Epigenetic Marking of the Zebrafish Developmental Program - Current Topics in Developmental Biology - page 86

Maternal-degraded
cluster

(i)

Maternal-zygotic
cluster

(ii)

(iii)

Pre-MBT cluster

(iv)

MBT cluster

(v) Post-MBT cluster

Egg Pre-MBT MBT Post-MBT

Development

Figure 3.1 Gene expression cohorts detected during zebrafish development through
the MBT period. Profiles are drawn based on our RNA-seq data ( Aanes et al., 2011 ).

at the MBT, upon ZGA ( Fig. 3.1 , panel ii). This “maternal-zygotic” cluster
primarily encodes proteins with housekeeping and cellular homeostasis func-
tions, the need for which increases as development proceeds. The third cohort
includes a cluster of transcripts detected in pre-MBT stages (pre-MBT cluster),
before ZGA onset; this cluster exhibits two distinct profiles: one subcluster is
upregulated throughout pre-MBT stages and after theMBT, while the other is
only transiently upregulated during pre-MBT development ( Fig. 3.1 , panel
iii). Strikingly, the pre-MBT cluster encodes molecules involved in posttrans-
lational modification, protein processing, transport and degradation, and in
gene expression regulation. Interestingly, closer examination of these pre-
MBT transcripts reveals a plethora of mRNAs expressed at various levels and
encoding histone lysine methyltransferases, DNA methyltransferases (Dnmts),
and nonmethylated and methylated CpG-binding proteins ( Table 3.1 ;
Fig. 3.2 A and B). Detection of these transcripts prior to ZGA onset likely