Bruno-like protein is localized to zebrafish germ plasm during the early cleavage stages

Maternally supplied germ plasm is essential for germ lineage establishment in many species, but the molecular details are still largely unknown, especially in vertebrates, and identification of novel factors that localize to germ plasm is desirable. We previously reported that one of the components of zebrafish germ plasm is mRNA of the bruno-like (brul) gene, a homologue of bruno, which, in Drosophila, is known to participate in germ lineage establishment. Here, we show that not only mRNA but also protein of brul is localized to the zebrafish germ plasm at the ends of the cleavage furrows. In 4- and 8-cell stage embryos, Brul protein is localized to the periphery of the blastomeres, as well as to the ends of the cleavage furrows, forming numerous minute particles. These particles appear at the cortex of the fertilized egg within 10 min after fertilization. Surprisingly, these distinctive localizations, as well as the minute particles, completely disappeared by the 16-cell stage, although relatively weak expression was detected ubiquitously throughout embryogenesis. This is the first report of a protein that localizes to the germ plasm in zebrafish.