How fruit flies sniff out their environments

Hexbyte Glen Cove

Credit: Hong lab

Fruit flies—Drosophila melanogaster—have a complicated relationship with carbon dioxide. In some contexts, CO2 indicates the presence of tasty food sources as sugar-fermenting yeast in fruit produces the molecule as a by-product. But in other cases, CO2 can be a warning to stay away, signaling an oxygen-poor or overcrowded environment with too many other flies. How do flies tell the difference?

Now, a new study reveals that fruit fly olfactory neurons—those responsible for sensing chemical “smells” such as CO2—have the ability to talk to each other through a previously undiscovered pathway. The work provides insights into the fundamental processes by which communicate with one another and also gives new clues to solving the longstanding mysteries about and CO2.

The research was conducted in the laboratory of Elizabeth Hong (BS ’02), assistant professor of neuroscience and Chen Scholar of the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech. A paper describing the study appears in the journal Current Biology on September 6.

“CO2 is an important but complex signal found in all sorts of different situations in the natural environment, and it illustrates a core challenge neurobiologists face in understanding the brain: How does the brain process the same sensory signal in different contexts to allow the animal to respond appropriately?” says Hong. “We tackle this question using the fly olfactory system, one of the best-studied and well-characterized sensory circuits. And even still, with this research, we discovered a surprising new phenomenon in how the brain processes sensory signals.”

Olfaction, or the sense of smell, was the original sensory system to evolve in all animals. Though humans are primarily visual, the majority of animals use olfaction as the main method of understanding their environments: sniffing out food, avoiding predators, and finding mates. Fruit flies are a particularly manageable model for understanding the biological mechanisms underlying the : a fruit fly only has about 50 different odorant receptors, whereas a human has around 400 to 500, and mice have more than a thousand.

A fly’s “nose” is its two antennae. These antennae are coated with thin hairs called sensilla, and inside of each sensillum are the olfactory neurons. Odors—like CO2 or the volatile esters produced by rotting fruit—diffuse into tiny pores on the sensilla and bind onto corresponding receptors on the olfactory neurons. Neurons then send signals down the sensillum and into the brain. Though we don’t have antennae, an analogous process happens in your own nose when you lean in to catch a whiff of delicious cooking or recoil from bad smells.

In fruit flies, while most odors activate around 20 different types of sensory neurons at once, CO2 is unusual in that it only activates a single type. Using a combination of genetic analysis and functional imaging, researchers in the Hong laboratory discovered that the output cables, or axons, of the CO2-sensitive olfactory neurons actually can talk to other olfactory neural channels—specifically, the neurons that detect esters, molecules that smell particularly delicious to a fruit fly.

Neurobiology graduate student Pratyush Kandimalla works to tether a fly for experiments. Credit: Hong lab

However, this olfactory crosstalk depends on the timing of CO2 cues. When CO2 is detected in fluctuating pulses, such as a wind-borne cue from a distant food source, the CO2-sensing olfactory channel sends a message to the channels encoding esters, signaling to the brain that delicious food is upwind. However, if CO2 is continually elevated in the local environment, for instance from a rotting log, this crosstalk is quickly shutoff, and the CO2-sensitive neurons signal directly to the brain to avoid the source.

This is the first time that olfactory neurons have been shown to talk to one another between their axons, processing incoming information before these signals ever reach the brain. The results cut against the prevailing dogma in neuroscience that information processing is limited to the integration of inputs by neurons; the new findings show that signals are reformatted at the output end as well.

The scientists also discovered that how flies behave toward CO2 also depends on the timing of CO2 signals. “We found that the behavior of the animal is affected by the temporal structure of the CO2 signal,” says Hong. “When the fly walks into a cloud of elevated CO2, it tends to turn away from the direction it was traveling. But in an environment where CO2 is pulsing, the fly will run upwind toward the source of the odor. This difference in how flies behave toward fluctuating CO2, versus sustained CO2, parallels the dependence of the crosstalk from the CO2-sensing neurons to attraction-promoting food-sensing neurons.”

