Bridging form and function: the first comprehensive comparison of the male and female fly brain

In Chapter 10, we learned how some behavioral patterns are genetically preordained to differ across the sexes. Female house mice care for pups while males range from indifferent to aggressive, and male fruit flies perform a stereotyped courtship ritual that females observe closely to discern their fitness. In each case, genetic differences specifying morphological sex also cause neural circuits to change their form, wiring the same sensory cues to different behavioral outputs in each sex to ensure the correct response.

Such sexual dimorphism — a product of divergent, sometimes competing, evolutionary pressures on each sex — has been extensively characterized in Drosophila. In the fly nervous system, sex is determined at the single-cell level by the expression of different isoforms of two transcription factors, doublesex (dsx) and fruitless (fru). These regulate an array of downstream genes to endow some neurons with sex-specific features: some may survive in only one sex, while others change their morphology, physiology, or connectivity to enable sex-specific computations. However, the question of how far the dimorphism actually extends has remained largely out of reach to researchers. How common is it for one cell to take on different forms across the sexes? Is such dimorphism largely restricted to the gross morphological form of neurons, or is there also dimorphism at a far more granular synaptic level? And, perhaps most intriguingly, how are large sex-specific modules incorporated into an existing, already extraordinarily complex, neural network without disrupting its normal function? 

To begin to address to these questions, Berg et al. undertook the herculean effort of reconstructing the entire central nervous system of an adult male fruit fly for the first time using FIB-SEM-based connectomics (discussed in Chapter 14). The authors painstakingly matched cell types from their dataset to existing connectomes of the female fly brain, and sorted each one into a category: isomorphic (present and equivalent in both sexes), dimorphic (present in both but noticeably different in shape), or sex-specific (found in only one sex). As expected, the overwhelming majority of cell types are simply isomorphic. The exceptions are where the story lives: the authors identify 289 male-specific cell types against 71 female-specific ones, with males possessing more than three times as many sex-specific neurons in the central brain as females. 

However, Berg et al.’s most elegant achievement is not their delineation of the extent of dimorphism, but their characterization of where it concentrates. The authors demonstrate that sensory neurons and motor output are almost entirely isomorphic, while dimorphic and sex-specific neurons are aggregated in the central brain's higher-order, integrative layers. Sex differences thus appear to primarily modify the circuits that link sensation to action, leaving sensory transduction and motor implementation largely untouched. The authors walk through examples of this higher order dimorphism in multiple modalities. Visually, Drosophila has no obvious anatomical analog of the enlarged "love spot" seen in some other flies, but the connectome reveals a functional one: a small set of frontally biased visual projection neurons feed disproportionately into male-specific and dimorphic circuitry, rerouting information from the frontal visual field, where a courting male keeps the female, toward courtship-relevant pathways. Across olfaction and taste, pheromone-responsive pathways are dimorphic mainly downstream of largely isomorphic first-order sensory neurons. The periphery thus appears to largely stay constant while the interpretation and routing of cues is tinkered with.

Another striking finding is how dimorphism isn't limited to growing an entirely new arbor. While some cell types differ between sexes through brand-new branches, others just shift or expand existing ones. Most intriguingly, plenty of connectivity changes happen with no visible change in shape at all — a phenomenon the authors call "secondary dimorphism." Such secondary dimorphism is surprisingly common, touching roughly 7% of male central brain neurons. Song-tuned vpoEN neurons are a striking case: indistinguishable in shape between the sexes, they relay the same auditory signal to vaginal plate opening circuitry in females but to male-specific pC1 neurons in males. Correspondence between dimorphism and fru/dsx expression is also strong but not absolute — about 90% of male-specific central brain neurons express these genes, but roughly 10% do not; over 2,000 fru/dsx+ neurons look perfectly isomorphic, suggesting these genes sometimes tune physiology rather than wiring; and while fru+ neurons preferentially connect with one another in tightly coupled hubs, some sex-specific types are instead sparsely woven into the shared network. Sex-specific circuitry is therefore largely modular, but not sealed off: evolution appears to have flexibly integrated both dimorphic and isomorphic circuit elements to efficiently enable both shared and divergent neural computations. 

None of this comes with a promise that the hard work is done. The authors are candid that each sex is represented by a single animal, and that separating genuine sex differences from individual and technical variability required comparing cell types rather than raw wiring diagrams. What Berg et al. have built is a structural scaffold: turning "this circuit is wired differently" into "this is why the behavior differs" still needs simulation and functional experiments, and linking molecular identity to connectomic cell types remains its own unsolved puzzle. But as a scaffold, it's quite extraordinary — enabling us to ask where, precisely, the form of neural circuits is reshaped to produce sex-specific function.

Figure 1: A complete connectome of the male Drosophila central nervous system has been constructed (top left), enabling comparisons at the single-cell level to existing female connectomes (top right). Most neurons in the male connectome correspond closely to cell types found in the female connectomes, as shown in the bottom right image. A subset of neurons, however, does not appear to have a corresponding cell type in the female brain. For example, the neuron AVLP710m found in males most closely corresponds to AVLP016 in the female brain – but, AVLP016 differs dramatically in morphology and is matched to another male cell type (bottom middle). AVLP710m thus does not exist in the female brain. Finally, some neurons, like AOTU008 shown in the bottom right image, exist in both male and female brains but look different: in males, but not in females, AOTU008 extends a downward branch in each hemisphere (black arrows). 

Reference: 

Berg, S., Beckett, I. R., Costa, M., Schlegel, P., Januszewski, M., Marin, E. C., Nern, A., Preibisch, S., Qiu, W., Takemura, S. Y., Fragniere, A. M. C., Champion, A. S., Adjavon, D. Y., Cook, M., Gkantia, M., Hayworth, K. J., Huang, G. B., Katz, W. T., Kämpf, F., Lu, Z., … Jefferis, G. S. X. E. (2026). Sexual dimorphism in the complete Drosophila male central nervous system connectome. Cell, 189(18), 5504–5526.e15. https://doi.org/10.1016/j.cell.2026.08.015