P20
Analyses of the mechanisms of epithelial morphogenesis during the tailformation in the embryos of ascidian Halocynthia roretzi
Ayaki Nakamoto*1, Gaku Kumano1
1

Research Center for Marine Biology, Asamushi, Graduate School of Life Sciences, Tohoku
University, Japan, anakamoto@m.tohoku.ac.jp (A. N.), kumano@m.tohoku.ac.jp (G. K.)

ABSTRACT
Morphogenesis is one of the important processes in animal development. To generate a newmorphological structure or a new tissue, embryonic cells need to behave properly at righttiming and right place. One of the remarkable morphognesis duirng ascidian development is
the tail formation. In the initial stage of tail formation (i.e., late neurula stage), the boundary
between trunk and tail regions is morphologically appeared as small waist (in japanese“Kubire”) at the specific position along anterior-posterior axis. Tail region then undergoes
extensive elongation along the anterior-posterior axis. At the tadpole larval stage, the lengthof tail is 4-5 times longer than that of the trunk region. The series of morphogenetic events
duirng ascidian tail formation are unique compared to other animals such as vertebrates,annelids, and insects. Therefore, to understand the mechansims of tail formation of ascidian
embryos would provide a novel principle about the mechanisms of morphogenesis.
In this study, we focus on the initail stage of tail shaping – the “Kubire” formation in theembryos of Halocynthia roretzi. We observed that “Kubire” was formed along with the cell
division of epithelium. We also found that epithelial cells divided along different orientation,and there was a clear boundary between these oriented cell divisions. In the trunk region,
cells divided around circumference of the embryo. In contralst, cells divided along
anterior-posterior axis in the tail region. Our current hypothesis is that “Kubire” is generated
by the different orientation of cell division, which occur on either side of a clear boundary in
the embryo.
To test this hypothesis experimentaly, we phamacologically disrupted the orientation of thecell division of epithelium and observed “Kubire” formation of the embryos. Whenembryos were treated with a dynein inhibitor Ciliobrevin D, which is known to block spindlerotation and thus disrupt spindle orientation in many model systems, the orientation of the cell
division was abnormal. Furthermore, these embryos failed to form “Kubire”. Thisobservation supports our hypothesis that cell division along different orientations generates a
new morphological structure.
An important condition which makes the hypothesis valid is that circumference of theembryo needs to be closed. “Kubire” is formed at the late neurula stage when neural tube isalmost closed. Interestingly, it has been known that neural tube is not closed in the embryos
which are incubated without egg membrane. It is also known that these embryos do notform “Kubire”. Therefore we currenly chareacterize the division pattern of epithelium cellsof the embryos whose neural tube is not closed in the absence of egg membrane. We will
also describe the orientation of cell division in the ventral epithelium during the “Kubire”
formation.

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P6Regulation of germline gene expression during ascidian embryogenesis
Kaori Miyaoku1, 2, Ayaki Nakamoto1, Hiroki Nishida2, Gaku Kumano*1*1

Research Center for Marine Biology, Asamushi, Graduate School of Life Sciences, Tohoku
University, Aomori, Japan, k-mok@dc.tohoku.ac.jp, anakamoto@m.tohoku.ac.jp,
kumano@m.tohoku.ac.jp
2Dapertment of Biological Science, Osaka University, Japan, hnishida@bio.sci.osaka-u.ac.jp

Germline is specialized lineage as it is the only lineage that transmits genetic information to
the next generation. In many animals, germline is separated from somatic line in early
embryogenesis. Transcriptional repression in early embryonic germline is important for theseparation from somatic line. As embryogenesis progresses, germline transcription is derepressed and zygotic transcription for germ cell formation, but not for somatic tissue formation,is started. The mechanisms by which germline transcription is regulated during embryogenesis
are not much known.
Ascidian’s germline cells inherit the centrosome-attracting body (CAB), which ismorphologically similar to germ plasm. They also inherit maternal mRNAs known as
Postplasmic/PEM RNAs which localize in the CAB. PEM, a member of Postplasmic/PEM
RNAs represses transcription in the germline globally and contributes to the separation fromsomatic line. However, functions of other Postplasmic / PEM RNAs in the germline formation
have not been analyzed. Therefore, using the ascidian, Halocynthia roretzi, we studied
functions of the RNAs and focused on their roles in germline transcriptional repression. Here,
we analyzed three Postplasmic/PEM RNAs: POPK-1; PEM; ZF-1.
Our results suggest that 1) POPK-1 contributes to repress germline transcription indirectly byregulating the proper CAB formation, which ensures that the sufficient amount of PEM mRNA
is localized to the CAB and translated for transcriptional repression, 2) decreasing PEM protein
level is necessary and sufficient for the de-repression of germline gene expression, indicatingthat PEM not only suppresses somatic gene expression, but also controls the timing of germline
zygotic transcription, 3) ZF-1 accelerates zygotic gene expression in the germline by decreasing
the PEM protein level.

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