Understanding fruit fly olfaction, particularly with respect to sensing CO2, is a long-standing goal for Caltech researchers. Decades ago, researchers in the laboratory of David Anderson— Seymour Benzer Professor of Biology; Tianqiao and Chrissy Chen Institute for Neuroscience Leadership Chair; Investigator, Howard Hughes Medical Institute; director, Tianqiao and Chrissy Chen Institute for Neuroscience—discovered that flies avoid CO2 as a chemical indicating an overcrowded environment. But recently, researchers in the lab of Michael Dickinson—Esther M. and Abe M. Zarem Professor of Bioengineering and Aeronautics and executive officer for Biology and Biological Engineering—discovered that flies can also be attracted to CO2, when using it to sniff out a source of food.

“Our work builds on these prior studies and provides one possible neural solution for how CO2 could be triggering opposing behaviors in flies in varying contexts. It has been a highlight of having my lab at Caltech to have the opportunity to directly interact with David’s and Michael’s labs and discuss the connections between our work and theirs,” says Hong.

The next major question is to understand how these parallel olfactory axons are talking to one another. The team ruled out most forms of classical chemical transmission that neurons use to communicate, and the mechanisms by which are able to send and receive messages between their axons are mysterious. Solving this problem may provide new insights into how animal brains detect and process sensory information.

More information:
Dhruv Zocchi et al, Parallel encoding of CO2 in attractive and aversive glomeruli by selective lateral signaling between olfactory afferents, Current Biology (2022). DOI: 10.1016/j.cub.2022.08.025

How fruit f

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Fruit flies prioritize mating over survival: study

Credit: Pixabay/CC0 Public Domain

Fruit flies continue to mate with each other even when infected with deadly pathogens—reveals a study by researchers at the University of Birmingham.

According to results published today in Proceedings of the Royal Society B, both male and female infected with bacterial pathogens show normal levels of courtship and success.

Mounting an is energetically ‘costly’, so infections are typically thought to reduce the amount of energy available for other activities such as mating. Surprisingly, however, this study demonstrated that infected fruit flies continued to engage in courtship and mating, regardless of whether either the male or the female fly was infected.

Dr. Carolina Rezaval, the research team leader at the University of Birmingham explains: “Animals have limited that need to be distributed among different activities, like fighting an infection or mating. We were interested to understand how animals prioritize and balance their investment in and reproduction.”

Saloni Rose, a Ph.D. student with Dr. Rezaval, tackled this question using the fruit fly Drosophila. By infecting both male and female fruit flies with different pathogens, ranging in type and severity, she made the surprising discovery that courtship and mating behaviors were similar in both infected and uninfected flies. This was also true when the flies’ was artificially activated using genetic manipulation. Moreover, uninfected flies mated equally frequently with both infected and healthy partners, suggesting that they do not select against mates who are infected.

Flies are not oblivious to infection, however. Previous studies have shown that infected flies can show abnormal locomotion, sleep and feeding behaviors. Consequently, this new study suggests that courtship and mating behaviors are prioritized, even when other behaviors are altered during the development of the infection.

When faced with a potential life threat, some animals respond by investing more into reproduction, likely in attempt to pass on genes to the next generation. This may well be what is happening with flies in the conditions tested in the lab. More work is needed to find out what is going on in the brain to maintain reproductive behaviors in the face of infection.

The team worked in collaboration with Professor Marc Dionne (Imperial College), Dr. Esteban Beckwith (IFIBYNE, Argentina) and Professor Robin May (Birmingham University).

More information:
Pre-copulatory reproductive behaviours are preserved in Drosophila melanogaster infected with bacteria, Proceedings of the Royal Society B Biological Sciences (2022).

Fruit flies prioritize mating over survival: study (2022, May 10)
retrieved 11 May 2022

This docum

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Hexbyte Glen Cove Blood-sucking flies may be following chemicals produced by skin bacteria to locate bats to feed on thumbnail

Hexbyte Glen Cove Blood-sucking flies may be following chemicals produced by skin bacteria to locate bats to feed on

Hexbyte Glen Cove

A Natal long-fingered bat (Miniopterus natalensis) parasitized by a male bat fly (Penicillidia fulvida) on the wall of a diatomite mine in Nakuru, Kenya. Credit: Holly Lutz

We humans aren’t the only animals that have to worry about bug bites. There are thousands of insect species that have evolved to specialize in feeding on different mammals and birds, but scientists are still learning how these bugs differentiate between species to track down their preferred prey. It turns out, the attraction might not even be skin-deep: a new study in Molecular Ecology found evidence that blood-sucking flies that specialize on bats may be locating their preferred hosts by following the scent of chemicals produced by bacteria on the bats’ skin.

