High Throughput Assay to Examine Egg-Laying Preferences of Individual Drosophila melanogaster

Increased sugar valuation contributes to the evolutionary shift in egg-laying behavior of the fruit pest Drosophila suzukii

Behavior evolution can promote the emergence of agricultural pests by changing their ecological niche. For example, the insect pest Drosophila suzukii has shifted its oviposition (egg-laying) niche from fermented fruits to ripe, non-fermented fruits, causing significant damage to a wide range of fruit crops worldwide. We investigate the chemosensory changes underlying this evolutionary shift and ask whether fruit sugars, which are depleted during fermentation, are important gustatory cues that direct D. suzukii oviposition to sweet, ripe fruits. We show that D. suzukii has expanded its range of oviposition responses to lower sugar concentrations than the model D. melanogaster, which prefers to lay eggs on fermented fruit. The increased response of D. suzukii to sugar correlates with an increase in the value of sugar relative to a fermented strawberry substrate in oviposition decisions. In addition, we show by genetic manipulation of sugar-gustatory receptor neurons (GRNs) that sugar perception is required for D. suzukii to prefer a ripe substrate over a fermented substrate, but not for D. melanogaster to prefer the fermented substrate. Thus, sugar is a major determinant of D. suzukii's choice of complex substrates. Calcium imaging experiments in the brain's primary gustatory center (suboesophageal zone) show that D. suzukii GRNs are not more sensitive to sugar than their D. melanogaster counterparts, suggesting that increased sugar valuation is encoded in downstream circuits of the central nervous system (CNS). Taken together, our data suggest that evolutionary changes in central brain sugar valuation computations are involved in driving D. suzukii's oviposition preference for sweet, ripe fruit.

An internal expectation guides Drosophila egg-laying decisions

To better understand how animals make ethologically relevant decisions, we studied egg-laying substrate choice in Drosophila. We found that flies dynamically increase or decrease their egg-laying rates while exploring substrates so as to target eggs to the best, recently visited option. Visiting the best option typically yielded inhibition of egg laying on other substrates for many minutes. Our data support a model in which flies compare the current substrate's value with an internally constructed expectation on the value of available options to regulate the likelihood of laying an egg. We show that dopamine neuron activity is critical for learning and/or expressing this expectation, similar to its role in certain tasks in vertebrates. Integrating sensory experiences over minutes to generate an estimate of the quality of available options allows flies to use a dynamic reference point for judging the current substrate and might be a general way in which decisions are made.

Function of Transient Receptor Potential-Like Channel in Insect Egg Laying

The transient receptor potential-like channel (TRPL) is a member of the transient receptor potential (TRP) channel family involved in regulating many fundamental senses, such as vision, pain, taste, and touch, in both invertebrates and vertebrates. Yet, the function of TRPL in other important biological processes remains unclear. We discover that TRPL regulates egg laying in two insect species, the brown planthopper, Nilaparvata lugens, and the fruit fly, Drosophila melanogaster. In both insects, trpl is expressed in the female reproductive organ. Loss of trpl leads to significantly defects in egg laying. In addition, TRPL is functionally interchangeable between the brown planthoppers and flies in egg laying. Altogether, our work uncovers a novel role played by TRPL in regulating egg laying and indicates TRPL as a potential pesticide target in brown planthoppers.

Monarda didyma Hydrolate Affects the Survival and the Behaviour of Drosophila suzukii

Simple Summary

During the steam distillation of aromatic plants, two main fractions are usually obtained: the hydrophobic essential oils and the hydrophilic fraction commonly known as hydrolate (HY). The essential oils are largely used in several industry fields, including the agricultural industry as biopesticides. Residual HYs, instead, are often discarded as by-products of little or no value. Our research pointed out that also HYs have biological activity, suggesting their potential use in plant-based strategy for the pest control. In more detail, we investigated the insecticidal properties of the hydrolate from Monarda didyma, scarlet beebalm, towards Drosophila suzukii. Using specific molecular and behavioural assays, we showed that M. didyma hydrolate affected the fitness and behaviour of D. suzukii, providing new insights in the D. suzukii control strategies through M. didyma hydrolate.

Abstract

Drosophila suzukii (Matsumara) is an herbivorous pest whose control in the field with conventional chemical is particularly difficult and has important drawbacks. Here, we investigated the insecticidal properties of hydrolate from Monarda didyma, scarlet beebalm, an aromatic herb in the Lamiaceae family. The identification of volatile organic compounds (VOCs) by CG–MS systems revealed that thymol (38%) and carvacrol (59%) were the most abundant VOCs in the hydrolate. M. didyma hydrolate did not show fumigant toxicity. Conversely, in contact assays, M. didyma hydrolate showed a LC₅₀ of 5.03 µL mL⁻¹, 48 h after the application on D. suzukii adults. Expression of detoxification genes increased in flies that survived the LC₅₀ application. Furthermore, toxicity persisted for 7 days after the treatment in the survival evaluation. Artificial diet assays with 100 and 1000 µL mL⁻¹ of M. didyma hydrolate resulted in a significant decrease in total food intake in both male and female D. suzukii adults. In addition, electropenetrography (EPG) showed that the D. suzukii females’ feeding behaviour was altered in hydrolate-treated diets. The hydrolate also caused a significant reduction in the number of eggs laid in two different oviposition assays. Overall, our findings provide a new perspective for the improvement of D. suzukii control strategies through M. didyma hydrolate.

