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1. Reproductive- and Social-State Plasticity of Multiple Sensory Systems in a Cichlid Fish

   https://doi.org/10.1093/icb/icab062  

   Maruska, Karen P ; Butler, Julie M ( May 2021 , Integrative and Comparative Biology)

   Synopsis Intra- and inter-sexual communications are vital to the survival and reproductive success of animals. In species that cycle in and out of breeding or other physiological condition, sensory function can be modulated to optimize communication at crucial times. Little is known, however, about how widespread this sensory plasticity is across taxa, whether it occurs in multiple senses or both sexes within a species, and what potential modulatory substances and substrates are involved. Thus, studying modulation of sensory communication in a single species can provide valuable insights for understanding how sensory abilities can be altered to optimize detection of salient signals in different sensory channels and social contexts. The African cichlid fish Astatotilapia burtoni uses multimodal communication in social contexts such as courtship, territoriality, and parental care and shows plasticity in sensory abilities. In this review, we synthesize what is known about how visual, acoustic, and chemosensory communication is used in A. burtoni in inter- and intra-specific social contexts, how sensory funtion is modulated by an individual’s reproductive, metabolic, and social state, and discuss evidence for plasticity in potential modulators that may contribute to changes in sensory abilities and behaviors. Sensory plasticity in females is primarily associated with the naturalmore »reproductive cycle and functions to improve detection of courtship signals (visual, auditory, chemosensory, and likely mechanosensory) from high-quality males for reproduction. Plasticity in male sensory abilities seems to function in altering their ability to detect the status of other males in the service of territory ownership and future reproductive opportunities. Changes in different classes of potential modulators or their receptors (steroids, neuropeptides, and biogenic amines) occur at both peripheral sensory organs (eye, inner ear, and olfactory epithelium) and central visual, olfactory, and auditory processing regions, suggesting complex mechanisms contributing to plasticity of sensory function. This type of sensory plasticity revealed in males and females of A. burtoni is likely more widespread among diverse animals than currently realized, and future studies should take an integrative and comparative approach to better understand the proximate and ultimate mechanisms modulating communication abilities across taxa.« less
2. Everything in Modulation: Neuromodulators as Keys to Understanding Communication Dynamics

   https://doi.org/10.1093/icb/icab102  

   Barkan, Charlotte L ; Leininger, Elizabeth C ; Zornik, Erik ( May 2021 , Integrative and Comparative Biology)

   Synopsis Across the animal kingdom, the ability to produce communication signals appropriate to social encounters is essential, but how these behaviors are selected and adjusted in a context-dependent manner are poorly understood. This question can be addressed on many levels, including sensory processing by peripheral organs and the central nervous system, sensorimotor integration in decision-making brain regions, and motor circuit activation and modulation. Because neuromodulator systems act at each of these levels, they are a useful lens through which to explore the mechanisms underlying complex patterns of communication. It has been clear for decades that understanding the logic of input–output decision making by the nervous system requires far more than simply identifying the connections linking sensory organs to motor circuits; this is due in part to the fact that neuromodulators can promote distinct and temporally dynamic responses to similar signals. We focus on the vocal circuit dynamics of Xenopus frogs, and describe complementary examples from diverse vertebrate communication systems. While much remains to be discovered about how neuromodulators direct flexibility in communication behaviors, these examples illustrate that several neuromodulators can act upon the same circuit at multiple levels of control, and that the functional consequence of neuromodulation can depend onmore »species-specific factors as well as dynamic organismal characteristics like internal state.« less
3. The connectome of an insect brain

   https://doi.org/10.1126/science.add9330  

   Winding, Michael ; Pedigo, Benjamin D. ; Barnes, Christopher L. ; Patsolic, Heather G. ; Park, Youngser ; Kazimiers, Tom ; Fushiki, Akira ; Andrade, Ingrid V. ; Khandelwal, Avinash ; Valdes-Aleman, Javier ; et al ( March 2023 , Science)

