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Evidence in zebrafish suggests that the polarity axis is prepositioned by the plane
of the last oogonial division (Elkouby et al. 2016 ). Similar to oogonia, zygotene
oocytes remain organized in nests and pairs of oocytes appear interconnected by a
cytoplasmic bridge that contains acetylated tubulin extending through it typical of a
mitotic midbody (Fig. 5.2a). As cytokinesis completes during abscission, the mid-
body microtubules are normally deacetylated, which was also observed in post-
zygotene stage oocytes. Interestingly the location of the midbody/cytoplasmic
bridge coincides with the location of the centrosome cytoplasm of both intercon-
nected oocytes, where the early Bb is forming (Fig. 5.2a). This suggests that polar-
ity is predisposed to be orthogonal to the last oogonial division plane, which
positions the centrosome and aligns the future AV axis. Polarization is then in effect
executed with the recruitment of Bb precursors to the centrosome by the bouquet
organization at zygotene stages.
Many questions remain open. It is unknown how the meiotic-vegetal center is
activated at zygotene to initiate telomere clustering and recruit Bb precursors. The
localization of Bb precursors required intact microtubules at the bouquet stage
(Elkouby et al. 2016 ); however, further elaboration of the localization mechanism
remains unknown. How Bb precursors continue to aggregate in the nuclear cleft and
what is the role of the nuclear cleft in this process are unknown. Lastly it is unclear if
follicle cells, either in the early cyst or during later folliculogenesis, contribute to
polarization. Theoretically, follicle cells could affect oocytes in the cyst by regulating
their division plane or express localized queues that could form signaling gradients.
5.3.4 Bucky Ball and Macf1: Genetic Entry Points to Oocyte
Polarity
Two maternal-effect mutants identified in zebrafish, bucky ball (buc) and macf1
(microtubule cross-linking factor 1), have provided the only known genes required
for AV polarity establishment in vertebrates (Fig. 5.2b) (Bontems et al. 2009 ; Gupta
et al. 2010 ; Marlow and Mullins 2008 ). In contrast to a normal egg with cytoplasm
in the animal pole blastodisc and yolk at the vegetal pole, buc and macf1 mutant
eggs show radially distributed cytoplasm around a centrally localized yolk.
Moreover, the primary defect in buc and macf1 mutants is evident much earlier,
specifically during the Bb localization pathway in stage I oocytes (discussed further
in the next section). buc mutant oocytes fail to form the Bb and RNAs that are nor-
mally vegetally localized are instead unlocalized and remain dispersed in the cyto-
plasm. In contrast, in macf1 mutants the Bb forms and RNAs localize to it; however,
they fail to anchor to the prospective vegetal cortex, and instead they remain in an
apparent persisting Bb (Gupta et al. 2010 ). Thus, Buc and Macf1 provide a molecu-
lar link to the early processes that establish AV polarity in the oocyte. The function
of these proteins is discussed further in Sect. 5.5 below.
M. Escobar-Aguirre et al.