2019/07/24
Hirosaki University
National Institute for Basic Biology
Keio University
Some species of the flatworm planarian can switch between two reproductive modes: asexual reproduction by fission and regeneration, and sexual reproduction by forming germ cells such as sperm and eggs to mate with other individuals. After fission, asexual individuals regenerate almost all their organs, but they do not produce germ cells or reproductive organs, thus maintaining their asexual state. In contrast, in sexual individuals, regeneration by fission stops, and they newly produce germ cells that they previously lacked. This switch in reproductive mode is made possible by the pluripotent stem cells that planarians possess throughout their bodies, from which all tissues, including germ cells, can be formed. Considering that many organisms produce germ cells during embryonic development (the process of forming a body from a fertilized egg), planarians are unusual creatures because they can newly form germ cells even as adults. Thus, the switch between asexual and sexual states in planarians is an interesting phenomenon, and how stem cells are controlled is thought to be the key.
A research group led by Associate Professor Kazuya Kobayashi of Hirosaki University conducted RNA sequencing, which allows for a comprehensive analysis of expressed genes, using the planarian *Dugesia ryukyuensis*, which can be experimentally induced to switch to a sexual state. This was the first step toward elucidating the mechanism of this reproductive mode switch. As a result, they were able to create a gene catalog consisting of 181,393 genes. Through comprehensive computer analysis, they also discovered that the expression of gene groups related to amino acid metabolism differs between asexual and sexual individuals. By focusing on these genes and conducting further analysis, they experimentally proved that serotonin, a metabolite of tryptophan, functions as an ovary-inducing factor. The facts that metabolism, such as amino acid metabolism and glycolysis, differs in pluripotent stem cells and germ cells compared to differentiated cells, and that a relationship between serotonin and germ cells has been suggested in other organisms, indicate that these are likely not phenomena specific only to planarians. Parasitic flatworms closely related to planarians, such as flukes and tapeworms, are known to also use both asexual and sexual reproduction, raising interest in whether the characteristics discovered in planarians in this study are common to other flatworms. The results of this research are expected to contribute to the elucidation of not only the reproductive strategies of flatworms but also the mechanisms of stem cell control and germ cell formation in a wide variety of organisms.
This research was published in the international scientific journal "Scientific Reports" on April 16, 2019.
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