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3.5. Localized mRNAs in zebrafish oocytes
Consistent with the many developmental similarities between
Xenopus
and
Teleost fish, maternally localized mRNAs are also prominent in zebrafish
oocytes and eggs. Early oocyte structure and oogenesis are fairlywell conserved
between fish and frogs. Teleost embryos undergo discoidal cleavage of the ani-
mal pole cytoplasm (
Ho, 1992a; Kimmel et al., 1995
), which is separated from
yolk cytoplasm that does not divide. During egg activation, the yolk-free cyto-
plasm streams toward the animal pole to form the blastodisc. Later cytoplasmic
rearrangements also occur during the 2-4 cell stages, and many maternal
RNAs are dynamically localized during cleavage, but these are not considered
here. Most cytoplasmic RNAs are ubiquitously distributed during oogenesis
but then become animally enriched during the general process of egg activa-
tion and cytoplasmic streaming (
Howley andHo, 2000
). These RNAs are thus
only secondarily localized as a result of this general cytoplasmic sorting.
During zebrafish oogenesis, several specific patterns of subcellular
mRNA localization are apparent (
Howley and Ho, 2000
;
Figs. 4.3 and
4.5
). One main class of RNAs is uniform in early oocytes but become local-
ized to the animal pole in stages II-IV oocytes (
cycB
,
gdf1
,
igfbp3
,
notch
,
acvr1b
,
pou5f
,
cpeb1
,
pabpc1
;
Bally-Cuif et al., 1998; Howley and Ho,
2000; Pogoda et al., 2000; Zhang et al., 1999
). This pathway may be
Figure 4.5 Patterns of mRNA localization in zebrafish. (A, B) In situ hybridization against
vasa and (C, D) dazl in zebrafish oocytes. Arrows point to mitochondrial cloud localiza-
tion, and arrowheads indicate vegetal cortical localization. By stage II/III, vasa is localized
around the cortex, whereas dazl remains vegetal. Scale bars¼50
m
m. Images contributed
by Mary C. Mullins, from
Gupta et al. (2010)
, used under the terms of the Creative Commons
Attribution License (CCAL).
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