Fungus manipulates fruit flies to climb and die in strategic locations

A recent study published in eLife by Carolyn Elya, a postdoctoral researcher at Harvard University, sheds light on the molecular and cellular mechanisms underlying the behavior manipulation caused by the parasitic fungus, Entomophthora muscae (E. muscae), in fruit flies.

In a previous study published in eLife in 2018, Elya initially observed an intriguing phenomenon called “summiting,” which involves infected fruit flies exhibiting altered behavior. While conducting her research on fruit fly-carrying microbes during her time as a graduate student at UC Berkeley, Elya set up decaying fruit as bait to capture wild fruit flies.

To her surprise, when she checked her traps, instead of finding captured fruit flies, she discovered lifeless zombie flies displaying a distinct abdominal banding pattern in a peculiar posture. Through DNA extraction and sequencing, Elya confirmed that the cause behind this behavior was E. muscae.

Summiting occurs during sunset when the infected flies ascend to higher positions and extend their proboscises, followed by a sticky droplet emerging from the proboscis that adheres the fly to the surface. Subsequently, the flies raise their wings away from their bodies and eventually perish.

Elya explains, “The climbing behavior is crucial as it positions the fly in a favorable location for the fungus to infect a maximum number of potential hosts. The fungus employs specialized and transient structures that puncture through the fly’s skin, releasing spores into the environment that remain viable for only a few hours. This process is ephemeral, so occupying an advantageous position is vital for survival.”

Zombie flies strike their final pose adhered to a wooden dowel. Credit: Carolyn Elya

During her time at UC Berkeley, Elya developed a laboratory model known as the Entomophthora muscae-Drosophila melanogaster ‘zombie fly’ system using a wild fungal isolate she discovered in her own backyard. This model enabled her to infect fruit flies, a common laboratory organism, with the fungus and culture it independently from the host flies in a medium designed to mimic their internal environment.

While the concept of summiting had been mentioned in scientific literature before, previous studies had only observed dead house flies and lacked insight into the behavior of flies in their final hours of life. Elya aimed to bridge this knowledge gap by creating a high-throughput behavioral assay that could automatically track the behavior of hundreds of infected flies. During this investigation into the summiting behavior, she made an unexpected finding.

Elya explains, “We discovered that summiting is not about climbing per se; it is actually a burst of locomotor activity that occurs approximately two and a half hours before the flies die.”

This breakthrough led Elya and her co-authors to combine her experimental system with the powerful fruit fly genetic toolkit available in their laboratory. By utilizing these tools and the newly developed behavior assay, they could identify the specific genes and neurons necessary for flies to exhibit summiting behavior.

“In our study, we found that the flies’ hormonal axes play a role in mediating summiting behavior. When we silenced these neurons, the flies showed significant difficulty in summiting,” says Elya. These neurons project to a neurohemal organ responsible for producing juvenile hormone, a hormone conserved in insects. “We believe that the fungus actually manipulates the activity of these neurons to induce the release of this hormone, which triggers the burst of locomotor activity observed in the flies.”

Zombie flies strike their final pose at the top of a vial. Credit: Carolyn Elya

Overall, these findings provide valuable insights into the mechanisms underlying summiting behavior and highlight the role of hormonal regulation and neuronal activity in the manipulation of host behavior by the parasitic fungus.

Using the behavioral dataset comprising numerous infected flies, Elya and her colleagues employed machine learning techniques to train a computer program to recognize and identify flies in the summiting state. This classifier tool provided valuable insights, revealing that the parasitic fungal cells systematically invade specific regions of the fly’s brain during summiting.

Interestingly, the research team also made a noteworthy observation regarding the compromised blood-brain barrier in infected flies. Ordinarily, the neurons in the fly’s brain are shielded from the circulating blood. However, the presence of the fungus leads to a breakdown of this barrier, allowing substances from the blood to potentially interact with the neurons in the brain. This breakdown has significant implications for the modulation of neural activity and is believed to be relevant to the behavioral changes induced by the fungus.

Elya states, “We believe this could play a crucial role in how the fungus manipulates behavior. In fact, we discovered that by extracting blood from flies exhibiting summiting behavior and transferring it to unaffected flies, we could induce increased locomotion. This indicates that certain factors circulating in the blood can drive summiting behavior.” However, the specific identity of these factors and whether they originate from the fungus or the fly itself remains unknown.

The experiments conducted by Elya and her team demonstrate that blood-borne factors have the ability to influence summiting behavior, although further research is needed to determine the precise nature of these factors and their source. Elya’s future plans involve developing transgenic techniques to investigate the fungal side of the interaction in addition to the existing perturbations that can be made in the flies. She emphasizes that numerous unanswered questions remain, and unraveling the mechanisms employed by the fungus continues to be an intriguing mystery.

Source: Harvard University

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