Sexual dimorphism in the visual system of flies: the divided brain of male Bibionidae (Diptera)

Complete mitochondrial genome of Plecialongiforceps Duda, 1933 (Diptera, Bibionidae) and its implications for a phylogeny of the family Bibionidae

Over the past decade, the prevalence of mass outbreaks involving non-native insects has sparked concerns about their potential negative impact on human inhabited areas and local environments. Plecialongiforceps Duda, 1933 (Diptera, Bibionidae) was recently recognized as an invasive pest in South Korea, causing public nuisance through mass outbreaks in the Seoul Metropolitan Area during early summer. In this study, we present the first complete mitochondrial genome of Plecialongiforceps, generated from the PacBio HiFi long-read sequencing data. Notably, the length of the circular genome is found to be larger than any annotated reference sequences of mitochondrial genomes for the infraorder Bibionomorpha, which is attributable to an unusually long A+T rich control region. We conducted a phylogenetic analysis of Bibionomorpha, focusing specifically on the family Bibionidae, using nearly all available mitochondrial genome data to elucidate relationships among genera within Bibionidae. Our phylogeny of Bibionomorpha recovered a strong monophyly of the family Bibionidae and its three subfamilies: Bibioninae (Bibio + Dilophus), Hesperininae (Hesperinus + Penthetria), and Pleciinae (Plecia), corroborating the recently proposed taxonomic classification system of Bibionidae. Furthermore, we discuss evolutionary trends within Bibionidae based on our well-supported higher relationships of the superfamily Bibionoidea.

The evolution and development of neural superposition

Visual systems have a rich history as model systems for the discovery and understanding of basic principles underlying neuronal connectivity. The compound eyes of insects consist of up to thousands of small unit eyes that are connected by photoreceptor axons to set up a visual map in the brain. The photoreceptor axon terminals thereby represent neighboring points seen in the environment in neighboring synaptic units in the brain. Neural superposition is a special case of such a wiring principle, where photoreceptors from different unit eyes that receive the same input converge upon the same synaptic units in the brain. This wiring principle is remarkable, because each photoreceptor in a single unit eye receives different input and each individual axon, among thousands others in the brain, must be sorted together with those few axons that have the same input. Key aspects of neural superposition have been described as early as 1907. Since then neuroscientists, evolutionary and developmental biologists have been fascinated by how such a complicated wiring principle could evolve, how it is genetically encoded, and how it is developmentally realized. In this review article, we will discuss current ideas about the evolutionary origin and developmental program of neural superposition. Our goal is to identify in what way the special case of neural superposition can help us answer more general questions about the evolution and development of genetically “hard-wired” synaptic connectivity in the brain.

Visual ecology of flies with particular reference to colour vision and colour preferences

The visual ecology of flies is outstanding among insects due to a combination of specific attributes. Flies' compound eyes possess an open rhabdom and thus separate rhabdomeres in each ommatidium assigned to two visual pathways. The highly sensitive, monovariant neural superposition system is based on the excitation of the peripheral rhabdomeres of the retinula cells R1-6 and controls optomotor reactions. The two forms of central rhabdomeres of R7/8 retinula cells in each ommatidium build up a system with four photoreceptors sensitive in different wavelength ranges and thought to account for colour vision. Evidence from wavelength discrimination tests suggests that all colour stimuli are assigned to one of just four colour categories, but cooperation of the two pathways is also evident. Flies use colour cues for various behavioural reactions such as flower visitation, proboscis extension, host finding, and egg deposition. Direct evidence for colour vision, the ability to discriminate colours according to spectral shape but independent of intensity, has been demonstrated for few fly species only. Indirect evidence for colour vision provided from electrophysiological recordings of the spectral sensitivity of photoreceptors and opsin genes indicates similar requisites in various flies; the flies' responses to coloured targets, however, are much more diverse.

