460
(Takahashi et al. 2000 ; Hilton et al. 2003 ). The pre-MBT expression of Xnr5/Xnr6
also requires Wnt signaling and VegT, is localized to early dorsal-vegetal blasto-
meres, and is inhibited by α-amanitin (Yang et al. 2002 ; Skirkanich et al. 2011 ). In
situ hybridization confirmed zygotic expression of Xnr5 in dorsal-vegetal blasto-
meres before the MBT (Takahashi et al. 2006 ). Furthermore, Yanai et al. using
microarrays reported a parallel increase in Xnr5 and Xnr6 expression between the
2-cell stage and the early to mid-blastula stage (stage 8) in X. laevis and X. tropicalis
(Yanai et al. 2011 ), and Collart et al. also identified Xnr5 and Xnr6 as pre-MBT
transcripts in X. tropicalis (Collart et al. 2014 ). Thus, Xnr5 and Xnr6 are transcribed
in a highly regulated manner before the MBT.
As discussed above, type II transcription can be sustained before the MBT if a
strong gene-specific activator is present. Thus, the zygotic expression of Xnr5/6
before the MBT may depend on maternal transcription factors that may in turn acti-
vate additional zygotic genes before the MBT. Xnr5/6 are direct targets of VegT
(Takahashi et al. 2000 ; Hilton et al. 2003 ), and other direct targets of VegT (Xanthos
et al. 2001 ), including mixer, bix4, derriere, and sox17α are also newly transcribed
between the seventh and eighth cleavage divisions (Skirkanich et al. 2011 ). Each of
these zygotic transcripts was detected at the 256-cell stage and rose exponentially,
with an increase of approximately 2 orders of magnitude by the onset of the MBT. The
pre-MBT expression of these mesendodermal genes required RNAPII and maternal
Fig. 9.2 Selected zygotic genes expressed before the MBT in zebrafish. A subset of zygotic genes
transcribed before the MBT in zebrafish (Heyn et al. 2014 ), supported by at least one other study,
is shown. The fold-increase in expression at 256-cell (two cycles before MBT) and 512-cell stage
relative to 128-cell was calculated from RNA-seq data (Supplemental Table S1) in Heyn et al.
(Note that Heyn et al. presented expression data as log 2 [FPKM] whereas here we are showing fold-
change in expression (FPKM) relative to FPKM at 128-cell (not absolute expression) to indicate
the rapid increase in expression from the 128-cell stage onward). All genes were present at >10
FPKM by 256 or 512-cell stage (Heyn et al. 2014 ) and an increase in the detection of each of these
genes at the 256-cell and 512-cell stages was also evident in supplemental data reported by Harvey
et al. ( 2013 ). Within this set of genes, pre-MBT histone H3 lysine-4 trimethylation, generally
indicative of “active” promoters, was reported for dld, gadd45bb, klf4, foxd5, and irx7 and pre-
MBT zygotic transcription of irx7 was also demonstrated by RT-PCR (Lindeman et al. 2011 ).
Pre-MBT expression of bozozok (also known as dharma) was first identified by in situ hybridiza-
tion (Leung et al. 2003 )
M. Zhang et al.