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5.1 Introduction
Studies in Xenopus have identified many animally and vegetally localized factors in
the egg that distribute either to the germ cells, the germ layers, or specify the axes
in the early embryo (Bubunenko et al. 2002 ; Claussen and Pieler 2004 ; Claussen
et al. 2011 ; Cuykendall and Houston 2010 ; Houston et al. 1998 ; Kaneshiro et al.
2007 ; Ku and Melton 1993 ; Kwon et al. 2002 ; Rebagliati et al. 1985 ; Weeks and
Melton 1987 ). Genetic screens in the zebrafish or knockdown approaches in
Xenopus have revealed the function for some of these factors and discovered new
ones. In the zebrafish, the bucky ball (buc) and macf1 genes were identified in a
maternal-effect mutant screen and provide the only known genetic entry points for
studying the early events of AV polarity establishment during oogenesis (Bontems
et al. 2009 ; Dosch et al. 2004 ; Gupta et al. 2010 ; Marlow and Mullins 2008 ; Wagner
et al. 2004 ). Vegetally-localized RNAs acting in DV axis formation include wnt8 in
zebrafish (Erter et al. 2001 ; Lekven et al. 2001 ; Varga et al. 2007 ) and wnt11 in
Xenopus (Cha et al. 2008 , 2009 ; Ku and Melton 1993 ; Tao et al. 2005 ), while veg-
etally localized VegT in Xenopus functions in germ layer formation (Clements et al.
1999 ; Xanthos et al. 2001 ). Genetic screens in zebrafish also identified glutamate
receptor interacting protein 2 (grip2a or hecate) (Ge et al. 2014 ) and syntabulin
(tokkaebi) (Nojima et al. 2010 ) as key vegetally-localized regulators of DV axis
formation in the egg. The repertoire of functionally important localized factors in
the egg will certainly continue to expand in the future.
In this chapter, we will review the RNA localization mechanisms in vertebrate
oocytes that are key to embryonic patterning, referring to some of the groundbreak-
ing studies in frog oocytes and incorporating the current genetic evidence from the
zebrafish. The mechanisms of RNA localization will be discussed in relation to how
they act in AV polarity establishment and its consequences in embryonic patterning,
germ cell specification, and fertilization.
5.2 Function of Localized Products
5.2.1 Determinants of the Embryonic Body Axes Originate
in the Polarized Oocyte
In vertebrates the main embryonic body axes are first formed and patterned during blas-
tula and gastrula stages. From a seemingly symmetrical sphere of totipotent cells, gas-
trulation transforms the embryo into the bilateral vertebrate body plan, comprised of
dorsal-ventral (DV), anterior-posterior (AP), and left-right axes. With the more dramatic
changes that follow, the patterning of these axes is fine-tuned with cells continuously
being specified and differentiating, with further morphogenesis governing organogene-
sis. The first cues that guide this magnificent process of development are, however,
already seeded in the developing oocyte, long before the formation of the zygote.
M. Escobar-Aguirre et al.