Holly Lutz, the paper’s lead author, got the idea for the project from previous research showing that mosquitoes seem to prefer some people over others. “You know when you go to a barbeque and your friend is getting bombarded by mosquitos, but you’re fine? There is some research to support the idea that the difference in mosquito attraction is linked to your skin microbiome—the unique community of living on your skin,” says Lutz, a research associate at Chicago’s Field Museum and a project scientist with the labs of Jack Gilbert (who co-authored this study) and Rob Knight at the University of California, San Diego. “Keeping in mind that some people are more attractive to mosquitoes than others, I wondered what makes insects attracted to some bats but not others.”

Lutz encountered plenty of bats during her Ph.D. work and postdoctoral residency at the Field Museum, on fieldwork trips to bat caves in Kenya and Uganda studying malaria. “In these caves, I’d see all these different bat species or even taxonomic families roosting side by side. Some of them were loaded with bat flies, while others had none or only a few. And these flies are typically very specific to different kinds of bats— you won’t find a fly that normally feeds on horseshoe bats crawling around on a fruit bat.” says Lutz. “I started wondering why the flies are so particular— clearly, they can crawl over from one kind of bat to another, but they don’t really seem to be doing that.”

Natal and African long-fingered bats (Miniopterus natalensis, M. africanus), Mauritian tomb bats (Taphozous mauritianus), and Noack's roundleaf bats (Hipposideros ruber) roosting together in a fossilized coral cave in Arabuko Sokoke Forest, Kenya. Credit: Holly Lutz

The flies in question are cousins of mosquitoes, and while they’re technically flies, most can’t actually fly. “They have incredibly reduced wings in many cases and can’t actually fly,” says Lutz. “And they have reduced eyesight, so they probably aren’t really operating by vision. So some other sensory mechanisms must be at play, maybe a sense of smell or an ability to detect chemical cues.”

​​”How the flies actually locate and find their bats has previously been something of a mystery,” says Carl Dick, a research associate at the Field Museum, professor of biology at Western Kentucky University, and one of the study’s co-authors. “But because most bat flies live and feed on only one bat species, it is clear that they somehow find the right host.”

Furthermore, bat flies transmit malaria between bats, and the malaria parasites are host-specific as well. It’s an intricate, complex system with important parallels to other vector-borne pathways for disease transmission, such as malarial and viral transmission among humans by anopheline mosquitoes. Previous research has shown that different bacterial species associated with skin or even the disease status of individual humans can influence feeding preferences of blood-seeking mosquitoes.

Closeup of a bat fly (Penicillidia fulvida). Credit: Holly Lutz

Lutz suspected that, similarly to what’s been observed in humans, the bats’ skin microbiomes may be playing a role in attracting the flies seeking them out. Skin— whether it belongs to a human or a bat— is covered with tiny microorganisms that help protect the body from invading pathogens, bolster the immune system, and break down natural products like sweat. Host species evolve alongside their skin microbiomes, leading to different species being home to different sets of bacteria.

All these different kinds of bacteria produce a unique bouquet of airborne chemicals as they metabolize nutrients. And, according to Lutz’s hypothesis, different kinds of insects are attracted to different chemical signals, which could help explain why some bats are more attractive to blood-sucking flies than others— just like your friend at the barbeque.

To test this hypothesis, Lutz examined dozens of bats from a variety of species. “We went into a ton of different caves where they roost and used long bat nets, which are basically like super sturdy butterfly nets, to catch them,” says Lutz. She and her colleagues took skin and fur samples from the bats’ bodies and wings in order to examine both the bats’ DNA and the microbes living on their skin. The researchers also examined the bats for flies. “You brush the bats’ fur with your forceps, and it’s like you’re chasing the fastest little spider,” says Lutz. “The flies can disappear in a split second. They are fascinatingly creepy.”

