There are cases in which the most surprising findings of scientific research come by chance. Kei Jokura’s latest discovery was one of them.
Jokura is a bioscientist and works at the University of Exeter (United Kingdom) and the National Institutes of Natural Sciences in Okazaki (Japan). Every morning, for months, the researcher went down to the port of the Marine Biological Laboratory of the University of Chicago to meet the animal that, phylogenetically, is the most distant from humans: the comb jellyfish. There he collected several specimens and took them back to the laboratory to study them. One day, during the collection, a couple of them were injured and the scientist placed them together in a tank to recover. The next morning, when he returned, he noticed that both individuals had merged to become one.
This coincidence led to an investigation that magazine Current Biology published this Monday, and is the first evidence of how a species of ctenophore, the Mnemiopsis leidyiis capable of fusing after an injury to have a better chance of surviving. The animals synchronized their nervous systems, coordinated their muscle contractions, and coupled their digestive tracts to share food. Jokura summarizes the finding: “We discovered that they have virtually no allorecognition system and that their nerves fuse easily to form a functional electrical connection.”
Allorecognition, explains the researcher, is the ability of an individual to distinguish between their own tissues and those of others. This phenomenon is common in animals and acts as a defense mechanism. It is possible, he adds, that this species of ctenophore lacks the genes necessary to distinguish between individuals of the same species. “Although it is not clear how it evolved, allorecognition may have played a role in reproduction or in preventing cannibalism,” he suggests. Perhaps, comb jellyfish never had this system integrated, or perhaps they lost it throughout their 600 million years of evolutionary history in order to survive. The lack of allorecognition, the study details, may provide clues about how this ability—and others, such as the immune system and neural networks—evolved in more advanced animals.
“It’s very surprising,” exclaims Arnau Sebé Pedrós, a biologist and doctor in Genetics who works at the Center for Genomic Regulation, in Barcelona, on the phone. “Allorecognition is one of the most ancient biological mechanisms in the world. This study questions the concept of the individual among animals,” he adds.
To move from chance to the scientific method, Jokura’s team designed an experiment: they removed parts of the lobes of several ctenophores and placed them together in pairs. In nine out of ten cases, the injured individuals became one within a few hours of contact and survived for at least three weeks. When the researchers prodded the animals, the fused body reacted with a start. In this type of organisms, muscle movements are controlled by an atypical nervous system, composed of networks of syncytial neurons, that is, without traditional synapses. It has not yet been possible to determine how these neuronal structures can organize complex networks capable of integrating with each other in times of need, such as after an injury.
What could be calibrated was that, at the beginning of the fusion, both organisms had independent movements. Two hours later, 95% of the muscle contractions were already synchronized, which gave the researchers to understand that the neural networks of both individuals had integrated to coordinate their movements.
“We are debating whether these animals invented the nervous system,” says Sebé Pedrós. The Spanish biologist wonders where the genetic limits are in this capacity for integration between two independent individuals, the answer to which could come in future research. “It is likely that the individuals who merged in this analysis are genetically very close, like brothers. It would be interesting to try the fusion with populations of comb jellyfish from different parts of the world,” he points out. Perhaps, the allorecognition of the Mnemiopsis leidyi exists, but has a certain permissiveness when it comes to genetically close individuals and allows that interindividual cooperation.
shared food
The digestive tract of the ctenophores was another key point of the research and the results also surprised the scientists. Upon closer inspection, the authors found that this system had become integrated between the two fused individuals. That is, they shared the food they received. Using fluorescently labeled brine shrimp (a tiny crustacean they feed on), they were able to track the food and see it make its way through the fused channels. This process suggests that the digestive systems are functionally connected. But not everything is so linear and simple. Excretion was not synchronous, and the organism’s feces came out through separate anuses and at different times, indicating that some aspects of its digestive control remain independent, even though they are physically united.
The usefulness of this study, in addition to reporting quite unprecedented behavior in the animal world, could have implications in the analysis of regeneration and immune systems. “I am very interested in how fused nervous systems couple electrically. Identifying the genes that facilitate this neuronal fusion could lead to important advances in the regeneration of neurons and have medical applications,” says Jokura. Furthermore, this rapid fusion capacity, which allows sharing physiological functions without activating immune rejection mechanisms, could shed light on regenerative processes in other animals, including humans.
The scientist is confident that delving into these findings could also be useful in research on transplants and regenerative treatments, where the integration of tissues and nerves is crucial for the patient’s survival. For his part, Sebé Pedrós says that “these are bugs that science has known about for more than a century and continue to give surprises year after year.” Carrying out molecular, genetic and cellular studies on them “could change global paradigms of certain fundamental principles of biology.”
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