43Allopolyploidization , being a radical, rapid mode of speciation , produces a new
species in a single step, a novel taxon that is isolated genetically from its two parental
species. Besides, newly formed allopolyploid is a hybrid species with two or more
different genome s enveloped within the same nucleus. This situation exerts a major
genetic stress on the newly formed allopolyploids that must overcome several imme-
diate challenges in order to survive in nature (Levy and Feldman 2002 , 2004 ;
Feldman and Levy 2005 , 2009 , 2011 ). Overcoming these challenges is achieved
through genomic plasticity, namely through the immediate triggering of a variety of
cardinal genetic and epigenetic changes that affect genome structure and gene
expression. Consequently, a newly formed allopolyploid species must secure an
exclusive intra- genomic pairing at meiosis that will lead to increased fertility i.e., to
prevent pairing between the homoeologous chromosomes of the different genomes
that are still closely related (Sears 1954 ; Morris and Sears 1967 ), and orchestrate
inter-genomic gene expression and DNA replication, i.e., expression of a number of
genes is quickly altered or silenced to enable harmonic inter-genomic coexistence
along with reduction in genome size (Levy and Feldman 2002 , 2004 ; Feldman and
Levy 2005 , 2009 , 2011 , 2012 ; Feldman et al. 2013 ). Meeting these challenges that
are achieved through either genetic or epigenetic alterations in the DNA or in chro-
matin structure is presumably critical for the survival of the newly formed allopoly-
ploids, ensures their increased fi tness and successful establishment in nature as
competitive entities.
Further divergence of the homoeologous chromosomes, i.e., related chromo-
somes of the different genome s, is rapidly achieved through elimination of a number
of DNA sequences, mainly low-copy and, to some extent, high-copy, noncoding and
coding sequence (Feldman et al. 1997 ; Liu et al. 1998a , b ; Ozkan et al. 2001 ; Shaked
et al. 2001 ; Han et al. 2003 , 2005 ; Salina et al. 2004 ; Baum and Feldman 2010 ; Guo
and Han 2014 ). Such rapid elimination was observed also in Triticale , a synthetic
allopolyploid between wheat and rye (Boyko et al. 1984 , 1988 ; Ma and Gustafson
2005 , 2006 ; Bento et al. 2011 ). These sequences exist in all the diploid progenitors
of the allopolyploid species of the wheat group, whereas in the allopolyploids they
exist in only one pair of homologues (Feldman et al. 1997 ; Ozkan et al. 2001 ).
Evidently, these sequences were eliminated from one of the two genomes in tetra-
ploids and from two of the three genomes in hexaploids. This elimination is rapid
and occurs during or soon after the formation of the allopolyploids and was desig-
nated as revolutionary changes (Feldman et al. 1997 ; Ozkan et al. 2001 ). No further
elimination of sequences occurs during the successive generations of the allopoly-
ploids. The elimination of sequences is reproducible as the same sequences were
eliminated in newly formed and in natural allopolyploids having the same genomic
combinations (Ozkan et al. 2001 ). It was concluded that instantaneous elimination
of DNA sequences in the fi rst generations of the newly formed allopolyploids of
Triticum and Aegilops , is one of the major and immediate responses of the wheat
genome to allopolyploidization.
The extent of DNA elimination was estimated by determining the amounts of
nuclear DNA in natural allopolyploids and in their diploid progenitors, as well as in
newly synthesized allopolyploids and their parental plants (Ozkan et al. 2003 ;
2 Origin and Evolution of Wheat and Related Triticeae Species