Joint control of Drosophila male courtship behavior by motion cues and activation of male-specific P1 neurons

Sexual behaviors in animals are governed by inputs from multiple external sensory modalities. However, how these inputs are integrated to jointly control animal behavior is still poorly understood. Whereas visual information alone is not sufficient to induce courtship behavior in Drosophila melanogaster males, when a subset of male-specific fruitless (fru)- and doublesex (dsx)-expressing neurons that respond to chemosensory cues (P1 neurons) were artificially activated via a temperature-sensitive cation channel (dTRPA1), males followed and extended their wing toward moving objects (even a moving piece of rubber band) intensively. When stationary, these objects were not courted. Our results indicate that motion input and activation of P1 neurons are individually necessary, and under our assay conditions, jointly sufficient to elicit early courtship behaviors, and provide insights into how courtship decisions are made via sensory integration.

Neuronal encoding of object and distance information: a model simulation study on naturalistic optic flow processing

We developed a model of the input circuitry of the FD1 cell, an identified motion-sensitive interneuron in the blowfly's visual system. The model circuit successfully reproduces the FD1 cell's most conspicuous property: its larger responses to objects than to spatially extended patterns. The model circuit also mimics the time-dependent responses of FD1 to dynamically complex naturalistic stimuli, shaped by the blowfly's saccadic flight and gaze strategy: the FD1 responses are enhanced when, as a consequence of self-motion, a nearby object crosses the receptive field during intersaccadic intervals. Moreover, the model predicts that these object-induced responses are superimposed by pronounced pattern-dependent fluctuations during movements on virtual test flights in a three-dimensional environment with systematic modifications of the environmental patterns. Hence, the FD1 cell is predicted to detect not unambiguously objects defined by the spatial layout of the environment, but to be also sensitive to objects distinguished by textural features. These ambiguous detection abilities suggest an encoding of information about objects-irrespective of the features by which the objects are defined-by a population of cells, with the FD1 cell presumably playing a prominent role in such an ensemble.

Turning males on: activation of male courtship behavior in Drosophila melanogaster

The innate sexual behaviors of Drosophila melanogaster males are an attractive system for elucidating how complex behavior patterns are generated. The potential for male sexual behavior in D. melanogaster is specified by the fruitless (fru) and doublesex (dsx) sex regulatory genes. We used the temperature-sensitive activator dTRPA1 to probe the roles of fru(M)- and dsx-expressing neurons in male courtship behaviors. Almost all steps of courtship, from courtship song to ejaculation, can be induced at very high levels through activation of either all fru(M) or all dsx neurons in solitary males. Detailed characterizations reveal different roles for fru(M) and dsx in male courtship. Surprisingly, the system for mate discrimination still works well when all dsx neurons are activated, but is impaired when all fru(M) neurons are activated. Most strikingly, we provide evidence for a fru(M)-independent courtship pathway that is primarily vision dependent.

Anisometric brain dimorphism revisited: Implementation of a volumetric 3D standard brain in Manduca sexta

Lepidopterans like the giant sphinx moth Manduca sexta are known for their conspicuous sexual dimorphism in the olfactory system, which is especially pronounced in the antennae and in the antennal lobe, the primary integration center of odor information. Even minute scents of female pheromone are detected by male moths, facilitated by a huge array of pheromone receptors on their antennae. The associated neuropilar areas in the antennal lobe, the glomeruli, are enlarged in males and organized in the form of the so-called macroglomerular complex (MGC). In this study we searched for anatomical sexual dimorphism more downstream in the olfactory pathway and in other neuropil areas in the central brain. Based on freshly eclosed animals, we created a volumetric female and male standard brain and compared 30 separate neuropilar regions. Additionally, we labeled 10 female glomeruli that were homologous to previously quantitatively described male glomeruli including the MGC. In summary, the neuropil volumes reveal an isometric sexual dimorphism in M. sexta brains. This proportional size difference between male and female brain neuropils masks an anisometric or disproportional dimorphism, which is restricted to the sex-related glomeruli of the antennal lobes and neither mirrored in other normal glomeruli nor in higher brain centers like the calyces of the mushroom bodies. Both the female and male 3D standard brain are also used for interspecies comparisons, and may serve as future volumetric reference in pharmacological and behavioral experiments especially regarding development and adult plasticity. J. Comp. Neurol. 517:210-225, 2009. (c) 2009 Wiley-Liss, Inc.

