Germ-Cell Formation in Solitary Ascidians: Coexistence of Preformation and Epigenesis

Part of the Diversity and Commonality in Animals book series (DCA)


In metazoans, primordial germ cells (PGCs) are the only type of cells that transmit genetic material into the next generation and are therefore vital for species preservation. PCGs are formed in two ways: they originate from cells that inherit maternal determinants in the germ plasm (preformation), or arise epigenetically in the early embryonic stages or the adult stage through cell-cell interaction (epigenesis). The epigenetic mode of PGC formation has been proposed to be ancient, but it can change dramatically during evolution. Several groups of animals have independently evolved the preformation mode, which is therefore polyphyletic. Although several conserved mechanisms and molecules involved in the maintenance and differentiation (gametogenesis) of germ cells have been identified, the principles and evolutionary paths of PGC specification remain largely unknown.

In ascidians, which are chordate siblings of vertebrates, the embryos contain post-plasm, a specific cytoplasm that accumulates a series of specific maternal components including germ-cell determinants, and is thus the equivalent of the germ plasm. Our previous studies showed that in the Ciona robusta (Ciona intestinalis type A) embryo, PGCs originate from the descendants of the posterior-most blastomeres that inherit the post-plasm at the ~110-cell stage. However, PGCs are also reported to form epigenetically in this species. When preformed PGCs are surgically removed from tadpole larvae, PGCs re-appear in the gonads after metamorphosis and can develop into functional gametes. Therefore, C. robusta appears to have an epigenetic mode of PGC formation, in addition to the better-known preformation mechanism. Because of this unique feature, Ciona is an ideal system for investigating two modes of PGC formation in a single chordate species.

We previously analyzed the molecular functions of evolutionarily conserved germline-related genes in C. robusta during early development, and found that they have conserved roles in germ-cell maintenance. Furthermore, recent advances in genome-editing technology will enable us to perform comparative analyses of the molecular mechanisms involved in two modes of PGC formation in C. robusta. Here, we introduce this unique and fascinating system for PGC formation in solitary ascidians, and provide future perspectives to further elucidate its evolutionary path in ascidians and other metazoans.


Primordial germ cells Germ plasm Ascidian Ciona intestinalis type A Ciona robusta predetermination Epigenesis 



We thank Gretchen Lambert and Shiori Nakazawa for their critical reading of the manuscript, and Rikako Iemura for English proofreading. We also thank MEXT National Bio-Resource Project for providing living adults of the adult C. robusta (C. intestinalis type A). This study was supported by JSPS KAKENHI (19770203, 15K14530), MEXT KAKENHI (26114508, 26114513), the Hayashi Memorial Foundation for Female Natural Scientists, and the Naito Foundation.


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Copyright information

© Springer Japan KK, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Sugashima Marine Biological Laboratory, Graduate School of ScienceNagoya UniversityTobaJapan
  2. 2.Department of Germline Development, Institute of Molecular Embryology and GeneticsKumamoto UniversityKumamotoJapan
  3. 3.Graduate School of Pharmaceutical SciencesKumamoto UniversityKumamotoJapan

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