How female fruit flies know when to say ‘yes’

A fundamental question in neurobiology is how animals, including humans, make decisions. A new study publishing in the open access journal PLOS Biology on October 7 reveals how fruit fly females make a very important decision: to either accept or reject male courtship. This decision appears to be generated by a very small number of excitatory neurons that use acetylcholine as their neurotransmitter located in three brain regions. This study provides the framework to understand how decisions are generated and suggests that a decision is reached because that option is literally the most exciting.

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Fruit fly research may reveal what happens in female brains during courtship and mating

What are the complex processes in the brain involved with choosing a mate, and are these processes different in females versus males? It’s difficult to study such questions in people, but researchers are finding clues in fruit flies that might be relevant to humans and other animals. Three different studies on the topic are being published in the Cell Press journals Neuron (1, 2) and Current Biology.

Work over the past 100 years has largely focused on the overt courtship behaviors that male flies direct toward females. However, the female ultimately decides whether to reject the male or copulate with him. How does the female make this decision? In one Neuron paper, researchers report that they have identified two small groups of neurons in the female brain that function to modulate whether she will mate or not with a male based on his distinct pheromones and courtship song. In this paper, a team led by Dr. Bruce Baker of the Howard Hughes Medical Institute’s Janelia Farm Research Campus in Virginia also reports that these neurons are genetically distinct from the previously identified neurons that function to drive the elaborate courtship ritual with which a male woos a female. “An understanding of the neural mechanisms underlying how sensory information elicits appropriate sexual behaviors can be used as a point of comparison for how similar sexual behavior circuits are structured and function in other species,” says Dr. Baker.

In the Current Biology study, Dr. Leslie Vosshall of The Rockefeller University in New York City and her team found that a small group of neurons in the abdominal nerve cord and reproductive tract—called Abdominal-B neurons—is necessary for the female to pause her movement and interact with a courting male. When the neurons are inactivated, the female ignores the male and keeps moving, but when the neurons are activated, the female spontaneously pauses. “Sexual courtship is a duet—the male and female send signals back and forth until they reach the point that copulation proceeds,” says Dr. Jennifer Bussell, the lead author of the study. “Pausing to interact with a male, rather than avoiding him, is a crucial step in any female’s behavior leading to copulation. Tying a group of neurons to this particular response to males will allow us to dissect in detail how female mating circuitry functions.”

In another Neuron paper, researchers studied the effects of a small protein called sex peptide that is transferred along with sperm from males to females and is detected by sensory neurons in the uterus. Sex peptide changes the female’s behavior so that she is reluctant to mate again for about10 days. The investigators traced the neuronal pathway that is modulated when the uterus’s sensory neurons detect sex peptide. “Thanks to our work, we think the sex peptide signal goes to a region of the fly’s brain that is the homolog of the hypothalamus, which has been know for many years to be central in controlling sexual receptivity in vertebrates,” explains co-lead author Dr. Mark Palfreyman of the Research Institute of Molecular Pathology in Vienna, Austria. This region of the brain links the nervous system to the endocrine, or hormonal, system. “Of course, these models will still need to be tested and our work only provides an initial glimpse, but our study opens the possibility that analogous neuroendocrine systems control sexual receptivity from flies to vertebrates,” adds senior author Dr. Barry Dickson, who was also a co-author on the Current Biology paper published by Dr. Vosshall.

Quick Getaway: How Flies Escape Looming Predators

When a fruit fly detects an approaching predator, the fly can launch itself into the air and soar gracefully to safety in a fraction of a second. But there’s not always time for that. Some threats demand a quicker getaway. New research from scientists at Howard Hughes Medical Institute’s Janelia Research Campus reveals how a quick-escape circuit in the fly’s brain overrides the fly’s slower, more controlled behavior when a threat becomes urgent.

“The fly’s rapid takeoff is, on average, eight milliseconds faster than its more controlled takeoff,” says Janelia group leader Gwyneth Card. “Eight milliseconds could be the difference between life and death.”

Card studies escape behaviors in the fruit fly to unravel the circuits and processes that underlie decision making, teasing out how the brain integrates information to respond to a changing environment. Her team’s new study, published online June 8, 2014, in the journal Nature Neuroscience, shows that two neural circuits mediate fruit flies’ slow-and-stable or quick-but-clumsy escape behaviors. Card, postdoctoral researcher Catherine von Reyn, and their colleagues find that a spike of activity in a key neuron in the quick-escape circuit can override the slower escape, prompting the fly to spring to safety when a threat gets too near.