Dimorphic olfactory lobes in the arthropoda

Specialized olfactory lobe glomeruli relating to sexual or caste differences have been observed in at least five orders of insects, suggesting an early appearance of this trait in insect evolution. Dimorphism is not limited to nocturnal species, but occurs even in insects that are known to use vision for courtship. Other than a single description, there is no evidence for similar structures occurring in the Crustacea, suggesting that the evolution of dimorphic olfactory systems may typify terrestrial arthropods.

An exceptionally well-preserved Eocene dolichopodid fly eye: function and evolutionary significance

The exceptionally preserved eyes of an Eocene dolichopodid fly contained in Baltic amber show remarkable detail, including features at micrometre and submicrometre levels. Based on this material, we establish that it is likely that the neural superposition compound eye existed as far back as 45 Ma. The ommatidia have an open rhabdom with a trapezoidal arrangement of seven rhabdomeres. Such a structure is uniquely characteristic of the neural superposition compound eye of present-day flies. Optical analysis reveals that the fossil eyes had a sophisticated and efficient optical system.

Hindwings are unnecessary for flight but essential for execution of normal evasive flight in Lepidoptera

In Lepidoptera, forewings and hindwings are mechanically coupled and flap in synchrony. Flight is anteromotoric, being driven primarily by action of the forewings. Here we report that lepidopterans can still fly when their hindwings are cut off, a procedure reducing their total wing surface, on average, by nearly one half. However, as we demonstrate by analysis of three-dimensional flight trajectories of a moth and a butterfly (Lymantria dispar and Pieris rapae), hindwing removal causes lepidopterans to incur a loss in both linear and turning acceleration, so that they are unable to exercise their normal flight maneuverability. Without hindwings they still are able to zigzag aerially (the ablation has no effect on their turning radius in flight) but at lesser speed and therefore less evasively. Consequently, hindwings in the expanded state in which they occur in lepidopterans seem to contribute in an essential way to lepidopteran survival. Moths in today's world, we argue, may rely on their evasive flight primarily to avoid capture by bats, whereas butterflies, which we propose advertise their evasiveness collectively through shared aposematism, may depend upon it primarily for defense against birds. Aerial agility thus may be the chief adaptive asset derived by lepidopterans from possession of oversize hindwings.

Free flight maneuvers of stalk-eyed flies: do eye-stalks affect aerial turning behavior?

The eyes of stalk-eyed flies (Diopsidae) are positioned at the end of rigid peduncles projected laterally from the head. In dimorphic species the eye-stalks of males exceed the eye-stalks of females and can exceed body length. Eye-stalk length is sexually selected in males improving male reproductive success. We tested whether the long eye-stalks have a negative effect on free-flight and aerial turning behavior by analyzing the morphology and free-flight trajectories of male and female Cyrtodiopsis dalmanni. At flight posture the mass-moment-of-inertia for rotation about a vertical axis was 1.49-fold higher in males. Males also showed a 5% increase in wing length compared to females. During free-flight females made larger turns than males (54 +/- 31.4 vs. 49 +/- 36.2 degrees , t test, P < 0.033) and flew faster while turning (9.4 +/- 5.45 vs. 8.4 +/- 6.17 cm s(-1), ANOVA, P < 0.021). However, turning performance of both sexes overlapped, and turn rate in males even marginally exceeded turn rate in females (733 +/- 235.3 vs. 685 +/- 282.6 deg s(-1), ANCOVA, P < 0.047). We suggest that the increase in eye-span does result in an increase in the mechanical requirements for aerial turning but that male C. dalmanni are capable of compensating for the constraint of longer eye-stalks during the range of turns observed through wingbeat kinematics and increased wing size.

Characterisation of a blowfly male-specific neuron using behaviourally generated visual stimuli

The pursuit system controlling chasing behaviour in male blowflies has to cope with extremely fast and dynamically changing visual input. An identified male-specific visual neuron called Male Lobula Giant 1 (MLG1) is presumably one major element of this pursuit system. Previous behavioural and modelling analyses have indicated that angular target size, retinal target position and target velocity are relevant input variables of the pursuit system. To investigate whether MLG1 specifically represents any of these visual parameters we obtained in vivo intracellular recordings while replaying optical stimuli that simulate the visual signals received by a male fly during chasing manoeuvres. On the basis of these naturalistic stimuli we find that MLG1 shows distinct direction sensitivity and is depolarised if the target motion contains an upward component. The responses of MLG1 are jointly determined by the retinal position, the speed and direction, and the duration of target motimotion. Coherence analysis reveals that although retinal target size and position are in some way inherent in the responses of MLG1, we find no confirmation of the hypothesis that MLG1 encodes any of these parameters exclusively.

