Plant Molecular Biology 43: 147–161, 2000.
M.A. Matzke and A.J.M. Matzke (Eds.), Plant Gene Silencing.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Genomic imprinting in plants: observations and evolutionary implications
Mary Alleman and John Doctor
Department of Biological Sciences, Duquesne University, Pittsburgh, PA 15282, USA (e-mail: firstname.lastname@example.org,
Key words: Arabidopsis, epigenetic, genomic imprinting, maize, review
The epigenetic phenomenon of genomic imprinting occurs among both plants and animals. In species where
imprinting is observed, there are parent-of-origin effects on the expression of imprinted genes in offspring. This
review focuses on imprinting in plants with examples from maize, where gene imprinting was ﬁrst described, and
Arabidopsis. Our current understanding of imprinting in plants is presented in the context of cytosine methylation
and imprinting in mammals, where developmentally essential genes are imprinted. Important considerations in-
cludethestructureand organizationofimprintedgenes and the role of regional,differentialmethylation.Imprinting
in plants may be related to other epigenetic phenomena including paramutation and transgene silencing. Finally,
we discuss the role of gene structure and evolutionary implications of imprinting in plants.
Genomic imprinting is an epigenetic phenomenon in
which the activity of a gene is reversibly modiﬁed de-
pendingon the sexof the parent that transmits it. Thus,
contrary to the expectations of simple Mendelian in-
heritance, imprinting results in non-equivalentexpres-
sion of maternally and paternally derived alleles in an
individual;usuallyone of the alleles is suppresseddur-
ing development. This suppression is correlated with
increased cytosine methylation or chromatin-mediated
epigenetic silencing (Kooter et al., 1999). In the case
of two alleles that have a different inherent phenotype
(big ‘A’ versus little ‘a’), the recessive a allele may
appear to be dominant in an A/a heterozygote where
the dominantallele A is silenced by imprinting. Conse-
quently, the terms dominant and recessive in the usual
Mendelian genetic sense are problematic at imprinted
Using the barometer of many college freshman bi-
ologytextbooks, imprintingis no longereither ignored
or viewed as a genetic curiosity, but is now described
in the context of its importance during development.
The increased attention to imprinting among biolo-
gists is in large part due to the recognition, in the
1980s, of its importance during mammalian devel-
opment (Monk, 1988). Also increasing the level of
interest, imprinting appears to underlie several human
genetic diseases (Clarke, 1990; Hall, 1990; Lalande,
1996; Nicholls, 1998; Tilghman, 1999)and is involved
in the proper development of the seed in cereal crops
(Kermicle and Alleman, 1990).
Though this review focuses on genomic imprinting
in plants, it is instructive ﬁrst to consider imprint-
ing in other organisms, particularly the mouse, Mus
musculus. Regardless of whether the discussion of im-
printing focuses on mouse, maize, or other species, a
number of intriguing questions remain:
(1) When and how are imprinted alleles modiﬁed in
the parental germ line?
(2) Are imprinting ‘marks’ necessary for genomic
imprinting in both animals and plants?
(3) How do imprints result in parent-speciﬁc gene
expression during development?
(4) Is there a set of organism-independent require-
ments for a gene to show imprinting?
(5) What is the evolutionary signiﬁcance of imprint-