187Vegetal pole specification, as executed by the Bb, is essential for embryonic
development, as indicated by the bucky ball (buc) mutant in zebrafish (Fig. 5.2b)
(Bontems et al. 2009 ; Heim et al. 2014 ; Marlow and Mullins 2008 ). Buc oocytes fail
to form the Bb; Bb mRNAs are not localized to the oocyte cortex and consequently
the vegetal cortex is not specified. Instead, animal mRNAs are expanded radially, and
these oocytes develop forming a radial animal pole. Instead of formation of a single
micropyle, the animal specialized structure that enables sperm entry, buc mutant
eggs form multiple micropyles. Another consequence of the radial animal fate is that
embryonic cell divisions occur radially around the yolk, instead of restricted to a
defined animal pole. This phenotype is catastrophic to embryogenesis, and embryos
die at cleavage stages (Dosch et al. 2004 ; Marlow and Mullins 2008 ).
5.4.2 Early Balbiani Body-Dependent RNA Localization
Few proteins have been implicated in localizing mRNAs to the Bb. Indeed Rbpms2
(Hermes) is the only one known to play such a role during oogenesis. In frogs and
fish, Rbpms2 was shown to localize to the vegetal cortex through the early Bb
pathway and interact with vegetally localized RNAs (Kosaka et al. 2007 ; Marlow
and Mullins 2008 ; Zearfoss et al. 2004 ). In frogs, Rbpms2 binds to nanos (for-
merly called Xcat-2) and Xvelo (buc homolog) RNAs among others (Nijjar and
Woodland 2013 ; Song et al. 2007 ; Zearfoss et al. 2004 ), whereas in zebrafish it has
only been shown to bind with buc RNA (Heim et al. 2014 ). Similarly, in both spe-
cies Rbpms2 was shown to interact with Buc/Xvelo protein (Heim et al. 2014 ;
Nijjar and Woodland 2013 ).
Much more is known about the cis-acting Bb localizing elements (LE) in various
mRNAs. The first LE that was identified was the mitochondrial cloud (Bb) localiz-
ing element (MCLE) in the 3′UTR of the Xenopus nanos mRNA (Zhou and King
1996a). Deletion analysis showed that 250 nucleotides (nts) in the proximal end of
the 3′UTR is required for nanos localization to the Bb. Specifically, six repeats of
the motif UGCAC within this region are required for its Bb localization (Zhou and
King 1996b). Within the UGCAC, the CAC sequence was proposed as a core motif,
required generally for localizing mRNAs. It was proposed that variations in the sur-
rounding sequences facilitate specific protein binding and/or activities that would
drive specific localization (Betley et al. 2002 ). Indeed, CAC motifs were also
required for Xpat and Xnif mRNA localization to the Bb (Betley et al. 2002 ;
Claussen and Pieler 2004 ).
Interestingly, the nanos MCLE motif can drive a reporter to localize to the Bb
in zebrafish oocytes (Kosaka et al. 2007 ). However, endogenous zebrafish Bb
mRNAs, like dazl, vasa, and nanos, contain no or a few sparse MCLE sequence
motifs, and even these are found outside the region required for localization of
these transcripts to the Bb (Kosaka et al. 2007 ). A similar endogenous MCLE
sequence therefore is unlikely to localize these zebrafish transcripts to the Bb.
Deletion analysis of the zebrafish dazl mRNA revealed a 375nt fragment in the
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development