Steering a virtual blowfly: simulation of visual pursuit

The behavioural repertoire of male flies includes visually guided chasing after moving targets. The visuomotor control system for these pursuits belongs to the fastest found in the animal kingdom. We simulated a virtual fly, to test whether or not experimentally established hypotheses on the underlying control system are sufficient to explain chasing behaviour. Two operating instructions for steering the chasing virtual fly were derived from behavioural experiments: (i) the retinal size of the target controls the fly's forward speed and, thus, indirectly its distance to the target; and (ii) a smooth pursuit system uses the retinal position of the target to regulate the fly's flight direction. Low-pass filters implement neuronal processing time. Treating the virtual fly as a point mass, its kinematics are modelled in consideration of the effects of translatory inertia and air friction. Despite its simplicity, the model shows behaviour similar to that of real flies. Depending on its starting position and orientation as well as on target size and speed, the virtual fly either catches the target or follows it indefinitely without capture. These two behavioural modes of the virtual fly emerge from the control system for flight steering without implementation of an explicit decision maker.

Neuroarchitecture of the color and polarization vision system of the Stomatopod haptosquilla

The apposition compound eyes of stomatopod crustaceans contain a morphologically distinct eye region specialized for color and polarization vision, called the mid-band. In two stomatopod superfamilies, the mid-band is constructed from six rows of enlarged ommatidia containing multiple photoreceptor classes for spectral and polarization vision. The aim of this study was to begin to analyze the underlying neuroarchitecture, the design of which might reveal clues how the visual system interprets and communicates to deeper levels of the brain the multiple channels of information supplied by the retina. Reduced silver methods were used to investigate the axon pathways from different retinal regions to the lamina ganglionaris and from there to the medulla externa, the medulla interna, and the medulla terminalis. A swollen band of neuropil-here termed the accessory lobe-projects across the equator of the lamina ganglionaris, the medulla externa, and the medulla interna and represents, structurally, the retina's mid-band. Serial semithin and ultrathin resin sections were used to reconstruct the projection of photoreceptor axons from the retina to the lamina ganglionaris. The eight axons originating from one ommatidium project to the same lamina cartridge. Seven short visual fibers end at two distinct levels in each lamina cartridge, thus geometrically separating the two channels of polarization and spectral information. The eighth visual fiber runs axially through the cartridge and terminates in the medulla externa. We conclude that spatial, color, and polarization information is divided into three parallel data streams from the retina to the central nervous system.

Anatomical organization of retinotopic motion-sensitive pathways in the optic lobes of flies

Anatomical methods have identified conserved neuronal morphologies and synaptic relationships among small-field retinotopic neurons in insect optic lobes. These conserved cell shapes occur across many species of dipteran insects and are also shared by Lepidoptera and Hymenoptera. The suggestion that such conserved neurons should participate in motion computing circuits finds support from intracellular recordings as well as older studies that used radioactive deoxyglucose labeling to reveal strata with motion-specific activity in an achromatic neuropil called the lobula plate. While intracellular recordings provide detailed information about the motion-sensitive or motion-selective responses of identified neurons, a full understanding of how arrangements of identified neurons compute and integrate information about visual motion will come from a multidisciplinary approach that includes morphological circuit analysis, the use of genetic mutants that exhibit specific deficits in motion processing, and biomimetic models. The latter must be based on the organization and connections of real neurons, yet provide output properties similar to those of more traditional theoretical models based on behavioral observations that date from the 1950s. Microsc. Res. Tech. 62:132-150, 2003.