A pair of neurons—called giant fibers—in the fruit fly brain has long been suspected to trigger escape. Researchers can provoke this behavior by artificially activating the giant fiber neurons, but no one had actually demonstrated that those neurons responded to visual cues associated with an approaching predator, Card says. She was curious how the neurons could be involved in the natural behavior if they didn’t seem to respond to the relevant sensory cues, so she decided to test their role.

Genetic tools developed in the lab of Janelia executive director Gerald Rubin enabled Card’s team to switch the giant fiber neurons on or off, and then observe how flies responded to a predator-like stimulus. They conducted their experiments in an apparatus developed in Card’s lab that captures videos of individual flies as they are exposed to a looming dark circle. The image is projected onto a hemispheric surface and expands rapidly to fill the fly’s visual field, simulating the approach of a predator. “It’s really like a domed IMAX for the fly,” Card explains. A high-speed camera records the response at 6,000 frames per second, allowing Card and her colleagues to examine in detail the series of events that make up the fly’s escape.

To ensure their experiments were relevant to fruit flies’ real-world experiences, Card teamed with fellow Janelia group leader Anthony Leonardo to record and analyze the trajectories and acceleration of damselflies—natural predators of the fruit fly—as they attacked. They designed their looming stimulus to mimic these features. “We wanted to make sure we were really challenging the animal with something that was like a predator attack,” Card says.

By analyzing more than 4,000 flies, Card and her colleagues discovered two distinct responses to the simulated predator: long and short escapes. To prepare for a steady take-off, flies took the time to raise their wings fully. Quicker escapes, in contrast, eliminated this step, shaving time off the take-off but often causing the fly to tumble through the air. 

When the scientists switched off the giant fiber neurons, preventing them from firing, flies still managed to complete their escape sequence. “On a surface level evaluation, silencing the neuron had absolutely no effect,” Card says. “You can do away with this neuron that people thought was fundamental to this escape behavior, and flies still escape.” Shorter escapes, however, were completely eliminated. Flies without active giant fiber neurons invariably opted for the slower, steadier escape. In contrast, when the scientists switched giant fiber neurons on in the absence of a predator-like stimulus, flies enacted their quick-escape behavior. The evidence suggested the giant fiber neurons were involved only in short escapes, while a separate circuit mediated the long escapes.

Card and her colleagues wanted to understand how flies decide when to sacrifice stability in favor of a quicker response. To learn more, Catherine von Reyn, a postdoctoral researcher in Card’s lab, set up experiments in which she could directly monitor activity in the giant fiber neurons. Surprisingly, she discovered that the giant fibers were not only active in short-mode escape, but also during some of the long-mode escapes. The situation was more complicated than their genetic experiments had suggested. “Seeing the dynamics of the electrophysiology allowed us to understand that the timing of the spike is important is determining the fly’s choice of escape behavior,” Card says.  

Based on their data, Card and von Reyn propose that a looming stimulus first activates a circuit in the brain that initiates a slow escape, beginning with a controlled lift of the wings. When the object looms closer, filling more of the fly’s field of view, the giant fiber activates, prompting a more urgent escape. “What determines whether a fly does a long-mode or a short-mode escape is how soon after the wings go up the fly kicks its legs and it starts to take off,” Card says. “The giant fiber can fire at any point during that sequence. It might not fire at all—in which case you get this nice long, beautifully choreographed takeoff. It might fire right away, in which case you get an abbreviated escape.” The more quickly an object approaches, the sooner the giant fiber is likely to fire, increasing the probability of a short escape.

Card remains curious about many aspects of escape behavior. How does a fly calculate the orientation of a threat and decide in which direction to flee, she wonders. What makes a fly decide to initiate a takeoff as opposed to other evasive maneuvers? The relatively compact circuits that control these sensory-driven behaviors provide a powerful system for exploring the mechanisms that animals use to selecting one behavior over another, she says. “We think that you can really ask these questions at the level of individual neurons, and even individual spikes in those neurons.”

Brain traffic jams that can disappear in 30 seconds

Motorists in Los Angeles, San Francisco and other gridlocked cities could learn something from the fruit fly.

Scientists have found that cellular blockages, the molecular equivalent to traffic jams, in nerve cells of the insect’s brain can form and dissolve in 30 seconds or less.

The findings, presented in the journal PLOS ONE, could provide scientists much-needed clues to better identify and treat neurodegenerative diseases such as Alzheimer’s and Huntington’s.

“Our research suggests that fixed, permanent blocks may impede the transport of important cellular components and, ultimately, lead to cellular degeneration and death,” says lead researcher Shermali Gunawardena, PhD, an assistant professor of biological sciences in the University at Buffalo’s College of Arts and Sciences. “Conversely, blocks that resolve themselves may be benign.”

