The germ line is specified in many animal embryos by maternal RNAs and proteins that are localized in a region of the egg called the germ plasm. The equivalent region in ascidian embryos seems to be the postplasm at the posterior pole but this region also contains somatic-cell determinants. On ,Shirae-Kurabayashi et al. propose that sea squirt postplasm regulates both germ- and somatic-cell differentiation through an asymmetric cell division that segregates the two types of determinants. Using CiVH, a homologue of the Drosophila germline-specific gene vasa, and other postplasm components as markers, the researchers show that the postplasm-containing blastomeres, the B7.6 cells, divide asymmetrically to form two distinct daughter cells: B8.11 and B8.12. The postplasmic components mainly segregate into the B8.11 cells, which later associate with the gut wall, but CiVH RNA and protein segregate into the B8.12 cells, which are incorporated into the gonad. This redistribution of specific maternal molecules into the B8.12 cells, the researchers suggest, drives germ-cell specification in ascidians.
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