sex-specific outputs remain poorly understood. Previous transcriptomic atlases of circadian neurons identified significant heterogeneity but largely ignored sex as a variable. Furthermore, the specific connectivity between circadian neurons and downstream sexually dimorphic circuits (regulated by doublesex and fruitless) has not been functionally mapped. The study aims to:

• Identify sexually dimorphic circadian neuron subtypes at single-cell resolution.
• Determine the molecular basis of this dimorphism, specifically focusing on neural connectivity molecules.
• Functionally validate the synaptic connections between these dimorphic clock neurons and downstream sex-specific circuits.

2. Methodology

The authors employed a multi-modal approach combining high-throughput genomics, connectomics, and functional neurophysiology:

• Single-Cell RNA Sequencing (scRNA-seq):

  • Sample Preparation: Male and female flies expressing GFP in all circadian neurons (Clk856-Gal4, DN3-spGal4) were collected at two circadian time points (ZT06 and ZT18). GFP-positive neurons were isolated via Fluorescence-Activated Cell Sorting (FACS).
  • Sequencing: Libraries were generated using the 10X Genomics platform.
  • Data Integration: The authors integrated a new "sex-informed" dataset (separate male/female processing) with a previously published "sex-inferred" dataset (pooled processing with genetic barcoding). Sex was assigned to cells based on the expression of the male-specific lncRNA roX1 (cutoff: >10 transcripts per 10k UMIs for males; <2.3 for females).
  • Analysis: Unsupervised clustering (Seurat) and differential gene expression (DGE) analysis were performed to identify sex-biased clusters and genes. Gene Ontology (GO) enrichment was conducted using PANGEA.

• Connectomics and Circuit Mapping:

  • Trans-TANGO: A modified restricted trans-synaptic labeling tool was used. DvPDF-Gal4 drove presynaptic ligand expression in LNd neurons, while dsx-LexA restricted postsynaptic labeling to doublesex-expressing neurons.
  • Connectome Analysis: Synaptic connectivity data from the FAFB (female) and MCNS (male) whole-brain connectomes were analyzed to quantify synaptic weights between LNd subtypes (LNd_a/b/c) and dsx-positive neurons (pC1, pCd).

• Functional Validation (Optogenetics & Calcium Imaging):

  • Setup: DvPDF-Gal4 drove the expression of the red-light activatable channelrhodopsin (CsChrimson) in LNd neurons. dsx-LexA drove the expression of the calcium indicator GCaMP6s in downstream neurons.
  • Stimulation: Red light (630 nm) was used to activate LNd neurons while recording calcium transients in dsx-positive postsynaptic neurons.
  • Perturbation: Genetic knockdown (RNAi) of specific Cell Adhesion Molecules (CAMs)—dpr9 (male-enriched) and dpr3 (female-enriched)—was performed in LNd neurons to test their necessity for functional connectivity.

3. Key Contributions and Results

A. Identification of Sexually Dimorphic Circadian Clusters

• Re-clustering: The integration of datasets revealed that two previously distinct LNd clusters (LNd_1 and LNd_2) are actually a single cell type (E3 LNds) separated by sex.
• Dimorphic Subtypes: Four specific clusters showed pronounced sexual dimorphism:
  1. LNd_c (E3 LNds): Cry-negative lateral neurons.
  2. DN3-9a: A specific DN3 cluster.
  3. DN3-9b: A related DN3 cluster.
  4. DN1p-4: A specific DN1p cluster.
• Cell Number Bias: The DN3-9a cluster showed a 3:1 male-to-female cell ratio, consistent with previous reports of higher DN3 numbers in males.

B. Molecular Basis: Enrichment of Connectivity Molecules

• Differential Expression: The dimorphic clusters exhibited significantly higher numbers of differentially expressed genes (DEGs) compared to non-dimorphic clusters.
• Gene Categories: The top enriched Gene Ontology terms were GPCR signaling, neuropeptides, and Cell Adhesion Molecules (CAMs).
• Key Molecules:
  • Male LNd: Enriched for NPF (Neuropeptide F) and the CAM dpr9.
  • Female LNd: Enriched for Proctolin and the CAM dpr3.
  • DN3s: Enriched for sNPF, Dh44, and CG33639.
• Specificity: There was minimal overlap in sex-specific genes between different cell types, suggesting distinct regulatory mechanisms for each dimorphic output.

C. Circuit Connectivity: LNds as Specialized Linkers

• Anatomical Asymmetry: Connectome analysis revealed that Cry-negative E3 LNds (LNd_c) are poorly connected to the core circadian network but possess robust, asymmetric output connections to dsx-expressing pC1 and pCd neurons.
• Sex Differences:
  • Males: LNd_c neurons form strong, repeated connections to pCd-1, pC1b, and pC1c.
  • Females: Connections are less extensive but remain the dominant circadian input to pCd-1 and pC1b.
• Functional Validation: Optogenetic stimulation of LNd neurons elicited significant calcium transients in downstream dsx-positive pC1 and pCd neurons in both sexes, confirming a functional circuit.

D. Mechanism of Wiring: Role of dpr9 and dpr3

• Knockdown Experiments:
  • Knocking down male-enriched dpr9 in male LNds significantly reduced calcium signaling in downstream dsx neurons.
  • Knocking down female-enriched dpr3 in female LNds similarly abolished functional connectivity.
  • Cross-knockdowns (e.g., dpr3 in males) had no significant effect.
• Conclusion: These sex-enriched CAMs are essential for establishing or maintaining the functional synaptic strength between the circadian clock and dimorphic behavioral circuits.

4. Significance

• Mechanistic Insight: The study provides the first molecular and circuit-level explanation for how the circadian clock regulates sexually dimorphic behaviors. It identifies specific "linker" neurons (E3 LNds) that bridge the clock to sex-specific outputs.
• Molecular Logic: It demonstrates that sexual differentiation of neural circuits is mediated not just by transcription factors (fru, dsx) but by the cell-type-specific expression of connectivity molecules (CAMs like dpr9 and dpr3).
• Reinterpretation of Cell Types: The work corrects previous transcriptomic classifications, showing that some "distinct" cell types were actually sex-separated populations of the same neuron type.
• Broader Implications: The findings suggest that sexual dimorphism in behavior arises from specific wiring differences sculpted by sex-enriched adhesion molecules, offering a potential model for understanding sex differences in neural circuits across species.

5. Limitations and Future Directions

• Behavioral Phenotypes: Despite the clear reduction in functional connectivity upon CAM knockdown, the authors did not observe significant defects in standard mating assays (latency/duration). This suggests potential redundancy in the circuit or that these molecules regulate more subtle, context-dependent behaviors not captured in the current assays.
• Developmental Timing: While the study identifies the adult state, the precise developmental window where fru isoforms regulate dpr expression to wire these circuits remains to be fully elucidated.