She continues: “This is an important distinction that could help researchers decide which kind or type of blocks to focus on when developing drugs and other forms of therapy for some of these debilitating diseases.”

Scientists have long known that many essential cellular components are transported along tracts of nerve cells called neuronal pathways, and that these movements are required for the growth, function and maintenance of neurons. Only recently, however, have they been able to understand the many proteins that help control these movements.

In the UB study, researchers examined isolated nerve cells from fruit fly larvae. Neuronal pathways of these larvae are similar to neuronal pathways in humans.

Traditionally, researchers have identified blockages through still images of dead larvae. These images provide a snapshot only, instead of a depiction of the behavior of the accumulated components over distinct periods of time.

UB researchers altered the approach by analyzing the neuronal pathways of living larvae. Unlike the still images, this method shows how the transport of components changes as neuronal pathways evolve over time.

The researchers found that certain blockages form and dissolve rather quickly. For example, one blockage appeared and disappeared within 29 seconds. Its relatively short life, Gunawardena said, indicates that the blockage is likely benign and not harmful to the cell.

The distinction is significant, she said, because it could allow researchers to focus on permanent blockages that likely halt cellular movement and may pose more serious health risks.

Researchers also looked at how the transport of essential materials over several days contributed to the growth of neurons. If transport was disrupted, growth of the neuron was compromised. As the neuron grew, the movement of some components carrying synaptic proteins increased while other components did not show significant changes.

This suggests that the transport of components in neuronal pathways is linked to the growth and function of the nerve cell.

Taken together, the findings suggest that more research must be conducted to better understand the spatial and temporal characteristics of how essential materials are transported within neurons of living organisms. This, in turn, will provide clues into how defects in this system can lead to neurodegenerative diseases and, perhaps, better ways to identify and treat these ailments.

Fruit flies ‘think’ before they act

Oxford University neuroscientists have shown that fruit flies take longer to make more difficult decisions.

In experiments asking fruit flies to distinguish between ever closer concentrations of an odour, the researchers found that the flies don’t act instinctively or impulsively. Instead they appear to accumulate information before committing to a choice.

Gathering information before making a decision has been considered a sign of higher intelligence, like that shown by primates and humans.

'Freedom of action from automatic impulses is considered a hallmark of cognition or intelligence,' says Professor Gero Miesenböck, in whose laboratory the new research was performed. 'What our findings show is that fruit flies have a surprising mental capacity that has previously been unrecognised.'

The researchers also showed that the gene FoxP, active in a small set of around 200 neurons, is involved in the decision-making process in the fruit fly brain.

The team reports its findings in the journal Science. The group was funded by the Wellcome Trust, the Gatsby Charitable Foundation, the US National Institutes of Health and the Oxford Martin School.

The researchers observed Drosophila fruit flies make a choice between two concentrations of an odour presented to them from opposite ends of a narrow chamber, having been trained to avoid one concentration.

When the odour concentrations were very different and easy to tell apart, the flies made quick decisions and almost always moved to the correct end of the chamber.

When the odour concentrations were very close and difficult to distinguish, the flies took much longer to make a decision, and they made more mistakes.

The researchers found that mathematical models developed to describe the mechanisms of decision making in humans and primates also matched the behaviour of the fruit flies.

The scientists discovered that fruit flies with mutations in a gene called FoxP took longer than normal flies to make decisions when odours were difficult to distinguish – they became indecisive.

The researchers tracked down the activity of the FoxP gene to a small cluster of around 200 neurons out of the 200,000 neurons in the brain of a fruit fly. This implicates these neurons in the evidence-accumulation process the flies use before committing to a decision.

Dr Shamik DasGupta, the lead author of the study, explains: ‘Before a decision is made, brain circuits collect information like a bucket collects water. Once the accumulated information has risen to a certain level, the decision is triggered. When FoxP is defective, either the flow of information into the bucket is reduced to a trickle, or the bucket has sprung a leak.’

Fruit flies have one FoxP gene, while humans have four related FoxP genes. Human FoxP1 and FoxP2 have previously been associated with language and cognitive development. The genes have also been linked to the ability to learn fine movement sequences, such as playing the piano.

'We don't know why this gene pops up in such diverse mental processes as language, decision-making and motor learning,' says Professor Miesenböck. However, he speculates: 'One feature common to all of these processes is that they unfold over time. FoxP may be important for wiring the capacity to produce and process temporal sequences in the brain.'

Professor Miesenböck adds: ‘FoxP is not a “language gene”, a “decision-making gene”, even a “temporal-processing” or “intelligence” gene. Any such description would in all likelihood be wrong. What FoxP does give us is a tool to understand the brain circuits involved in these processes. It has already led us to a site in the brain that is important in decision-making.’