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PLoS One. 2015 Nov 20;10(11):e0143525. doi: 10.1371/journal.pone.0143525. eCollection 2015.

Unusually Large Number of Mutations in Asexually Reproducing Clonal Planarian Dugesia japonica.

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Global COE Program: Evolution and Biodiversity, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo-ku, Kyoto, Japan.
Department of Biophysics, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo-ku, Kyoto, Japan.
Cellular and Structural Physiology Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, Japan.
Center for Developmental Biology, RIKEN, 2-2-3 Minatojima-Nakamachi, Chuo-ku, Kobe, Hyogo, Japan.
Advanced Center for Computing and Communication, RIKEN, 2-1 Hirosawa, Wako, Saitama, Japan.
Graduate School of Life Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo, Japan.


We established a laboratory clonal strain of freshwater planarian (Dugesia japonica) that was derived from a single individual and that continued to undergo autotomous asexual reproduction for more than 20 years, and we performed large-scale genome sequencing and transcriptome analysis on it. Despite the fact that a completely clonal strain of the planarian was used, an unusually large number of mutations were detected. To enable quantitative genetic analysis of such a unique organism, we developed a new model called the Reference Gene Model, and used it to conduct large-scale transcriptome analysis. The results revealed large numbers of mutations not only outside but also inside gene-coding regions. Non-synonymous SNPs were detected in 74% of the genes for which valid ORFs were predicted. Interestingly, the high-mutation genes, such as metabolism- and defense-related genes, were correlated with genes that were previously identified as diverse genes among different planarian species. Although a large number of amino acid substitutions were apparently accumulated during asexual reproduction over this long period of time, the planarian maintained normal body-shape, behaviors, and physiological functions. The results of the present study reveal a unique aspect of asexual reproduction.

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