Agriculture Reference
In-Depth Information
fertilization, meiosis, and the next fertilization not only re
ects the
number of divisions but also other factors leading to genetic and epi-
genetic errors, since the age of egg cells is associated with many, often
epigenetic ontogenic deviations (Zoghbi and Beaudet 2007). It is known
that both egg and sperm cells have numerous individual differences in
epigenetic marks and in mammalian male gametes virtually each cell is
epigenetically distinct (Flanagan et al. 2006).
Considering the special circumstances in plants of promiscuous germ
or stem cell production, and persistent unprotected exposure of these
cells to a changing environment, it is no surprise that many of the earliest
observations of non-Mendelian, or transgenerational epigenetic inheri-
tance, were made with plants. In fact, a peloric mutant in Linaria was
first described by Linnaeus and its high degree of instability was
essentially unexplained until it was shown to be an epigenetic-based
phenotype change more than 200 years later (Cubas et al. 1999; Tsaftaris
et al. 2008). Because replication of the discoveries of these kinds of
variants in animal systems was slow to be attempted, their universal
acceptance or even understanding was diminished and long delayed.
For instance, the culturing of plant somatic stem cells with immortal,
pluripotent, or totipotent properties and displaying non-Mendelian
transgenerational genetic variation (Kaeppler et al. 2000; Kubis et al.
2003; Madlung and Comai 2004; Grant-Downton and Dickensen 2005,
2006; Henderson and Jacobsen 2007) predated animal studies by several
decades. Much of the phenotypic variation resulting from cultured
pluripotent and totipotent plant cells has long been considered and
often demonstrated to be epigenetic (Larkin and Scocroft 1981; Watad
et al. 1991; Jaligot et al. 2000; Kaeppler et al. 2000; Kubis et al. 2003;
Ceccarelli et al. 2006; Tanurdzic et al. 2008; Miguel andMarum 2011). In
fact, genes controlling epigenetic changes, including parts of the gene
silencing pathways, are altered during culture of plant cells (Rodriquiez-
Enriquez et al. 2011). In contrast, somatic nuclei could not be manip-
ulated in mammals to produce an adult organism (cloning) until the
1990s (Roemer et al. 1997; Wilmut et al. 1997; Bourc
his et al. 2001), after
which they were also found to display epigenetic variation (Jaenisch and
Gurdon 2007).
The germ lineage in plants also is less clear. Regeneration of plants by
morphogenesis often occurs from pluripotent cell clusters (Sugimoto
et al. 2011). Genetic transformation experiments on plant somatic cells
indicate that transgenic chimeras are common, pointing to a multi-
cellular origin of the germ line (Meins 1996) and not usually from a
strict germ cell lineage from an initial polar cell such as an egg cell.
Nontransformed sectors of chimeras cannot pass on transgenic traits to
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