193Kinesin-1 has been identified to interact with Buc protein and shown to function
in the germ line of the embryo. Kinesin-1 (Kif5Ba) acts in the accumulation of GP to
the cleavage furrows in the 4-cell stage embryo (Campbell et al. 2015b). Kif5Ba is
required for the recruitment of Buc to the cleavage furrows along with several RNA
components of the GP. Consistent with this important role in GP assembly, kif5ba
mutants do not form PGCs and develop into sterile adults. Interestingly, kif5ba RNA
does not localize to the Bb in stage I oocytes and the kif5ba mutant does not cause
defects in oocyte polarity (Campbell et al. 2015b). Thus, Buc remains as the only
known factor to have GP assembly functions in the oocyte and early embryo.
5.5.2 Macf1: Cytoskeleton Integration Underlies AV Polarity
Establishment
Microtubules (MT), actin, and intermediate-filament (IF) cytoskeletal components
are all detected in different compartments during oocyte progression. Cytokeratins,
for instance, are detected first in the Bb (Gard et al. 1997 ) and in the cytoplasm as
oogenesis progresses in Xenopus. On the other hand, polymerized MTs are not
detected in the Bb and show intricate cytoplasmic networks in late-stage Xenopus
oocytes (Gard 1991 , 1992 , 1999 ; Gard et al. 1995 ). Actin distributes cortically and
is present also intranuclearly (Gard 1999 ; Gupta et al. 2010 ; Marlow and Mullins
2008 ). Microtubule actin cross-linking factor 1 (Macf1), a member of the spectra-
plakin family, functionally integrates the cytoskeleton and is required for maintain-
ing the Bb structure and for guiding the Bb and associated RNAs to the cortex for
vegetal anchoring (Fig. 5.2b) (Gupta et al. 2010 ). Paradoxically, disruption of MT
and actin filaments in early stage I oocytes does not affect Bb structure nor the
localization of RNAs to the Bb through the early localization pathway. Nevertheless,
Macf1 function suggests that the cytoskeleton is relevant for the early pathway of
RNA localization. Thus, elucidating Macf1 function and the cytoskeleton-
dependent mechanism of RNA localization will shed light into the unknown pro-
cesses of cytoskeleton integration that are required for AV polarity establishment
(Gard et al. 1997 ).
The functions of spectraplakins in cell polarity have been reported in a variety of
cell types (Alves-Silva et al. 2012 ; Applewhite et al. 2010 ; Bottenberg et al. 2009 ;
Karakesisoglou et al. 2000 ; Kodama et al. 2003 ; Lin et al. 2005 ; Sanchez-Soriano
et al. 2009 ; Sonnenberg and Liem 2007 ; Wu et al. 2008 , 2011 ). Shot, the Macf1
homolog in flies, is required for the organization of the fusome and for oocyte pro-
gression. The fusome is a membranous structure that extends between the cytoplas-
mic bridges of the 16 cells of the oogonial cyst and which functions in oocyte
specification (Roper and Brown 2004 ). Shot localizes to the fusome and functions
to recruit MT for the transport of factors, including centrosomes, toward the oocyte.
Shot in Drosophila probably acts much earlier during oogenesis than Macf1 in
zebrafish, since the macf1 mutant phenotype is visible only around mid-stage I of
oogenesis, long after the oogonial cyst stage (Gupta et al. 2010 ).
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development