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First published online 5 October 2005
doi: 10.1242/dev.02049

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POPK-1/Sad-1 kinase is required for the proper translocation of maternal mRNAs and putative germ plasm at the posterior pole of the ascidian embryo

¹ Department of Biological Sciences, Graduate School of Science, Osaka University, Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
² Department of Biological Sciences, Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama 226-8501, Japan
³ Faculty of Integrated Arts and Sciences, Tokushima University, 1-1 Minami-Josanjima, Tokushima 770-8502, Japan

^* Author for correspondence (e-mail: yorinaka{at}bio.sci.osaka-u.ac.jp)

Accepted 19 August 2005

Maternal mRNAs localized to specific regions in eggs play important roles in the establishment of embryonic axes and germ layers in various species. Type I postplasmic/PEM mRNAs, which are localized to the posterior-vegetal cortex (PVC) of fertilized ascidian eggs, such as the muscle determinant macho-1 mRNA, play key roles in embryonic development. In the present study, we analyzed the function of the postplasmic/PEM RNA Hr-POPK-1, which encodes a kinase of Halocynthia roretzi. When the function of POPK-1 was suppressed by morpholino antisense oligonucleotides, the resulting malformed larvae did not form muscle or mesenchyme, as in macho-1-deficient embryos. Epistatic analysis indicated that POPK-1 acts upstream of macho-1. When POPK-1 was knocked down, localization of every Type I postplasmic/PEM mRNA examined, including macho-1, was perturbed, showing diffuse early distribution and eventual concentration into a smaller area. This is the probable reason for the macho-1 dysfunction. The postplasmic/PEM mRNAs such as macho-1 and Hr-PEM1 are co-localized with the cortical endoplasmic reticulum (cER) and move with it after fertilization. Eventually they become highly concentrated into a subcellular structure, the centrosome-attracting body (CAB), at the posterior pole of the cleaving embryos. The suppression of POPK-1 function reduced the size of the domain of concentrated cER at the posterior pole, indicating that POPK-1 is involved in the movement of postplasmic/PEM RNAs via relocalization of cER. The CAB also shrank. These results suggest that Hr-POPK-1 plays roles in concentration and positioning of the cER, as well as in the concentration of CAB materials, such as putative germ plasm, in the posterior blastomeres.

Key words: Ascidian embryo, Halocynthia roretzi, RNA localization, postplasmic/PEM RNA, Cortical endoplasmic reticulum, Germ plasm, POPK-1 kinase, Sad-1 kinase

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