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Reproductive Strategies in Planarians: Insights Gained from the Bioassay System for Sexual Induction in Asexual Dugesia ryukyuensis Worms

  • Takanobu Maezawa
  • Kiyono Sekii
  • Masaki Ishikawa
  • Hikaru Okamoto
  • Kazuya Kobayashi
Chapter
Part of the Diversity and Commonality in Animals book series (DCA)

Abstract

Some freshwater planarians (Platyhelminthes, Turbellaria, Seriata, and Tricladida) reproduce asexually by transverse fission and subsequent regeneration. Depending on environmental conditions, some asexual worms also develop complex hermaphroditic reproductive organs from planarian pluripotent stem cells, or neoblasts. Acquired sexual worms then mate, and eventually lay a cocoon filled with several fertilized eggs and a large number of yolk gland cells. The mechanisms underlying the switch from an asexual to a sexual state and the differentiation of germ cells from adult stem cells are of interest because they represent fundamental aspects of both reproductive biology and developmental biology. To study this mechanism, an experimental system was established in a triploid asexual strain, the OH strain of Dugesia ryukyuensis (Kobayashi et al. 1999). In this assay system, asexual worms acquire sexuality and cease asexual reproduction by transverse fission when experimentally dosed with sex-inducing substances produced by sexually mature planarians. Acquired sexual worms are then able to produce sex-inducing substances by themselves to maintain a sexual state, without the intake of sex-inducing substances. Interestingly, triploid acquired sexual worms sexually produce four types of offspring, namely, diploid asexual worms, diploid innate sexual worms, triploid asexual worms, and triploid innate sexual worms. Although the relationship between sexuality and ploidy has not yet been clarified, innate sexuality noticeably differs to acquired sexuality because worms never convert to an asexual state. This review discusses insights obtained from the study of these intricate biological phenomena to help elucidate the mechanisms used for reproductive strategies in planarians, including that of switching from an asexual to a sexual state.

Keywords

Germ cells Planarian Dugesia ryukyuensis Asexual reproduction Sexual reproduction Sexual induction Sex-inducing substance 

Notes

Acknowledgements

We thank Dr. Yuni Nakauchi’s group (Yamagata University) for providing invaluable assistance with collecting B. brunnea and S. auriculata. We also thank Dr. Hidefumi Orii (University of Hyogo), Dr. Kimitoshi Sakamoto (Hirosaki University), and Dr. P. A. Newmark (University of Wisconsin-Madison) for their kind gifts of D. japonica worms and sexual specimens of S. mediterranea (Dr. Orii), G. dorotocephala worms (Dr. Sakamoto), and asexual specimens of S. mediterranea (Dr. Newmark). We also thank Miss Sachiko Arioka for producing 3D images. This study was supported in part by a Grant-in-Aid for Scientific Research (Nos. 26114501 [KK], 15K07121 [KK], and 25650103 [TM]) from the Ministry of Science, Culture, Sports and Education, Japan, The NAITO Foundation (KK), The Sumitomo Foundation (TM), and Ryobi Teien Memory Foundation (TM).

Supplementary material

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Copyright information

© Springer Japan KK, part of Springer Nature 2018

Authors and Affiliations

  • Takanobu Maezawa
    • 1
  • Kiyono Sekii
    • 2
  • Masaki Ishikawa
    • 2
  • Hikaru Okamoto
    • 2
  • Kazuya Kobayashi
    • 2
  1. 1.Advanced Science Course, Department of Integrated Science and TechnologyNational Institute of Technology, Tsuyama CollegeOkayamaJapan
  2. 2.Department of Biology, Faculty of Agriculture and Life ScienceHirosaki UniversityHirosakiJapan

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