Worms welcome!
Some shrink and grow, others regenerate body parts. Some age and die, others seem to live forever. Flatworms occur on land and in water, on all continents on earth. At the MPI-NAT, they give our researchers from the Department of Tissue Dynamics and Regeneration of Jochen Rink insights into questions of evolution, regeneration, and perhaps immortality.
A worm sucks on the glass wall of the aquarium with its tail, while the head crawls forward, stretching the body – until it finally tears apart. This spectacle is frequently observed in the Animal Facility of our institute: For this is how the flatworm Schmidtea mediterranea reproduces asexually. Within two weeks, a healthy animal grows from the pieces of a dissected worm. A group of special stem cells called neoblasts make this possible. The offspring of these cells can form any tissue and replace all body parts as needed – this even applies to the head, eye, and brain.
Research on the worm
With their special properties, flatworms raise many questions that the team of Jochen Rink’s Department of Tissue Dynamics and Regeneration wants to answer. “What is of burning interest to us is: How do the neoblasts in the body of a dissected animal ‘know’ which organs are missing and which tissues therefore have to be regenerated?”, the director says. In addition, not all flatworm species can regenerate. Some have lost this ability in the course of evolution. Why?
To get to the bottom of the flatworm mysteries, the team works in a variety of research areas: from molecular and computational biology to genomics, taxonomy, and field research. For example, the department has already been able to demonstrate a crucial molecular switch that significantly influences regenerative capacity – the so-called ‘Wnt signal transduction pathway’. When this is switched ‘on’, planarians always regenerate a tail; in contrast, if the molecular switch is set to ‘off’, a head always forms. This is true even in the planarian species Dendrocoelum lacteum, which is normally unable to regenerate a head in the posterior parts of its body. But if the scientists switched off the Wnt signaling pathway in the worm, even this species was able to regrow its head completely after loss.
Another fascinating feature of flatworms is that they have no fixed body size. A single animal of the species Schmidtea mediterranea can range from 0.5 to over 25 millimeters in size during its lifetime, depending on the food supply. “We were able to confirm that the animals grow and shrink by mainly changing the number of cells, but not the size,” Rink explains. “The same individuals can thus consist of 5,000 to several million cells at different times.” Further, his group discovered that the rate of growth and shrinkage depends on the size of the animal. This leads to another open question: How does the worm, or rather its neoblasts, ‘know’ how big it is?
Illuminating regeneration
To better understand how the cell dynamics and communication of flatworms work, the researchers are developing a method to make these processes visible. The scientists’ ambitious goal: To insert a gene for a fluorescent protein into the animals’ genome to visualize regenerated tissue. “We want to watch the stem cells later through the microscope as they regenerate body parts,” Rink says. Unlike fruit flies or zebrafish, gene modifications have not yet been successful in flatworms. “We hope to successfully apply the technique in one to two years. Our dream is to then be able to watch the molecular and cellular processes surrounding regeneration live and in color.”
About turbo worms and flamenco
For the department, our institute’s Animal Facility breeds about 60 aquatic flatworm species, which are housed in a few aquariums and about 250 plastic boxes. “It gets exciting when a new species arrives,” says Jens Krull, the collection manager. Animal keeper Sara Rudert adds: “Then it is time for detective work, because each species has different requirements. Since a majority of the species do not exist in any other lab, we have to figure everything out ourselves.” Her colleague Jana Fahrenbach pulls out a box of light-colored animals just a few millimeters long. “These are our ‘turbo worms.’ Compared to many other species, which tend to drift sluggishly in the water, these really step on the gas.” For each species, the keepers create a precise plan about optimal water quality, diet, and temperature. Only when it comes to food do most flatworms agree: Pureed calf’s liver tastes best!
Where the journey goes
The institute’s flatworm collection is highly international. There are animals from India, Japan, the USA, Brazil, Iceland, Australia, and several European countries. It is not known how many flatworm species there are worldwide. “Discovering new species is frequent as a flatworm specialist,” says Miquel Vila Farré, scientific curator of the species collection. The trained zoologist advices his colleagues in the department on the biology of the animals. “I have scientifically described more than ten species so far in my career.” He has named his personal favorite Phagocata flamenca, because the curved flanks of the worm are reminiscent of the flowing dress of a flamenco dancer.
In early 2020, the team set up a field station at Lake Baikal in Siberia (Russia), which Rink describes as the “Galapagos of flatworms.” The plan was to collaborate closely with their Russian colleagues. But now the Russian war of aggression in Ukraine has made cooperation impossible – the project is on hold. Instead, the director is looking optimistically to another corner of the world: He suspects that there are various unexplored flatworm species in East Africa. An excursion to Australia is also planned for this year. There, the scientists will search for new species to systematically answer their research questions. The flatworm collection at the Fassberg will thus continue to grow. (jw / kr / cr)
