Segregates Asymmetrically before Germline Specification. Here, we describe theasymmetric segregation of zebrafish vasa RNA, which distinguishesgerm cell precursors from somatic cells in cleavage stage embryos. Analysis of hybrid fish between Danio rerio and Danio feegradei demonstrates that zygotic vasa transcription is initiated shortly after the loss of unequal vasa mRNA segregation. Blocking DNA replication indicates that the change in vasa RNA segregation is dependent on a maternal program.
Asymmetric segregation is impaired in embryos mutant for the maternal effect gene nebel .
Furthermore, ultrastructural analysis of vasa RNA particles reveals that vasa RNA, but not Vasa protein, localizes to a subcellular structure that resembles nuage, a germ plasm organelle. The structure is initially associated with the actin cortex, and subsequent aggregation is inhibited by actin depolymerization. Later, the structure is found in close proximity of microtubules. We previously showed that its translocation to the distal furrows is microtubule dependent. Triggeredby maternal signals, the pattern of germ plasm segregation changes,which results in the expression of primordial germ cell specificgenes such as vasa and, consequently, in germline fate commitment.