197of MEG-1 and MEG-3 by the MBK-2/DYRK kinase led to dissolution of granules,
whereas their dephosphorylation by the phosphatase PP2A induces their condensation
(Wang et al. 2014 ). Indeed, MBK-2/DYRK is localized to the anterior half of the
embryo, while PP2A is ubiquitously distributed. This results in the condensation/dis-
solution gradient of P granules along the anteroposterior axis (Wang et al. 2014 ).
Intriguingly, the Bucky ball protein, which is required for Bb formation, is an IDP
(Jamieson-Lucy and Mullins, unpublished data) (Bontems et al. 2009 ; Toretsky and
Wright 2014 ). Since the Bb is a large aggregate of mRNP granules, and given the con-
servation of a hydrogel-like structure of mRNP granules, it is tempting to speculate that
Bucky ball functions as a polymerizing substrate, providing the scaffold upon which
different Bb components amass. Buc and other Bb precursors are initially enriched in
a region around the centrosome at zygotene stages, and aggregation of the Bb precur-
sors into the mature Bb occurs in the nuclear cleft at subsequent stages (Elkouby et al.
2016 ). Interestingly, Bb mitochondria initially localize normally in the nuclear cleft in
buc−/− oocytes and only disperse later, suggesting that oocyte polarity is initiated nor-
mally in the absence of Buc (Elkouby et al. 2016 ). It is possible that Buc accumulation
over a specific threshold in the nuclear cleft may be required for Bb precursor conden-
sation into a single aggregate granule. This would argue for a scaffolding role of Buc
that is similar to the C. elegans MEG proteins. It will be important in the future to
identify potential regulators of Buc condensation-dissolution, possibly by controlling
its phosphorylation state. Such a condensed hydrogel structure would also explain the
extremely slow turnover and diffusion of mRNAs within the mature Bb and between it
and the cytoplasm in Xenopus (Chang et al. 2004 ), as discussed above.
5.8 Potential Contribution of Germ Granules/Nuage
to the Balbiani Body
Nuage is another universal feature of germ cells that has some relationship to the
Bb. The nuage consists of mRNPs seen in TEM as electron-dense material that
when associated with mitochondria has also been termed “mitochondrial cement.”
The nuage is now established to function in the piRNA pathway and across different
species harbors several piRNA pathway proteins such as the Argonaute family,
Tudor domain proteins, Maelstrom, Vasa, and GasZ (Findley et al. 2003 ; Juliano
et al. 2011 ; Ku and Lin 2014 ; Ma et al. 2009 ).
Nuage granules are usually distributed in the perinuclear cytoplasm. This is the
case in mouse developing sperm and oocytes, as well as in primordial germ cells
and young oocytes (≤ 20 μm in diameter) in zebrafish (Houwing et al. 2007 , 2008 ;
Huang et al. 2011 ; Juliano et al. 2011 ; Kamminga et al. 2010 ; Ku and Lin 2014 ; Ma
et al. 2009 ). In zebrafish, nuage containing granules have been termed “germ gran-
ules” because of their germ cell specificity and because they contain the germplasm
protein Vasa. These granules dissociate in zebrafish oocytes at early diplotene stages
(Houwing et al. 2007 , 2008 ; Huang et al. 2011 ; Kamminga et al. 2010 ) and are next
detected at early embryonic cleavage stages (Strasser et al. 2008 ). They contain
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development