How do these parasitic worms turn brown shrimp into bright orange “zombies”?

Zoom in / Orange amphipods caught the attention (and interest) of Brown University graduate students who did field research.

David Johnson

Comb the salt marshes at the mouth of Plum Island in Massachusetts, and you’re likely to spot bright orange shrimp lurking among the vegetation and debris. This unusual coloration is a sign that the shrimp have been infected by a parasitic worm, which also appears to affect the shrimp’s behaviour. Injured shrimp usually become sluggish and spend more time exposed in open swamps, making them easier to pick up for hungry birds. Now biologists at Brown University have sequenced the DNA of these shrimp to focus on the molecular mechanisms behind the changes, according to a recent study published in the journal Molecular Ecology.

“This may be an example of a parasite manipulating an intermediate host to ensure transmission between hosts,” said co-author David Rand of Brown University, comparing how malaria spreads to humans through mosquito bites. Rabies could be another relevant example: it drives infected individuals “crazy,” so they bite others and infect the next host. Learning the molecular mechanisms of these types of host-parasite interactions could have important implications for how we manage pathogens in general. , and in humans.

Parasites that control and alter the behavior of their hosts are well known in nature. Most notably, there is a family of parasitic zombie fungi called cordyceps– More than 400 different species, each targeting a specific type of insect, be it ants, dragonflies, cockroaches, aphids or beetles. In reality, The last of us Neil Druckmann, the game’s co-creator, said the premise was inspired in part by an episode of a BBC nature documentary. Planet Earth (narrated by Sir David Attenborough) “Zombie” depicts the ant in vivid detail. Scientists are eager to study cordyceps To learn more about the origins and complex mechanisms behind these types of pathogen-based diseases.

It’s a very terrifying process. First, the fungus infiltrates the host’s exoskeleton and brain via airborne spores, which then fall to the ground. When a foraging ant encounters spores, the spores stick to the ant’s body, hiding inside. Once inside, the spores sprout long tendrils called mycelium that eventually reach the brain and release chemicals that make the miserable host a slave to the fungus. The chemicals force the host to move to the location most suitable for the fungus to thrive and grow. The fungus then slowly feeds on the host, spreading new spores throughout the body as a final insult. These buds burst, releasing more spores into the air, which infect more unsuspecting hosts.

This latest study involves a parasitic worm (a trematode) instead of a fungus, targeting a specific species of bog-dwelling brown shrimp (amphipoda) instead of insects. In collaboration with the Marine Biology Laboratory in Woods Hole, Massachusetts, Rand has taken his students on annual field trips to the mouth of Plum Island since 2013. Many of these students will no doubt be amazed by Plum Island’s eerie orange color. Grill OrchestraAs a result of a worm infection Levincinella birdie. But the mechanism by which the worm was able to change the color and behavior of its host shrimp remained a mystery.

BEFORE AND AFTER: Amphipods infected with parasitic trematodes change color from light gray or brown to orange and move to more exposed areas of the salt marshes.
Zoom in / BEFORE AND AFTER: Amphipods infected with parasitic trematodes change color from light gray or brown to orange and move to more exposed areas of the salt marshes.

David Johnson

L Birdy It is an avian fluke (or shell), which means it ultimately targets birds, but there are three distinct stages in its life cycle. Infected swamp birds expel parasite eggs in their feces, which are then ingested by the snails. These snails, in turn, give birth to free-swimming worm larvae, which can then penetrate the worm’s exoskeleton. Oh cricket. So technically it’s shrimp L BirdyThe second intermediate host. After 25-30 days, the larvae become large cysts inside the shrimp’s body cavity. That’s when the color turns from brown to neon orange, and the shrimp switch to spending more time in the exposed areas of the salt marsh. This makes the shrimp more vulnerable to hungry birds. The birds consume the shrimp, become infected, and the cycle begins again.

First, rand et al. Two complete genome sequences were generated Oh cricketUsing DNA extracted from one male leg and one female leg, as the leg tissue is unlikely to be infected with flux DNA. They then sequenced the DNA of 24 infected shrimp (marked by a conspicuous orange color and at least one cyst in the body cavity) and 24 uninfected shrimp, all collected from the banks of a creek in the Plum Island estuary.

Results: Infection with flukes activates gene transcription in shrimp related to pigmentation and the ability of shrimp to detect external stimuli. At the same time, many copies of genes involved in the immune response are inhibited, which could explain why 99% of shrimp in exposed swamps at low tide become infected. rand et al. It suggests that these altered gene expressions provide the parasitic flukes with an evolutionary advantage, giving them a greater chance of increasing their numbers. “The injured amphipods become the ducks of the predators,” Rand said, in this case the birds. “This allows the parasite to spread to a newer, larger, more robust host organism, and to continue to reproduce and spread its own species.”

“These are counterintuitive findings given the apparent changes in pigmentation of amphipods as a result of infection, and their increased tendency to be found in exposed areas of their salt marsh habitat,” the researchers wrote. “While we acknowledge that prediction of organismal behavior from transcriptional profiles spans multiple important causal relationships, the phenotype-genotype correlations identified here highlight potential genetic targets driving host manipulation in the wild.”

Molecular Ecology, 2023. DOI: 10.1111/mec.17093 (about DOIs).

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