Eye-shine reflects from thousands of Egyptian fruit bats (Rousettus aegyptiacus) sampled by Lutz and her team at Kitum Cave in Mount Elgon National Park, Uganda. Credit: Holly Lutz

“The flies are exquisitely evolved to stay on their bat,” says Dick. “They have special combs, spines, and claws that hold them in place in the fur, and they can run quickly in any direction to evade the biting and scratching of the bats, or the efforts by researchers to capture them.”

The researchers then analyzed the specimens back at the Field Museum’s Pritzker DNA Laboratory. “Once we were back at the lab, we extracted all the DNA from the bacteria and sequenced it. We basically created libraries of all the bacteria associated with each individual skin sample. Then we used bioinformatics methods to characterize the bacteria there and identify which ones are present across different bat groups, comparing bats that were parasitized by flies to those that were not,” says Lutz.

The team found that the different bat families had their own unique combinations of skin bacteria, even when the bats were collected from different locations. “The goal of this study was to ask, ‘Are there differences in the microbiome of these different bats, and are there bacteria that are common among bats that have parasites versus those that don’t?'” says Lutz. “Getting these results was really exciting— this paper is the culmination of years of thinking and wondering and sampling.”

One of the bat species studied in this project, Hipposideros caffer. Credit: Holly Lutz

There are still some big questions to answer, however. “We weren’t able to collect the actual chemicals producing cue- – secondary metabolites or volatile organic compounds— during this initial work. Without that information, we can’t definitively say that the bacteria are leading the flies to their hosts. So, next steps will be to sample bats in a way that we can actually tie these compounds to the bacteria” says Lutz, “In science, there is always a next step.”

In addition to explaining how blind, flightless flies are able to be so picky with which they feed on, the study gets at bigger-picture questions of how different organisms coexist. “We live in these complex communities where different types of life are always bumping into each other and interacting and sometimes depending on each other or eating each other,” says Lutz. “In a healthy natural state, these organisms partition themselves so they can coexist. But as habitats are destroyed, organisms are forced to share resources or start utilizing new ones.” Animals that used to be able to give each other a wide berth might no longer be able to, and that can lead to new diseases spreading from one organism to another.

“Humans are affecting these ecosystems, and these ecosystems can in turn affect us,” says Lutz. “That’s why it’s important to study them.”

More information:
HL Lutz et al, Associations between Afrotropical bats, eukaryotic parasites, and microbial symbionts, Molecular Ecology (2021). DOI: 10.1111/mec.16044

Blood-sucking flies may be following chemicals produced by skin bacteria to locate bats to feed on (2021, July 30)
retrieved 1 August 2021

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Hexbyte Glen Cove For female flies, mating requires the right musical backdrop thumbnail

Hexbyte Glen Cove For female flies, mating requires the right musical backdrop

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Credit: CC0 Public Domain

A new study traces the neural circuit that makes a female fly receptive to a mating advance.

The female fruit fly has discerning standards for her : She’ll only signal that she’s ready to reproduce once a male fly has serenaded her with the proper courtship song.

Now, new research from scientists at HHMI’s Janelia Research Campus identifies the that controls female flies’ receptivity to mating. Led by research scientists Kaiyu Wang and Fei Wang, in the lab of group leader Barry Dickson, the team traced the pathway that directs the fly’s vaginal plates to open—a sign she’s ready to partner up.

“We’ve identified almost the whole circuit,” says Kaiyu Wang.

The pathway integrates messages from two different sets of : neurons that encode whether the fly has already mated, and neurons that are tuned to the sound of the specific courtship song sung by males of her species. If she’s paired up recently, or if she’s wooed by a different species of fly, the neurons that open her vaginal plates don’t fire and the plates remain closed, the team reports November 25 in the journal Nature. But when the fly is still seeking a mate and she hears the song of her species, those signals combined trigger the vaginal plates to open.

The next step is to figure out exactly how the fly’s brain encodes the male courtship , and how it distinguishes between songs of different species.

More information:
Kaiyu Wang et al. Neural circuit mechanisms of sexual receptivity in Drosophila females, Nature (2020). DOI: 10.1038/s41586-020-2972-7

For female flies, mating requires the right musical backdrop (2020, November 26)
retrieved 26 November 2020

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part may be reproduced without the written permission. The content is provided for information purp

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