Molecular control limiting sensitivity of sweet taste neurons in Drosophila

To assess the biological value of environmental stimuli, animals' sensory systems must accurately decode both the identities and the intensities of these stimuli. While much is known about the mechanism by which sensory neurons detect the identities of stimuli, less is known about the mechanism that controls how sensory neurons respond appropriately to different intensities of stimuli. The ionotropic receptor IR76b has been shown to be expressed in different Drosophila chemosensory neurons for sensing a variety of chemicals. Here, we show that IR76b plays an unexpected role in lowering the sensitivity of Drosophila sweet taste neurons. First, IR76b mutants exhibited clear behavioral responses to sucrose and acetic acid (AA) at concentrations that were too low to trigger observable behavioral responses from WT animals. Second, IR76b is expressed in many sweet neurons on the labellum, and these neurons responded to both sucrose and AA. Removing IR76b from the sweet neurons increased their neuronal responses as well as animals' behavioral responses to sucrose and AA. Conversely, overexpressing IR76b in the sweet neurons decreased their neuronal as well as animals' behavioral responses to sucrose and AA. Last, IR76b's response-lowering ability has specificity: IR76b mutants and WT showed comparable responses to capsaicin when the mammalian capsaicin receptor VR1 was ectopically expressed in their sweet neurons. Our findings suggest that sensitivity of Drosophila sweet neurons to their endogenous ligands is actively limited by IR76b and uncover a potential molecular target by which contexts can modulate sensitivity of sweet neurons.

Sweet neurons inhibit texture discrimination by signaling TMC-expressing mechanosensitive neurons in Drosophila

Integration of stimuli of different modalities is an important but incompletely understood process during decision making. Here, we show that Drosophila are capable of integrating mechanosensory and chemosensory information of choice options when deciding where to deposit their eggs. Specifically, females switch from preferring the softer option for egg-laying when both options are sugar free to being indifferent between them when both contain sucrose. Such sucrose-induced indifference between options of different hardness requires functional sweet neurons, and, curiously, the Transmembrane Channel-like (TMC)-expressing mechanosensitive neurons that have been previously shown to promote discrimination of substrate hardness during feeding. Further, axons of sweet neurons directly contact axons of TMC-expressing neurons in the brain and stimulation of sweet neurons increases Ca²⁺ influx into axons of TMC-expressing neurons. These results uncover one mechanism by which Drosophila integrate taste and tactile information when deciding where to deposit their eggs and reveal that TMC-expressing neurons play opposing roles in hardness discrimination in two different decisions.

H2O2-Sensitive Isoforms of Drosophila melanogaster TRPA1 Act in Bitter-Sensing Gustatory Neurons to Promote Avoidance of UV During Egg-Laying

The evolutionarily conserved TRPA1 channel can sense various stimuli including temperatures and chemical irritants. Recent results have suggested that specific isoforms of Drosophila TRPA1 (dTRPA1) are UV-sensitive and that their UV sensitivity is due to H₂O₂ sensitivity. However, whether such UV sensitivity served any physiological purposes in animal behavior was unclear. Here, we demonstrate that H₂O₂-sensitive dTRPA1 isoforms promote avoidance of UV when adult Drosophila females are selecting sites for egg-laying. First, we show that blind/visionless females are still capable of sensing and avoiding UV during egg-laying when intensity of UV is high yet within the range of natural sunlight. Second, we show that such vision-independent UV avoidance is mediated by a group of bitter-sensing neurons on the proboscis that express H₂O₂-sensitive dTRPA1 isoforms. We show that these bitter-sensing neurons exhibit dTRPA1-dependent UV sensitivity. Importantly, inhibiting activities of these bitter-sensing neurons, reducing their dTRPA1 expression, or reducing their H₂O₂-sensitivity all significantly reduced blind females' UV avoidance, whereas selectively restoring a H₂O₂-sensitive isoform of dTRPA1 in these neurons restored UV avoidance. Lastly, we show that specifically expressing the red-shifted channelrhodopsin CsChrimson in these bitter-sensing neurons promotes egg-laying avoidance of red light, an otherwise neutral cue for egg-laying females. Together, these results demonstrate a physiological role of the UV-sensitive dTRPA1 isoforms, reveal that adult Drosophila possess at least two sensory systems for detecting UV, and uncover an unexpected role of bitter-sensing taste neurons in UV sensing.