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localized to the vegetal pole of the egg and zygote (Fig. 5.1a) (Ku and Melton 1993 ;
Lu et al. 2011 ; Nojima et al. 2004 , 2010 ). Upon fertilization in Xenopus and egg
activation in zebrafish, microtubules reorganize to pave the way for these mRNAs to
translocate asymmetrically in an animal direction, resulting in their new localization
specifically on the future dorsal side of the embryo (Fig. 5.1b) (Heasman 2006 ; Ku
and Melton 1993 ; Lu et al. 2011 ; Tao et al. 2005 ). Prior to this cortical rotation, these
mRNAs are translationally silent. While wnt8 translation regulation has yet to be dis-
sected in zebrafish, wnt11 mRNA in Xenopus was shown to load onto polysomes
only after detaching from the microtubules at its dorsal destination during cleavage
stages (Schroeder et al. 1999 ). A similar mechanism is expected for zebrafish Wnt8.
The coordinated localization and translation of these mRNAs ensures their dorsally
localized protein product and results in a dorsal to ventral Wnt gradient (Fig. 5.1b).
Two factors identified in maternal-effect screens in zebrafish, syntabulin (tokkaebi)
and grip2a (hecate), are also localized to the vegetal pole of the egg and play key roles
in dorsal determinant translocation following egg activation (Ge et al. 2014 ; Nojima
et al. 2010 ). Consistent with this role, females mutant for these genes produce embryos
that are ventralized. Syntabulin is a microtubule-motor linker protein that can bind the
Kinesin I motor heavy chain, Kif5b (Nojima et al. 2010 ). Syntabulin protein localizes
to the vegetal pole of the egg and translocates asymmetrically in a microtubule-depen-
dent manner, consistent with it playing a role in translocating dorsal determinants on
the microtubule array. The grip2a mRNA also localizes to the vegetal pole and itself is
translocated asymmetrically like the wnt transcripts following egg activation (Fig. 5.1b)
(Ge et al. 2014 ). In eggs of grip2a mutant females, the vegetal pole microtubule net-
work is compromised and the asymmetric translocation of wnt8 and grip2a itself fails
(Ge et al. 2014 ). The vegetal localization of grip2a and syntabulin are conserved in
Xenopus, as well as the function of Syntabulin (Colozza and De Robertis 2014 ; Nojima
et al. 2010 ). However, grip2a is, in addition, later localized to PGCs in Xenopus (Ge
et al. 2014 ; Kaneshiro et al. 2007 ; Kirilenko et al. 2008 ; Tarbashevich et al. 2007 ).
The wnt8, wnt11, syntabulin, and grip2a mRNAs are all localized to the vegetal
pole of the egg (Ge et al. 2014 ; Kirilenko et al. 2008 ; Ku and Melton 1993 ; Lu et al.
2011 ). Moreover, with the exception of wnt11 (Ku and Melton 1993 ), their localiza-
tion to the vegetal pole occurs early in oogenesis and is executed by the Balbiani
body (Bb) (Kirilenko et al. 2008 ; Lu et al. 2011 ; Tarbashevich et al. 2007 ). These
mRNAs localize to the Bb and the future vegetal pole of the early oocyte, where
they remain anchored throughout oogenesis and in the egg. Thus, the AV axis of the
oocyte is key to forming the dorsal organizer and subsequently establishing the
basic vertebrate embryonic body plan (Fig. 5.1).
5.2.2 Germ Layer Formation and Its Regulation
by Oocyte Polarity
All the organs and cell types in an animal body originate from the three embryonic germ
layers, the ectoderm, mesoderm, and endoderm during early development. The specifi-
cation of the germ layers is intimately related to the AV axis and layered upon the DV
M. Escobar-Aguirre et al.