25morphological and eco-geographical characteristics (Table 2.1 ). Taxonomists who
dealt with the classifi cation of Aegilops and Triticum , e.g., Zhukovsky ( 1928 ), Eig
( 1929a ), Hammer ( 1980 ), Gupta and Baum ( 1986 ), Van Slageren ( 1994 ), and others,
kept the two genera separated. Cytogeneticists (e.g., Kihara 1954 ; Kihara et al.
1959 ) also kept the two genera separated.
The various genera of this tribe are classifi ed in two subtribes; Hordeineae, the
barley lineage, (nine genera) and the Triticineae, the wheat lineage, (nine genera)
(Clayton and Renvoize 1986 ; Table 2.1 ; Fig. 2.1 ). [The genus Brachypodium
P. Beauv , diverged approximately 32–39 MYA from the Triticeae ( Bossolini et al.
2007 ; International Brachypodium Initiative 2010 ), is not included in this tribe
(Catalán et al. 1995 ; Hasterok et al. 2004 )].
Based on their geographical distributions, Sakamoto ( 1973 , 1991 ) classifi ed the
genera into two major groups: the “Mediterranean” group and the “Arctic-
Temperate” group (Table 2.2 ). This classifi cation was supported by the study of
Hsiao et al. ( 1995 ) and Fan et al. ( 2013 ). The Arctic-Temperate group, distributing
in the Arctic-Temperate regions of the world, have evolved into many endemic spe-
cies in each area. Five of the genera are perennial with two or three spikelets at each
rachis node [ Elymus (several species have solitary spikelets at each rachis node) ,
Histrix Hordelymus, Psathyrostachys and Sitanion ]. The genus Agropyron is an
exception that has only solitary spikelets. Only Hordeum includes perennial and
annual species. One noteworthy characteristic of this group is the extensive inter-
generic and inter-specifi c hybridization over the whole distribution area.
Fig 2.1 Relationships in the Triticeae (modifi ed from Clayton and Renvoize 1986 ). Perennial
genera = white ; Annual genera = gray ; Annual and perennial genera = patterned
2 Origin and Evolution of Wheat and Related Triticeae Species