Alien Introgression in Wheat Cytogenetics, Molecular Biology, and Genomics

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Little is known about processes that lead to speciation in the tribe. The progeni-
tors of the Triticeae were probably all diploid (palepolyploid), perennial, and allog-
amous. From an originally paniculate infl orescence the group developed a spike
with three, later mostly one spikelet at each rachis node (Runemark and Heneen
1968 ; Sakamoto 1973 ). These authors recognized fi ve steps in the development of
the various Triticeae genera: (1) evolvement of diploid (paleopolyploid), cross-
pollinated perennial genera in the Temperate-Arctic zones; (2) intensifi ed autopoly-
ploidization and allopolyploidization and geographical spread; (3) evolvement of
the diploid (palepolyploid) annual genera in the hot and dry summer area of the east
Mediterranean and Southwest Asia regions; (4) evolvement of self-pollinated gen-
era; (5) intensifi ed allopolyploidization of the self-pollinated annuals, especially in
the genera Aegilops , Triticum , and Eremopyrum. Since all the primitive genera dis-
tribute in the Arctic-Temperate zone, this is probably the center of origin of the tribe
while the Mediterranean is the center of variation.
Much of the critical diversifi cation of the tribe, mainly at the diploid level, started
during the Oligocene in the middle of the Tertiary (Fan et al. 2013 ; Table 2.3 ).

Table 2.3 Geological epochs in the Cenozoic era (65 million years ago to the present) and major
climatic, ecological, and Triticeae evolutionary events

Period Epoch

Million

years ago

Major climate and

ecological events

Events in the

evolution of the

Triticeae

Quaternary Holocene 0.01 to

present

Warmer climates;

conversion of many

grasslands and forests

into cultivated areas;

increase in human

population

Domestication of

wheat, barley , and

rye ; formation of

hexaploid (bread)

wheat and

domesticated forms

of tetraploid wheat

Pleistocene 1.8–0.01 Global cooling; four

major ice ages; most

temperate zones were

covered by glaciers

during the cool periods

and uncovered during the

warmer interglacial

periods

Formation of the

allotetraploid

species of Triticum

and presumably also

the allotetraploid

and allohexaploid

species of Aegilops

Pliocene 5.3–1.8 Cooler and drier global

climates; accumulation

of ice at the poles;

development seasonal

climate (cold and humid

winters and hot and dry

summers) in the east

Mediterranean and

Southwest Asia;

development of today’s

landscapes; further

spread of grasslands

Evolvement of the

diploid species of

Aegilops and

Triticum

(continued)

2 Origin and Evolution of Wheat and Related Triticeae Species