41hybrid between T. turgidum, and Ae. tauschii. They identifi ed six QTLs in Ae. tauschii
that are involved in hybrid genome doubling regulating nonreductional meiosis and
its subsequent unreduced gamete production processes. Therefore, one might
assume that there is a high potential for the frequent and recurrent formation of
allopolyploids in the wheat group. The discovery by Blakeslee ( 1937 ) that colchi-
cine can induce chromosome doubling opened new possibilities for the study of
wheat evolution through allopolyploidization. It also provided an easy method to
synthesize different wheat allopolyploids some of which have similar genomes to
natural allopolyploids and others have new genomic combinations (see examples in
Ozkan et al. 2001 ). Synthetic allopolyploids, either induced or occurring spontane-
ously, offer excellent tools to mimic the evolutionary speciation events that occurred
in nature and to test in a controlled manner the new features of the allopolyploid
genome compared to those of its parents.
The accumulating cytogenetic and molecular evidence have indicated that the
concept of genome stability, that is, the assumption that the genomes of the allopoly-
ploid species remain similar to those of their diploid parents, is not always the case
and that one genome in every allopolyploid remains relatively unchanged while the
second genome(s) has undergone considerable changes from that of its diploid
parent. These genomes were termed modifi ed genomes by Kihara ( 1954 ) and other
wheat cytogeneticists (Table 2.8 ). Every allopolyploid species of the genera Aegilops
and Triticum contains a relatively unchanged genome side by side with a modifi ed
one whose diploid origin has been intricate to trace (Zohary and Feldman 1962 ).
Nevertheless, genome analysis studies revealed that allotetraploid wheat (genome
BBAA) originated from hybridization events involving two diploid progenitors clas-
sifi ed in the genera Aegilops and Triticum. Genome B, which is a modifi ed genome,
derived from Ae. speltoides or from a closely related species to Ae. speltoides, which
is extinct or extant (Feldman et al. 1995 ), and underwent changes at the polyploid
level. Genome A, that has been modifi ed relatively little, was derived from T. urartu
(Chapman et al. 1976 ; Dvorak 1976 ). Allohexaploid wheat (genome BBAADD)
originated from hybridization between allotetraploid wheat and Ae. tauschii , the
donor of the D genome (Kihara 1944 ; McFadden and Sears 1944 , 1946 ).
The wheat group contains 18 allopolyploid species, 12 tetraploids, and 6 hexa-
ploids (Table 2.8 ). Modern classifi cation for the genus Triticum (Van Slageren
1994 ) recognizes two diploid species, T. monococcum L. and T. urartu Tum. ex
Gand., two tetraploid species, T. turgidum L. and T. timopheevii (Zhuk.) Zhuk., and
two hexaploid species, T. aestivum L. and T. zhukovskyi Men. & Er. The economi-
cally important wheats are T. aestivum (bread wheat, comprising 95 % of the global
wheat production) and T. turgidum (macaroni wheat).
Most possible allopolyploid combinations were produced synthetically indicat-
ing that most genomic combinations can be created. The reasons why some combi-
nations are missing in nature can be explained partly by the eco-geographical
isolation of the corresponding analyzers and possibly also by the low viability of
some combinations that might have hybrid weakness and consequently, could not
compete with their parental diploids and establish themselves in nature.
2 Origin and Evolution of Wheat and Related Triticeae Species