Chasing a dummy target: smooth pursuit and velocity control in male blowflies

Male blowflies chase and catch other flies in fast acrobatic flights. To unravel the underlying control system, we presented a black moving sphere instead of a real fly as a pursuit target. By varying the size and speed of the target, we were able to systematically analyse the decisive visual determinants that guide chasing behaviour. Flies pursue targets of a wide range of sizes and velocities. The percentage of pursuits resulting in target capture decreases with increasing target size and speed. Chasing male flies adjust their forward velocity depending on the retinal size of the target, indicating that retinal size is a relevant input variable of the control system. The chasing fly focuses the target with great accuracy in the frontal part of its visual field by means of a smooth pursuit control system using the retinal position of the target to determine the flight direction. We conclude that for a comprehensive understanding of chasing control different time lags in the control systems of angular and forward velocity together with the impact of inertia on fly movements need to be taken into account.

Flight performance and visual control of flight of the free-flying housefly ( Musca domestica L.) II. Pursuit of targets

The pursuit behaviour of houseflies has been analysed by the evaluation of movie films. On the floor, males, but not females, turn towards passing targets. Males as well as females pursue targets in the air. Male chasing seems to be functionally different from female tracking. Males attack targets in the air from below. They sometimes retract from the target fly after an approach. Thus, a chase may be divided into attacks, periods of pursuit and retreats. Males catch females, but not other males. The pursuer is therefore able to discriminate between the sexes. Close approach or contact with the target fly seems to be necessary to obtain the information. During pursuit both sexes increase the rate of turning. The male but not the female target fly performs evasive translatory reactions to the attacks (figure 4). Females do not catch other flies. They often react with a single turn in the direction of a passing object. They seldom follow the target, which is then normally positioned below the tracking fly. The rotations about the vertical and transverse axis (yaw and pitch) are visually controlled in both sexes. The horizontal and vertical error angle, as well as the horizontal and vertical retinal target velocity, influence the turning behaviour. At least in males, further, hitherto unknown, cues seem to be additionally involved in the control of the rotatory movements. The male control systems operate more precisely than those of the females. Rotations are characterized by steplike changes in angular orientation (‘ turns’) at high angular velocity. Smooth rotations at angular velocities less than about 200 deg s -1 seem not to play any role either in males or in females. ‘Sideways’ tracking, most probably mediated by rolling about the long axis, occurred in a single sequence only. A correlation between the translation velocity and the distance between pursuer and target is observed in the pursuit sequences of both sexes. This correlation is interpreted as a by-product of the organization of the flight motor. Therefore, neither males nor females control the translation velocity by the distance to the target. The discussion concentrates on the problems in characterizing the control systems and a comparison with data from optical and electrophysiological measurements. The behavioural differences between hoverflies and houseflies are attributed to the different flight motors.

Visual motion-detection circuits in flies: small-field retinotopic elements responding to motion are evolutionarily conserved across taxa

The Hassenstein-Reichardt autocorrelation model for motion computation was derived originally from studies of optomotor turning reactions of beetles and further refined from studies of houseflies. Its applicaton for explaining a variety of optokinetic behaviors in other insects assumes that neural correlates to the model are principally similar across taxa. This account examines whether this assumption is warranted. The results demonstrate that an evolutionarily conserved subset of neurons corresponds to small retinotopic neurons implicated in motion-detecting circuits that link the retina to motion-sensitive neuropils of the lobula plate. The occurrence of these neurons in basal groups suggests that they must have evolved at least 240 million years before the present time. Functional contiguity among the neurons is suggested by their having layer relationships that are independent of taxon-specific neurons, or the absence of orientation-specific motion-sensitive levels in the lobula plate.

Sexual differentiation in the terminal ganglion of the moth Manduca sexta: role of sex-specific neuronal death

In the insect Manduca sexta the genitalia on the terminal abdominal segments are sexually dimorphic structures but they arise during metamorphosis from segments that are monomorphic in the larva. The motoneurons in the terminal ganglion that innervate these structures were examined by cobalt backfills of peripheral nerves. In the larval stage the population of motoneurons innervating the terminal segments was identical in both sexes. By contrast, the motoneuron populations in the terminal ganglia of adult males and females were strikingly different. No new motoneurons were produced during metamorphosis. Rather, this difference was the result of sex-specific cell death which occurred primarily during the early stages of adult differentiation. Possible mechanisms underlying this sex-specific degeneration of neurons are discussed.