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Maize Genomics and Genetics 2012, Vol.3, No.3, 13
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Table 1 Imprinted genes in maize and their expression pattern
Allele-specific imprinted genes
Tissue-specific expression Reference
R
Endosperm
Kermicle (1970); Ludwig et al (1989)
Dzr
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1
Endosperm
Chaudhuri & Messing (1994)
Zein
Endosperm
Lund et al (1995a)
Alpha
-
tubulin
Endosperm
Lund et al (1995b)
Locus-specific
imprinted genes
Parental-specific
expression
Tissue-specific expression
ZmFie1
Maternal
Endosperm
Danilevskaya et al (2003); Gutierrez-Marcos et al (2006);
ZmFie2
Maternal
Endosperm
Hermon et al (2007); Haun and Springer (2008)
Nrp1
Maternal
Endosperm
Danilevskaya et al (2003); Gutie' rrez-Marcos et al (2006); Hermon
et al (2007)
Peg1
Paternal
Guo et al (2003); Haun and Springer (2008)
Meg1
Maternal
Endosperm
Gutierrez-Marcos et al (2003)
Mez1
Maternal
Endosperm
Gutie' rrez-Marcos et al (2004)
Mee1
Maternal
Embryo and Endosperm
Haun et al (2007); Haun and Springer (2008)
VIM5
Paternal
Endosperm
Jahnke and Scholten (2009)
YUC10
Paternal
Endosperm
Zhang et al (2011)
Figure 1 Fertilization events in the angiosperms
Note: Flowering plants undergo double fertilization and triple
fusion; Two sperm are produced from a single meiotic division
during micro-sporogenesis; One fertilizes the egg cell to
produce the diploid embryo; The other fertilizes the diploid
central cell to produce a triploid endosperm
paternal expression is completely absent throughout
seed development, which is repressed by DNA
methylation of CpG island (Danilevskaya et al., 2003;
Gutierrez-Marcos et al., 2006; Huan and Springer;
2008). No-apical meristem (NAM) related protein1
(
Nrp1
), a transcription factor, displayed gene-specific
imprinting and expressed only in endosperm (Guo et
al., 2003). Janhnke and Scholten (2009), has shown
the imprinting of maternally expressed in embryo 1
(
mee1
) gene of maize in both the embryo and
endosperm indicating the correlation of parent-of-
origin-specific expression with differential allelic
methylation. This differential methylation is maintained
in the endosperm, whereas the embryonic maternal
allele is demethylated on fertilization and remethylated
later in embryogenesis. Maternally expressed gene1
(
meg1
), which is a maize endosperm transfer cell-
specific gene shows maternal parent-of-origin ex-
pression pattern during early stages of endosperm
development but biallelic expression at later stages
(Gutierrez-Marcos et al., 2004). Polycomb group
(PcG) proteins, Maize Enhancer-of-zeste 1 (
Mez1
) is
imprinted while Mez2 and Mez3 are not. Mono-allelic
expression of maternal
Mez1
was seen in the
developing endosperm and was found to be present
throughout the development of endosperm. (Huan et
al., 2009). Differentially methylated region was found
at the transcription start site of
Mez1
and CpG and
CpNpG nucleotides were heavily methylated on the
non-expressed paternal allele while maternal allele
showed less methylation (Huan et al., 2007). Recent
advances in transcriptome profiling techniques like
deep sequencing have reported many PEGs and MEGs
in Maize endosperm (Wang et al., 2009). Waters et al.
(2011), reported 100 putative imprinted genes in
maize endosperm, including 54 maternally expressed
genes (MEGs) and 46 paternally expressed genes
(PEGs) using genome-wide allele-specific expression
profiling. Zhang et al., showed 179 genes (1.6% of