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Axis formation is one of the most important events occurring at the beginning of animal development. In the ascidian egg, the antero-posterior axis is established at this time owing to a dynamic cytoplasmic movement called cytoplasmic and cortical reorganisation. During this movement, mitochondria, endoplasmic reticulum (ER), and maternal mRNAs (postplasmic/PEM RNAs) are translocated to the future posterior side. Although accumulating evidence indicates the crucial roles played by the asymmetrical localisation of these organelles and the translational regulation of postplasmic/PEM RNAs, the organisation of ER has not been described in sufficient detail to date owing to technical difficulties. In this study, we developed three different multiple staining protocols for visualising the ER in combination with mitochondria, microtubules, or mRNAs in whole-mount specimens. We defined the internally expanded "dense ER" using these protocols and described cisterna-like structures of the dense ER using focused ion beam-scanning electron microscopy. Most importantly, we described the dynamic changes in the colocalisation of postplasmic/PEM mRNAs and dense ER; for example, macho-1 mRNA was detached and excluded from the dense ER during the second phase of ooplasmic movements. These detailed descriptions of the association between maternal mRNA and ER can provide clues for understanding the translational regulation mechanisms underlying axis determination during ascidian early embryogenesis., (© 2021. The Author(s).)

We have cloned a newly identified gene, designated CiNut, C iona i ntestinalis neural-tube-specific gene. CiNut shows weak similarity to known neural receptors such as adrenergic receptors. Moreover, seven transmembrane domains are predicted based on its amino acid sequence. Zygotic expression of CiNut starts at the gastrula stage, and is restricted to the entire neural tube in the neurula- and the tailbud-stage embryos. CiNut is thus thought to be a novel G-protein-coupled receptor important for neural tube formation, and should provide a useful tool for the analysis of the molecular mechanism of neural tube formation.

We have cloned a putative ascidian RNA-binding protein gene, CiRGG1, which has three RNA-recognition motifs and two arginine-glycine-glycine domains. Its deduced amino acid sequence has a weak similarity to nucleolin, which is one of the known components of messenger ribonucleoproteins. Its maternal transcript is transiently accumulated into the anterior side of 2-cell stage embryos. Zygotic expression of CiRGG1 is strongest in the brain, mesenchyme and endoderm of the trunk region of the tailbud stage embryo. This mRNA is the first example of an anterior-localized molecule in the ascidian 2-cell stage embryo.