Agriculture Reference
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1980; Mayr 1982; Richards 2006) and inheritance based on DNA
sequence changes is considered as hard genetics, conceptually analo-
gous to hardware and software of computers. The main litmus test for the
hard or soft nature of genetic changes has historically been the ability of
the phenotype to pass through mitosis only (soft) or also through the
sexual cycle. Russo et al. (1996) and others have pointed out that
epigenetic (non-DNA) changes can sometimes be transgenerational
and pass through meiosis during sexual reproduction (McClintock
1978; Martienssen and Colot 2001; Kakutani 2002; Tani et al. 2005;
Youngson and Whitelaw 2008), but the progeny appear in non-
Mendelian proportions. However, we should remember that classical
mutation DNA sequence changes may not always be inherited by the
strict Mendelian patterns either. Several processes, for example, gamete
lethality, result in non-Mendelian inheritance patterns. Likewise, pro-
cesses that we consider to be epigenetic, such as transposition, are
inherited in Mendelian fashion. There may be many unreported traits
that are transgenerational but do not followMendelian inheritance rules
and could thus be transgenerational soft genetics (Benkemoun and
Saupe 2006).
Transgenerational epigenetic traits have forced us to reconsider the
role of the environment as always being passive in the evolution of
phenotype changes. Signi
cant to this possibility, there are often impor-
tant but overlooked similarities between DNA sequence changes (muta-
tions) and transgeneration epigenetic changes. First, both may occur in
the germ cells to be passed through the meiotic cycle. Second, alterations
in both DNA sequence and epigenetic marks are robustly corrected and
copied during DNA replication by elaborate maintenance machinery
(Richards 2006). In fact, the error rate in editing and correcting of both
gene sequence and epigenetic marks is the likely process by which most
spontaneous genome (and phenotype) variation occurs. The role of the
environment in speci
cally controlling the occurrence of genetic versus
epigenetic errors that may be transgenerational could be fundamentally
different (Jablonka and Lamb 2007; Rando and Verstrepen 2007). For
example, environmental stress can increase changes in genome structure
by increasing homologous recombination and other types of rearrange-
ments in the genome (McClintock 1978; Wessler 1996; Walbot 1999;
Kirik et al. 2000; Cullis 2005; Puchta and Hohn 2010; Gao et al. 2012; Yao
et al. 2013). In fact, various stress factors and stress-mediating hormones
appear to activate speci
c retroelements based on promoter structures
associated with retroelements (Mhiri et al. 1997; Grandbastien 1998;
Takeda et al. 1999). Recently, stress-induced release of cryptic genetic
variation through the buffering of HSP9O has been reported (Sangster
