219localized RNAs and cortical rotation in Xenopus and zebrafish. However, it has not
been specifically demonstrated whether the exact RNAs implicated, including
trim36 and syntabulin (sybu), are in fact unlocalized in urodeles. Since these RNAs
are partially associated with the germ plasm, which is not found in urodeles
(Nieuwkoop and Sutasurya 1979 ), one might expect an absence of localization.
Urodeles may however localize important components posttranslationally.
Nevertheless the basic mechanisms of polarizing the egg and distributing dorsal
determinants appear conserved, but are not well understood in either case.
6.2.2 Cortical Rotation and Dorsal Determinant Transport
in Zebrafish
Axis formation in zebrafish similarly relies on asymmetric localization of dorsal
determinants and activation of Wnt/beta-catenin signaling. The polarizing mecha-
nisms and the similarity of these to classical amphibian cortical rotation are only
now becoming apparent, however. It has been traditionally thought that typical cor-
tical rotation does not occur in teleost fish. The origin of this assumption is mysteri-
ous, but likely can be attributed to initial observations on the importance of a
polarized dYSL in teleost axial patterning rather than formation of a gray crescent-
like clear crescent, which does occur in non-teleosts (Long 1983 ; Ho 1992 ). There
do not appear to have been any classical embryological studies directly addressing
either relative displacement of cytoplasm and cortex or the existence of transplant-
able vegetal cortical cytoplasm.
However, parallel microtubule arrays have been noted at the vegetal pole cortex
of the early post-fertilization (~20 min) medaka and zebrafish egg (Jesuthasan and
Stähle 1997 ). By 30 min post-fertilization, this array is offset from the vegetal pole,
giving bilateral symmetry to the egg. During cleavage of the blastoderm, a second
set of microtubule arrays forms along the animal–vegetal axis (Strähle and
Jesuthasan 1993 ; Solnica-Krezel and Driever 1994 ), and polarized transport of fluo-
rescent beads has been observed to move animally into the YSL and marginal blas-
tomeres (Jesuthasan and Stähle 1997 ). Disruption of either set of microtubules with
UV, cold, or nocadazole disrupts axis formation as well as epiboly (Strähle and
Jesuthasan 1993 ; Solnica-Krezel and Driever 1994 ; Jesuthasan and Stähle 1997 ).
Thus a two-step transport pathway is proposed (Fig. 6.4). Asymmetry is initially
established by the localization of determinants to the future dorsal vegetal side of
the egg, followed by generalized upward movement of material into the YSL and
marginal blastoderm cells (the dorsal determinants being carried along only on the
dorsal side). In support of this idea, a recent live imaging study has demonstrated for
the first time, as was tacitly assumed, that the plus ends of zebrafish vegetal cortical
microtubules are oriented dorsally as in frogs (Tran et al. 2012 ). Given the relation-
ship between movement of the cytoplasmic core and microtubule orientation in
Xenopus eggs (Olson et al. 2015 ), it is likely that relative cortical movement, at least
locally, is involved in orienting these microtubules in teleost eggs as well.
6 Vertebrate Axial Patterning: From Egg to Asymmetry