Base Recording: A Technique for Analyzing Responses of Taste Neurons in <i>Drosophila</i>

Insects taste the external world through taste hairs, or sensilla, that have pores at their tips. When a sensillum comes into contact with a potential ...

Yale University

The Connecticut Agricultural Experiment Station

Odor information is processed through multiple receptor-glomerular channels in the first order olfactory center, the antennal lobe (AL), then reformatted into higher brain centers and eventually perceived by the fly. To reveal the logic of olfaction, it is fundamental to map odor representations from the glomerular channels into higher brain centers.

Olfactory coding from the periphery to higher brain centers in the Drosophila brain.

Mate finding and recognition in animals evolves during niche adaptation and involves social signals and habitat cues. Drosophila melanogaster and related species are known to be attracted to fermenting fruit for feeding and egg-laying, which poses the question of whether species-specific fly odours contribute to long-range premating communication.

A Drosophila female pheromone elicits species-specific long-range attraction via an olfactory channel with dual specificity for sex and food.

Despite the comprehensive knowledge on odor coding, our understanding of the relationship between sensory input and behavioral output in Drosophila remains weak. Here, we measure the behavioral responses generated by larval and adult flies in response to 34 fruit odors and find that larval preference for fruit odors differs from that of adult flies. Next, we provide a functional analysis of the full repertoire of the peripheral olfactory system using the same comprehensive stimulus spectrum. We find that 90% and 53% of larval and adult olfactory receptors tested here, respectively, are involved in evaluating these fruit odors. Finally, we find that the total amount of olfactory neuronal activity correlates strongly positively with behavioral output in larvae and correlates weakly negatively in adult flies. Our results suggest that larval and adult flies have evolved different mechanisms for detection and computation of fruit odors, mechanisms likely mirroring the different lifestyles of both developmental stages.

The Olfactory Logic behind Fruit Odor Preferences in Larval and Adult Drosophila.

Taste systems detect a vast diversity of toxins, which are perceived as bitter. When a species adapts to a new environment, its taste system must adapt to detect new death threats. We deleted each of six commonly expressed bitter gustatory receptors (Grs) from Drosophila melanogaster. Systematic analysis revealed that requirements for these Grs differed for the same tastant in different neurons and for different tastants in the same neuron. Responses to some tastants in some neurons required four Grs, including Gr39a. Deletions also produced increased or novel responses, supporting a model of Gr-Gr inhibitory interactions. Coexpression of four Grs conferred several bitter responses to a sugar neuron. We then examined bitter coding in three other Drosophila species. We found major evolutionary shifts. One shift depended on the concerted activity of seven Grs. This work shows how the complex logic of bitter coding provides the capacity to detect innumerable hazards and the flexibility to adapt to new ones.

Molecular Logic and Evolution of Bitter Taste in Drosophila.

Insects use sex pheromones as a reproductive isolating mechanism to attract conspecifics and repel heterospecifics. Despite the profound knowledge of sex pheromones, little is known about the coevolutionary mechanisms and constraints on their production and detection. Using whole-genome sequences to infer the kinship among 99 drosophilids, we investigate how phylogenetic and chemical traits have interacted at a wide evolutionary timescale. Through a series of chemical syntheses and electrophysiological recordings, we identify 52 sex-specific compounds, many of which are detected via olfaction. Behavioral analyses reveal that many of the 43 male-specific compounds are transferred to the female during copulation and mediate female receptivity and/or male courtship inhibition. Measurement of phylogenetic signals demonstrates that sex pheromones and their cognate olfactory channels evolve rapidly and independently over evolutionary time to guarantee efficient intra- and inter-specific communication systems. Our results show how sexual isolation barriers between species can be reinforced by species-specific olfactory signals.

Large-scale characterization of sex pheromone communication systems in Drosophila.

Adaptations to anthropogenic domestic habitats contribute to the success of the mosquito  as a major global vector of several arboviral diseases. The species inhabited African forests before expanding into domestic habitats and spreading to other continents. Despite a well-studied evolutionary history, how this species initially moved into human settlements in Africa remains unclear. During this initial habitat transition, African  switched their larval sites from natural water containers like tree holes to artificial containers like clay pots. Little is known about how these natural versus artificial containers differ in their characteristics. Filling this knowledge gap could provide valuable information for studying the evolution of  associated with larval habitat changes. As an initial effort, in this study, we characterized the microenvironments of  larval sites in forest and domestic habitats in two African localities: La Lopé, Gabon, and Rabai, Kenya. Specifically, we measured the physical characteristics, microbial density, bacterial composition, and volatile chemical profiles of multiple larval sites. In both localities, comparisons between natural containers in the forests and artificial containers in the villages revealed significantly different microenvironments. We next examined whether the between-habitat differences in larval site microenvironments lead to differences in oviposition, a key behavior affecting larval distribution. Forest  readily accepted the artificial containers we placed in the forests. Laboratory choice experiments also did not find distinct oviposition preferences between forest and village  colonies. These results suggested that African  are likely generalists in their larval site choices. This flexibility to accept various containers with a wide range of physical, microbial, and chemical conditions might allow  to use human-stored water as fallback larval sites during dry seasons, which is hypothesized to have initiated the domestic evolution of .

