Epigenetic Marking of the Zebrafish Developmental Program - Current Topics in Developmental Biology - page 84

acquires transcriptional competence and initiates its own gene expression program.
We also address models accounting for the origin of epigenetic states detected in early
embryos. Fromtheseobservations, a concept of epigeneticprepatterningof theembryonic
gene expressionprogramprior to theonset of ZGA is emerging. The recent datacollectively
start shedding light on how ZGA may be programmed and regulated.

1. INTRODUCTION: SETTING THE STAGE FOR THE
ZEBRAFISH DEVELOPMENTAL GENE
EXPRESSION PROGRAM
The zebrafish ( Danio rerio ) has over the years proved to be a powerful
animal model for our understanding of the genetics of vertebrate develop-
ment. Zebrafish are small, hardy, and have a short generation interval; thus,
they are relatively easy to maintain in large quantities and at moderate costs
compared to rodents. More importantly, zebrafish lay on average more than
100 eggs in a clutch, development is rapid and external, and embryos are
optically clear. A reference review of zebrafish developmental stages has
been published by Kimmel, Ballard, Kimmel, Ullmann, and Schilling,
(1995) . Large-scale forward genetic screens, later supplemented with reverse
genetics strategies, have led to the identification of genes and pathways
essential for vertebrate development ( Alestr¨m, Holter, & Nourizadeh-
Lillabadi, 2006; Lawson & Wolfe, 2011 ).
Embryonic development results from a cascade of gene activation and
inactivation events triggered after fertilization, which results in the establish-
ment of complex gene regulatory networks ( Chan, Longabaugh, et al.,
2009 ). A particularity of anamniotes, including zebrafish, is a relatively long
developmental window after fertilization in the absence of on-going tran-
scription. In zebrafish, this window lasts for 3.3 h, during which the zygote
undergoes 10 rounds of cell divisions with an unusually rapid cell cycle time
of 15 min ( Kane & Kimmel, 1993 ). In the absence of noticeable G1 and

G2 phases, the embryo merely replicates its DNA in each cell cycle during
this period. Until the midblastula transition (MBT; 3.3-h postfertilization) at
the 1000-cell stage, zebrafish development is supported by proteins and
RNAs stored in the egg cytoplasm ( Kane & Kimmel, 1993; Tadros &
Lipshitz, 2009 ). At the MBT, the embryo turns on its own genes, a critical
step referred to as the maternal-to-zygotic transition, or MZT ( Tadros &
Lipshitz, 2009 ). Zygotic genome activation (ZGA) during the MZT, there-
fore, ensures that the embryo takes control over its developmental program.