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subsequent cooperativity resulting from interactions between products
of different alleles. This would also create the possibility to alter the
epigenetic program by an enormous number of combinations of differ-
ential expression levels between the two alleles present at each locus in
the diploid genome (Dilkes and Comai 2004). The two extreme situations
would be represented by the expression of only one or the other haploid
parental genomes, that is, one haploid genome suppressing the other
entirely or vice versa, as extreme versions of imprinting. Genome-wide
allele-speci
c expression has been observed (Chen 2007; Zhang and
Borevitz 2009; Autran et al. 2011). Speci
c differential expression of
alleles occurs in x-inactivation, allelic exclusion, monoreceptor expres-
sion in lymphoid immune cells, olfactory sensory function (Yang and
Kuroda 2007), mating locus cassettes in yeast (Egel 2004), paramutation
(Brink 1956; Chandler and Stam 2004), and in imprinting (Allis et al.
2007).
The most studied of these has been imprinting. The
first parental
imprinted loci were discovered in plant endosperm (Matzke and Scheid
2007), which is similar to the mammalian placenta in its developmental
origin and general function to nourish the embryo. Imprinting in mam-
mals is a critical epigenetic process that prevents either the male or
female parental haploid genomes alone from directing full somatic
development, a process called parthenogenesis that commonly occurs
in plants and some insects. In plants parthenogenic development can
proceed by several mechanisms (Elhiti et al. 2010). Unexpectedly,
nuclear compartmentation and even allelic physical location and inter-
actions may also play a role in allele-speci
c gene expression (Yang and
Kuroda 2007; Köhler et al. 2012). All of these examples further suggest
that the exploitation of genome duplication events for many additional
regulatory functions has occurred.
There are many observations that interactions between the alleles of
the two haploid genomes (haplotypes) in the diploid also have been
somehow associated with invasive genome mobile elements (Matzke
and Scheid 2007). Diploid generation by gamete fusion could therefore
be seen from the fascinating perspective of genome invasion. The
genome of the male gamete is actually invading the genome of the
female gamete (or vice versa) much as the retrovirus genome invades
that of its host. This is less eccentric if one considers what happens in
wide crosses such as interspecies or even intergenus crosses. Although
not as common in animals that have behavioral and other barriers against
wide crosses (an exception is the horse
donkey cross producing mules),
they occur rather frequently in plants (Muller 1925; Harlan et al. 1970;
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