"Zebrafish vasa RNA but Not Its Protein Is a Component of the Germ Plasm and Segregates Asymmetrically before Germline Specification", 

Holger Knaut ^a, Francisco Pelegri ^a, Kerstin Bohmann ^a, Heinz Schwarz ^b, and Christiane Nüsslein-Volhard ^a,
^a Max Planck Institut für Entwicklungsbiologie Abteilung Genetik, 72076 Tübingen, Germany,
^b Elektronenmikroskopie Labor, 72076 Tübingen, Germany,

Correspondence to: Holger Knaut, Max Planck Institut für Entwicklungsbiologie Abteilung Genetik, Spemannstrasse 35, 72076 Tübingen, Germany. Tel:49-7071-601-490 Fax:49-7071-601-384 E-mail:holger.knaut@tuebingen.mpg.de.

Abstract:

Work in different organisms revealed that the vasa gene productis essential for germline specification. Here, we describe theasymmetric segregation of zebrafish vasa RNA, which distinguishesgerm cell precursors from somatic cells in cleavage stage embryos.At the late blastula (sphere) stage, vasa mRNA segregation changes from asymmetric to symmetric, a process that precedes primordial germ cell proliferation and perinuclear localization of Vasa protein. 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. We propose that vasa RNA but not Vasaprotein is a component of the zebrafish germ plasm. 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.

1. "Nuclear RNA Species Activate DNA Transcription within Chromatin".

2. "A Model of Specific De-Repression within Interphase Chromatin".

3. "Selective Gene De-Repression by De-Repressor RNA".

4. "Oncogenes as Molecular Targets within Active Chromatin".

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euchromatin: "the most active portion of the genome within the cell nucleus".