520
and Smith 2015 ). An example of removal of maternal deposition of a specific
histone variant is histone H3.3, a replication-independent histone variant associated
with active genes, which transiently disappears from the maternal genome after
fertilization (Akiyama et al. 2011 ). Similarly, a maternal store of histone variant
macroH2A is eliminated from zygotes shortly after fertilization (Chang et al. 2005 ).
Remarkably, macroH2A is associated with transcriptional repression (Costanzi and
Pehrson 1998 ; Buschbeck et al. 2009 ) and acts as a barrier to induced pluripotency
(Gaspar-Maia et al. 2013 ).
The last example of maternal chromatin mark is an oocyte-specific linker histone
variant H1foo, which has homologues among vertebrates (Tanaka et al. 2001 ). The
mammalian H1foo shows highest expression levels in germinal vesicle oocytes, but
it is gradually lost after fertilization (Tanaka et al. 2001 ; McGraw et al. 2006 ), when
it is replaced with the somatic linker histone H1 (Gao et al. 2004 ). The oocyte-
specific linker histone B4 in Xenopus, a homologue of H1foo, also accumulates
during oogenesis and becomes eliminated around the MBT when it is replaced by
somatic-type linker histones (Dimitrov et al. 1993 ). Interestingly, the elimination of
oocyte-specific linker histones correlates with the time of embryonic genome acti-
vation in the mouse, bovine, and Xenopus. This implies that the elimination of
H1foo might be prerequisite for successful embryonic genome activation. This
notion is supported by impaired pluripotency upon ectopic expression of H1foo in
embryonic stem cells (Hayakawa et al. 2012 ).
10.5 Elimination of Other Parental Products
The last section will review controlled elimination of yolk and paternal mito-
chondria, as unique examples of clearance of parental structures associated with
nutrition/energy support.
10.5.1 Yolk Consumption
Yolk (reviewed in Anton 2013 ) is the common nutrient supply for externally devel-
oping embryos. Yolk is a supramolecular assembly of lipids and proteins, which
form dense, membrane-bound yolk platelets. The major yolk proteins are derived
from the conserved lipoprotein vitellogenin (Vg), which is not synthesized in the
oocyte but is internalized by the oocyte and then stored in specialized organelles,
the yolk platelets (Anton 2013 ). Although yolk platelets appear to be modified
lysosomes, the content of yolk platelets is not degraded until specific developmen-
tal stages, sometimes weeks after their formation. Degradation of yolk proteins
mainly serves as a source of amino acids during development (Anton 2013 ). Two
factors appear to be responsible for inducing yolk degradation in the developing
embryo: pH and enzymatic latency (Fagotto 1995 ).
P. Svoboda et al.