Larval sites of the mosquito  in forest and domestic habitats in Africa and the potential association with oviposition evolution.

Salt taste is one of the most ancient of all sensory modalities. However, the molecular basis of salt taste remains unclear in invertebrates. Here, we show that the response to low, appetitive salt concentrations in Drosophila depends on Ir56b, an atypical member of the ionotropic receptor (Ir) family. Ir56b acts in concert with two coreceptors, Ir25a and Ir76b. Mutation of Ir56b virtually eliminates an appetitive behavioral response to salt. Ir56b is expressed in neurons that also sense sugars via members of the Gr (gustatory receptor) family. Misexpression of Ir56b in bitter-sensing neurons confers physiological responses to appetitive doses of salt. Ir56b is unique among tuning Irs in containing virtually no N-terminal region, a feature that is evolutionarily conserved. Moreover, Ir56b is a "pseudo-pseudogene": its coding sequence contains a premature stop codon that can be replaced with a sense codon without loss of function. This stop codon is conserved among many Drosophila species but is absent in a number of species associated with cactus in arid regions. Thus, Ir56b serves the evolutionarily ancient function of salt detection in neurons that underlie both salt and sweet taste modalities.

Ir56b is an atypical ionotropic receptor that underlies appetitive salt response in Drosophila.

The agricultural pest  differs from most other  species in that it lays eggs in ripe, rather than overripe, fruit. Previously, we showed that changes in bitter taste sensation accompanied this adaptation (Dweck et al., 2021). Here, we show that  has also undergone a variety of changes in sweet taste sensation.  has a weaker preference than  for laying eggs on substrates containing all three primary fruit sugars: sucrose, fructose, and glucose. Major subsets of  taste sensilla have lost electrophysiological responses to sugars. Expression of several key sugar receptor genes is reduced in the taste organs of . By contrast, certain mechanosensory channel genes, including , are expressed at higher levels in the taste organs of , which has a higher preference for stiff substrates. Finally, we find that  responds differently from  to combinations of sweet and mechanosensory cues. Thus, the two species differ in sweet sensation, mechanosensation, and their integration, which are all likely to contribute to the differences in their egg-laying preferences in nature.

Sugar sensation and mechanosensation in the egg-laying preference shift of .

Tsetse flies transmit trypanosomes-parasites that cause devastating diseases in humans and livestock-across much of sub-Saharan Africa. Chemical communication through volatile pheromones is common among insects; however, it remains unknown if and how such chemical communication occurs in tsetse flies. We identified methyl palmitoleate (MPO), methyl oleate, and methyl palmitate as compounds that are produced by the tsetse fly  and elicit strong behavioral responses. MPO evoked a behavioral response in male-but not virgin female-.  males mounted females of another species, , when they were treated with MPO. We further identified a subpopulation of olfactory neurons in  that increase their firing rate in response to MPO and showed that infecting flies with African trypanosomes alters the flies' chemical profile and mating behavior. The identification of volatile attractants in tsetse flies may be useful for reducing disease spread.

A volatile sex attractant of tsetse flies.

Taste systems encode chemical cues that drive vital behaviors. We have elucidated noncanonical features of taste coding using an unconventional kind of electrophysiological analysis. We find that taste neurons of  are much more sensitive than previously thought. They have a low spontaneous firing frequency that depends on taste receptors. Taste neurons have a dual function as olfactory neurons: They are activated by most tested odorants, including ,-diethyl--toluamide (DEET), at a distance. DEET can also inhibit certain taste neurons, revealing that there are two modes of taste response: activation and inhibition. We characterize electrophysiological OFF responses and find that the tastants that elicit them are related in structure. OFF responses link tastant identity to behavior: the magnitude of the OFF response elicited by a tastant correlated with the egg laying behavior it elicited. In summary, the sensitivity and coding capacity of the taste system are much greater than previously known.

Hany K. M. Dweck

Department of Molecular,
Cellular and Developmental Biology,
Department of Molecular, Cellular and Developmental Biology

Yale University

Base Recording: A Technique for Analyzing Responses of Taste Neurons in Drosophila

Hany K. M. Dweck

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Molecular,Cellular and Developmental Biology,Department of Molecular, Cellular and Developmental Biology

Yale University

Base Recording: A Technique for Analyzing Responses of Taste Neurons in Drosophila

Department of Molecular,
Cellular and Developmental Biology,
Department of Molecular, Cellular and Developmental Biology