   INTRODUCTION A brainwide, synaptic-resolution connectivity map—a connectome—is essential for understanding how the brain generates behavior. However because of technological constraints imaging entire brains with electron microscopy (EM) and reconstructing circuits from such datasets has been challenging. To date, complete connectomes have been mapped for only three organisms, each with several hundred brain neurons: the nematode C. elegans , the larva of the sea squirt Ciona intestinalis , and of the marine annelid Platynereis dumerilii . Synapse-resolution circuit diagrams of larger brains, such as insects, fish, and mammals, have been approached by considering select subregions in isolation. However, neural computations span spatially dispersed but interconnected brain regions, and understanding any one computation requires the complete brain connectome with all its inputs and outputs. RATIONALE We therefore generated a connectome of an entire brain of a small insect, the larva of the fruit fly, Drosophila melanogaster. This animal displays a rich behavioral repertoire, including learning, value computation, and action selection, and shares homologous brain structures with adult Drosophila and larger insects. Powerful genetic tools are available for selective manipulation or recording of individual neuron types. In this tractable model system, hypotheses about the functional roles of specific neurons and circuit motifs revealedmore »by the connectome can therefore be readily tested. RESULTS The complete synaptic-resolution connectome of the Drosophila larval brain comprises 3016 neurons and 548,000 synapses. We performed a detailed analysis of the brain circuit architecture, including connection and neuron types, network hubs, and circuit motifs. Most of the brain’s in-out hubs (73%) were postsynaptic to the learning center or presynaptic to the dopaminergic neurons that drive learning. We used graph spectral embedding to hierarchically cluster neurons based on synaptic connectivity into 93 neuron types, which were internally consistent based on other features, such as morphology and function. We developed an algorithm to track brainwide signal propagation across polysynaptic pathways and analyzed feedforward (from sensory to output) and feedback pathways, multisensory integration, and cross-hemisphere interactions. We found extensive multisensory integration throughout the brain and multiple interconnected pathways of varying depths from sensory neurons to output neurons forming a distributed processing network. The brain had a highly recurrent architecture, with 41% of neurons receiving long-range recurrent input. However, recurrence was not evenly distributed and was especially high in areas implicated in learning and action selection. Dopaminergic neurons that drive learning are amongst the most recurrent neurons in the brain. Many contralateral neurons, which projected across brain hemispheres, were in-out hubs and synapsed onto each other, facilitating extensive interhemispheric communication. We also analyzed interactions between the brain and nerve cord. We found that descending neurons targeted a small fraction of premotor elements that could play important roles in switching between locomotor states. A subset of descending neurons targeted low-order post-sensory interneurons likely modulating sensory processing. CONCLUSION The complete brain connectome of the Drosophila larva will be a lasting reference study, providing a basis for a multitude of theoretical and experimental studies of brain function. The approach and computational tools generated in this study will facilitate the analysis of future connectomes. Although the details of brain organization differ across the animal kingdom, many circuit architectures are conserved. As more brain connectomes of other organisms are mapped in the future, comparisons between them will reveal both common and therefore potentially optimal circuit architectures, as well as the idiosyncratic ones that underlie behavioral differences between organisms. Some of the architectural features observed in the Drosophila larval brain, including multilayer shortcuts and prominent nested recurrent loops, are found in state-of-the-art artificial neural networks, where they can compensate for a lack of network depth and support arbitrary, task-dependent computations. Such features could therefore increase the brain’s computational capacity, overcoming physiological constraints on the number of neurons. Future analysis of similarities and differences between brains and artificial neural networks may help in understanding brain computational principles and perhaps inspire new machine learning architectures. The connectome of the Drosophila larval brain. The morphologies of all brain neurons, reconstructed from a synapse-resolution EM volume, and the synaptic connectivity matrix of an entire brain. This connectivity information was used to hierarchically cluster all brains into 93 cell types, which were internally consistent based on morphology and known function.« less
4. Sexual selection and the ascent of women: Mate choice research since Darwin

   https://doi.org/10.1126/science.abi6308  

   Rosenthal, Gil G. ; Ryan, Michael J. ( January 2022 , Science)

