Alien Introgression in Wheat Cytogenetics, Molecular Biology, and Genomics

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disappearance of the Tethys Sea and the elevation of the east Mediterranean region;
at that geological time temperate ecosystems diversifi ed and new ecological niches
opened (Table 2.3 ). Diversifi cation in the Mediterranean lineage of the Triticeae not
only stimulated the formation of many genera but also provided the opportunity for
the production of many allopolyploids (Fan et al. 2013 ).
Recent molecular studies of chloroplast DNA and nuclear genes have contributed
to a better understanding of the Triticeae phylogeny. However, there is no consensus
concerning the phylogenetic relationships between the various genera of the tribe
due to a limited number of studied samples (Kellogg and Appels 1995 ; Kellogg
et al. 1996 ; Mason-Gamer and Kellogg 1996 ; Escobar et al. 2011 ) or a small number
of genes that were analyzed (Petersen and Seberg 1997 ; Hsiao et al. 1995 ; Kellogg
and Appels 1995 ; Helfgott and Mason-Gamer 2004 ; Mason-Gamer 2005 ). Inter-
generic hybridizations and introgression events as well as incomplete sorting of
ancestral polymorphisms (Mason-Gamer 2005 ; Kawahara 2009 ; Escobar et al.
2011 ) may also contribute to the ambiguity of the phylogenetic relationships.
The eroding genetic basis of domesticated wheat have boosted further the moti-
vation of scientiss to explore the possibility of utilizing the gene pool of the various
Triticeae species (Feldman and Sears 1981 ). This vast genetic resource contains

Table 2.4 Time of beginning divergence of the Triticeae lineages in million years ago

Lineages

Beginning

divergence time Method of study Reference

Barley and

rye -wheat

10–14 Sequencing chloroplast DNA Wolfe et al. ( 1989 )

11–15 RFLP of BAC libraries Ramakrishna et al. ( 2002 )

11.0 Sequencing of two nuclear

genes

Huang et al. ( 2002a , b )

10.1 Sequencing of four nuclear

genes

Dvorak and Akhunov

( 2005 )

11.6 Sequencing of nuclear genes Chalupska et al. ( 2008 )

8–9 Sequencing of chloroplast

genome

Middleton et al. ( 2014 )

15 Sequencing of three

chloroplast genes

Marcussen et al. ( 2014 )

10.6 Sequencing chloroplast DNA Gornicki et al. ( 2014 )

Rye and wheat 3–4 Sequencing of chloroplast

genome

Middleton et al. ( 2014 )

7 Sequencing of three

chloroplast genes

Marcussen et al. ( 2014 )

Wheat and

Aegilops

2.5–4.5 Sequencing of two nuclear

genes

Huang et al. ( 2002b )

2.7 (1.44.1) Sequencing of four nuclear

genes

Dvorak and Akhunov

( 2005 )

2.1–2.9 S equencing of chloroplast

genome

Middleton et al. ( 2014 )

6.5 Sequencing of three

chloroplast genes

Marcussen et al. ( 2014 )

2 Origin and Evolution of Wheat and Related Triticeae Species