1855.4 Mechanisms of RNA Localization in the Oocyte
Once AV polarity is established, it will ultimately define the sperm entry site, the
embryonic axes, and underlie the mechanism of germ cell specification. Throughout
oogenesis, dorsal determinants and germplasm components distribute within dis-
tinct compartments of the oocyte reflecting its AV polarity. Early studies in Xenopus
identified several vegetally localized RNAs and determined their localization
dynamics during oogenesis (Cuykendall and Houston 2010 ; Houston et al. 1998 ).
Among them, Xcat/nanos (Forristall et al. 1995 ; Mosquera et al. 1993 ) and Vg1/
gdf1 (Yisraeli and Melton 1988 ) became models for unraveling RNA localization
mechanisms. nanos RNA localizes to the Bb in stage I oocytes and then translocates
via the Bb to the vegetal cortex, where it becomes docked during Bb disassembly at
the end of stage I in zebrafish (stage II in frog) of oogenesis. On the other hand, Vg1
localization to the vegetal pole begins during stage III and culminates in anchoring
to the vegetal cortex by stage VI in frog (Mowry and Cote 1999 ). The localization
patterns of nanos and Vg1 became signatures for defining two pathways of RNA
localization, an early pathway for RNAs that are transported via the Bb (also called
the METRO pathway) (Kloc and Etkin 1995 ; Wilk et al. 2005 ) and a later pathway
for RNAs, like Vg1, that localize after the Bb disassembles (Mowry and Cote 1999 ).
These two pathways are further distinct mechanistically in that the early pathway is
not known to require the cytoskeleton, whereas the late pathway requires microtu-
bules for successful localization of RNAs to the vegetal pole.
Although zebrafish and frog oocytes use similar pathways of RNA localization,
they also show differences in the localization patterns for some RNAs. For example,
nanos RNA localizes to the Bb initially, but in zebrafish it becomes unlocalized in
stage II oocytes after Bb disassembly (Kosaka et al. 2007 ). Another example is Vg1
RNA, which localizes in stage III oocytes to the vegetal pole in Xenopus, while in
zebrafish it is docked to the animal pole (Bally-Cuif et al. 1998 ; Marlow and Mullins
2008 ). Further, the RNA of the bucky ball homolog in frog Xvelo does not localize
to the Bb in stage I but is vegetally localized later in oogenesis (Claussen and Pieler
2004 ; Nijjar and Woodland 2013 ). In zebrafish, bucky ball localizes in stage I
oocytes to the Bb and vegetal cortex after Bb disassembly and then reappears at the
animal pole in later stages (Bontems et al. 2009 ). These differences open new ques-
tions regarding the evolutionary modifications to the RNA localization processes in
these organisms, which likely reflect evolutionary changes in the location of the
embryo blastomeres relative to the vegetal pole and yolk (yolk is separated in
zebrafish vegetally from, or in frog is integral to, the blastomeres).
In frogs and zebrafish, the early pathway, also known as the METRO pathway,
functions during stage I of oogenesis and relies on the Bb to translocate RNAs to
the oocyte vegetal cortex (Kloc and Etkin 1995 ; Kloc et al. 1996 ; Kosaka et al.
2007 ; Marlow and Mullins 2008 ). Transcripts that are localized via the Bb include
nanos, dazl, vasa, syntabulin, grip2, xlsirt, among others (Cuykendall and
Houston 2010 ; Ge et al. 2014 ; Houston 2013 ; Kosaka et al. 2007 ; Marlow and
Mullins 2008 ; Nojima et al. 2010 ). These RNAs localize to the Bb, translocate
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development