   BACKGROUND Charles Darwin’s  Descent of Man, and Selection in Relation to Sex  tackled the two main controversies arising from the Origin of Species:  the evolution of humans from animal ancestors and the evolution of sexual ornaments. Most of the book focuses on the latter, Darwin’s theory of sexual selection. Research since supports his conjecture that songs, perfumes, and intricate dances evolve because they help secure mating partners. Evidence is overwhelming for a primary role of both male and female mate choice in sexual selection—not only through premating courtship but also through intimate interactions during and long after mating. But what makes one prospective mate more enticing than another? Darwin, shaped by misogyny and sexual prudery, invoked a “taste for the beautiful” without speculating on the origin of the “taste.” How to explain when the “final marriage ceremony” is between two rams? What of oral sex in bats, cloacal rubbing in bonobos, or the sexual spectrum in humans, all observable in Darwin’s time? By explaining desire through the lens of those male traits that caught his eyes and those of his gender and culture, Darwin elided these data in his theory of sexual evolution. Work since Darwin has focused on howmore »traits and preferences coevolve. Preferences can evolve even if attractive signals only predict offspring attractiveness, but most attention has gone to the intuitive but tenuous premise that mating with gorgeous partners yields vigorous offspring. By focusing on those aspects of mating preferences that coevolve with male traits, many of Darwin’s influential followers have followed the same narrow path. The sexual selection debate in the 1980s was framed as “good genes versus runaway”: Do preferences coevolve with traits because traits predict genetic benefits, or simply because they are beautiful? To the broader world this is still the conversation. ADVANCES Even as they evolve toward ever-more-beautiful signals and healthier offspring, mate-choice mechanisms and courter traits are locked in an arms race of coercion and resistance, persuasion and skepticism. Traits favored by sexual selection often do so at the expense of chooser fitness, creating sexual conflict. Choosers then evolve preferences in response to the costs imposed by courters. Often, though, the current traits of courters tell us little about how preferences arise. Sensory systems are often tuned to nonsexual cues like food, favoring mating signals resembling those cues. And preferences can emerge simply from selection on choosing conspecifics. Sexual selection can therefore arise from chooser biases that have nothing to do with ornaments. Choice may occur before mating, as Darwin emphasized, but individuals mate multiple times and bias fertilization and offspring care toward favored partners. Mate choice can thus occur in myriad ways after mating, through behavioral, morphological, and physiological mechanisms. Like other biological traits, mating preferences vary among individuals and species along multiple dimensions. Some of this is likely adaptive, as different individuals will have different optimal mates. Indeed, mate choice may be more about choosing compatible partners than picking the “best” mate in the absolute sense. Compatibility-based choice can drive or reinforce genetic divergence and lead to speciation. The mechanisms underlying the “taste for the beautiful” determine whether mate choice accelerates or inhibits reproductive isolation. If preferences are learned from parents, or covary with ecological differences like the sensory environment, then choice can promote genetic divergence. If everyone shares preferences for attractive ornaments, then choice promotes gene flow between lineages. OUTLOOK Two major trends continue to shift the emphasis away from male “beauty” and toward how and why individuals make sexual choices. The first integrates neuroscience, genomics, and physiology. We need not limit ourselves to the feathers and dances that dazzled Darwin, which gives us a vastly richer picture of mate choice. The second is that despite persistent structural inequities in academia, a broader range of people study a broader range of questions. This new focus confirms Darwin’s insight that mate choice makes a primary contribution to sexual selection, but suggests that sexual selection is often tangential to mate choice. This conclusion challenges a persistent belief with sinister roots, whereby mate choice is all about male ornaments. Under this view, females evolve to prefer handsome males who provide healthy offspring, or alternatively, to express flighty whims for arbitrary traits. But mate-choice mechanisms also evolve for a host of other reasons Understanding mate choice mechanisms is key to understanding how sexual decisions underlie speciation and adaptation to environmental change. New theory and technology allow us to explicitly connect decision-making mechanisms with their evolutionary consequences. A century and a half after Darwin, we can shift our focus to females and males as choosers, rather than the gaudy by-products of mate choice. Mate choice mechanisms across domains of life. Sensory periphery for stimulus detection (yellow), brain for perceptual integration and evaluation (orange), and reproductive structures for postmating choice among pollen or sperm (teal). ILLUSTRATION: KELLIE HOLOSKI/ SCIENCE« less
5. Pheromone Perception in Fish: Mechanisms and Modulation by Internal Status

   https://doi.org/10.1093/icb/icad049  

   Bowers, Jessica M. ; Li, Cheng-Yu ; Parker, Coltan G. ; Westbrook, Molly E. ; Juntti, Scott A. ( June 2023 , Integrative And Comparative Biology)

   Pheromones are chemical signals that facilitate communication between animals, and most animals use pheromones for reproduction and other forms of social behavior. The identification of key ligands and olfactory receptors used for pheromonal communication provides insight into the sensory processing of these important cues. An individual’s responses to pheromones can be plastic, as physiological status modulates behavioral outputs. In this review, we outline the mechanisms for pheromone sensation and highlight physiological mechanisms that modify pheromone-guided behavior. We focus on hormones, which regulate pheromonal communication across vertebrates including fish, amphibians, and rodents. This regulation may occur in peripheral olfactory organs and the brain, but the mechanisms remain unclear. While this review centers on research in fish, we will discuss other systems to provide insight into how hormonal mechanisms function across taxa.