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Additional insight into microtubule assembly in zebrafish and further support for
the dual-range model of transport in the yolk cell has recently come from analysis of
maternal-effect mutants in zebrafish (Nojima et al. 2004 , 2010 ; Lyman Gingerich
et al. 2005 ; Ge et al. 2014 ). The mutants hecate (hec; Ge et al. 2014 ) and tokkaebi
(tkk; Nojima et al. 2010 ) are ventralized with near complete penetrance and harbor
disruptions in syntabulin (sybu) and glutamate receptor interacting protein 2a
(grip2a) loci, respectively. Parallel microtubule assembly at the vegetal pole is nor-
mal in tkk mutants but disrupted in hec mutants. Upward movement to the YSL/
Fig. 6.4 Dorsal determinant transport in zebrafish. (a) Sequence of events in wildtype embryos.
RNAs and other dorsal determinants are localized vegetally during oogenesis (blue). After fertil-
ization, cytoplasm streams to the animal pole, forming the future blastoderm. Microtubule assem-
bly initiates ~20 min post-fertilization at the vegetal pole of the yolk cell; localized RNAs and
Syntabulin protein (Sybu) are shifted toward the future dorsal side. Microtubule networks in the
lateral cortex facilitate global transport animal-ward, which on the dorsal side would contain axis
determinants. (b) In hecate (hec) mutants lacking Grip2a, maternal vegetal localization occurs, but
cortical rotation and microtubule assembly are deficient post-fertilization. This image is repro-
duced and modified from, Ge X, Grotjahn D, Welch E, Lyman-Gingerich J, Holguin C, Dimitrova
E, et al. (2014) Hecate/Grip2a acts to reorganize the cytoskeleton in the symmetry-breaking event
of embryonic axis induction. PLoS Genet 10(6): e1004422. doi:10.1371/journal.pgen.1004422,
under the terms of the Creative Commons Attribution License (CC BY 4.0)
D.W. Houston