Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 155

analogous to the frog late pathway, although there is some variation in the
timing of localization for certain transcripts (e.g., gdf1 ; Bally-Cuif et al.,
1998; Zhang et al., 1999 ), suggesting multiple overlapping pathways or dif-
ferences in the efficiency of the process between transcripts.
A second group of mRNAs including vasa , nos1 , and dnd1 localize ini-
tially to the mitochondrial cloud in stage I oocytes and then are relocalized to
the entire cortex of stage III oocytes and finally to a cortical band around the
animal pole of the ovulated egg ( Kosaka et al., 2007; Theusch et al., 2006 ).
During embryogenesis, these RNAs also become localized to the distal ends
of the first two cleavage furrows at 4-cell stage, ( Koprunner et al., 2001;
Weidinger et al., 2003; Yoon et al., 1997 ), a position consistent with histo-
logically identifiable germ plasm in the fish embryo ( Knaut et al., 2000 ). In a
variation of this pattern, eomesodermin ( eomes ; Bruce et al., 2003 ) is cortically
localized in stage III oocytes but absent from the early mitochondrial cloud.
A third category of RNAs includes dazl , which localizes initially to the
mitochondrial cloud and is then anchored to the vegetal pole. Following fer-
tilization, dazl remains at the vegetal pole but is then translocated through
“corridors” in the yolk cell to the blastodisc, accumulating in overlapping
but distinct regions of the germ plasm in the cleavage furrows ( Maegawa
et al., 1999 ). The pattern for bruno-like ( brul/celf3 ) is similar, except that it
localizes vegetally independently of the mitochondrial cloud ( Hashimoto
et al., 2004; Suzuki et al., 2000 ).
Finally, several RNAs show unique variations of these patterns. syntabulin
( sybu ) mRNAs localize to the mitochondrial cloud in stage I and translocate to
the vegetal cortex like dazl , but then remain cortical until the 16-cell stage and
are not found in germ plasmor later PGCs ( Nojima et al., 2010 ). Additionally,
buckyball , foxh1 ,and igf2bp3 transcripts localize to the mitochondrial cloud in
stage I oocytes but then translocate to the animal pole in later stage oocytes and
are uniformly distributed in embryos ( Bontems et al., 2009 ). Overall, these
observations reveal additional levels of complexity and reiteration of mRNA
localization during zebrafish oogenesis and early development. Since these
patterns are likely the product of modification during teleost oocyte/egg evo-
lution, these localized RNAs afford the opportunity to study the evolutionary
variation of RNA localization mechanisms.
3.6. RNA localization mechanisms in zebrafish
Much less is known about the biochemical mechanisms of RNA in zebrafish
oocytes than those of Xenopus . The localization of Igf2bp3 protein to the
animal pole of stage III/IV oocytes ( Zhang et al., 1999 ) suggests that many