Zebrafish vasa RNA but not its protein is a component of the germ plasm and segregates asymmetrically before germline specification

Work in different organisms revealed that the vasa gene product is essentia l for germline specification. Here, we describe the asymmetric segregation of zebrafish vasa RNA, which distinguishes germ cell precursors from somati c cells in cleavage stage embryos. At the late blastula (sphere) stage, vas a mRNA segregation changes from asymmetric to symmetric, a process that pre cedes primordial germ cell proliferation and perinuclear localization of Va sa protein. Analysis of hybrid fish between Danio rerio and Danio feegradei demonstrates that zygotic vasa transcription is initiated shortly after th e 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 eff ect gene nebel. Furthermore, ultrastructural analysis of vasa RNA particles reveals that vasa RNA, but not Vasa protein, localizes to a subcellular st ructure that resembles nuage, a germ plasm organelle. The structure is init ially associated with the actin cortex, and subsequent aggregation is inhib ited by actin depolymerization. Later, the structure is found in close prox imity of microtubules. We previously showed that its translocation to the d istal furrows is microtubule dependent. We propose that vasa RNA but not Va sa protein is a component of the zebrafish germ plasm. Triggered by materna l signals, the pattern of germ plasm segregation changes, which results in the expression of primordial germ cell-specific genes such as vasa and, con sequently, in germline fate commitment.