58
The higher expression of Q than q is in accord with the fi nding of Muramatsu
( 1963 ) that extra doses (fi ve or six) of q mimic the effect of Q in common wheat. In
addition to Q , Kerber and Rowland ( 1974 ) found the Tg (tenacious glumes) gene,
located on chromosome arm 2DS, confers tough glumes. Chromosome arm 2BS of
emmer also contains the Tg gene that determines tough glumes (Simonetti et al.
1999 ), and there is a possibility that also chromosome arm 2AS contains such a
gene and, therefore, the free- threshing trait might be determined in tetraploid wheat
by at least two complementary genes, Q and tg. Thus, at least several mutations
were required to produce the free- threshing character in ssp. parvicoccum
(Jantasuriyarat et al. 2004 ).
The genes determining tenacious glumes and soft glumes are two independent
loci that affect glume tenacity and spike threshability (Sood et al. 2009 ).
In addition to the above-mentioned classical domestication traits that were selected
in the process of durum evolution, other domestication traits were selected that were
advantageous to the farmer, such as plant erectness versus the prostrate grassy types,
increased number of seeds per spikelet, and reduced seed dormancy (Feldman 2001 ).
Several domestication-related QTLs that affected various traits were mapped (Peng
et al. 2003a , b ; Peleg et al. 2009 ) but the underlying genes were not identifi ed at the
molecular level. It is likely that some other QTLs were selected that are not easily
visible to the eye, such as resistance to abiotic and biotic stresses, physiological
parameters that contribute to yield (Peleg et al. 2009 ), increased grain size (Gegas
et al. 2010 ) as well as quality parameters (Levy and Feldman 1989b ) and in recent
decades, following the green revolution, adaptation to the new cultivation conditions
including chemical fertilizers and mechanical harvest.
The main achievements of the fi rst phase were therefore nonbrittleness of the
spikes, simultaneous ripening of grains, rapid and synchronous germination, and
possibly also larger grains, erect rather than prostrate culms, and free threshing
(naked grains).
During the second phase of evolution under cultivation wheat culture spread into
new areas—an event that required the adaptation of wheat to new climatic, edaphic
and biotic conditions. During the spread to Europe, for example, the plant became
taller and its leaves as well as spikes and grains increased in size. Photoperiodic and
thermoperiodic responses were modifi ed to achieve an optimum balance between
vegetative and reproductive phases: the vegetative period was extended to take
advantage of the longer summer days and rainy season, while the need for high
temperatures for maturation gradually disappeared. In addition, the grain- fi lling
period became longer and fl ag-leaf senescence was delayed. These latter adapta-
tions, allowing for larger amounts of carbohydrates to be assimilated and translo-
cated to the developing grains, greatly contributed to higher yields through an
increase in grain size and number.
This phase involved a long and continuous selection for various agronomic and
technological characters in the polymorphic fi elds of the traditional farmers. In such
fi elds, numerous genotypes were grown in mixtures as landraces. Most fi elds even
contained a mixture of cytotypes, representing tetraploid and hexaploid wheats, and
occasionally diploid wheat as well (Zeven 1980 ). Hence, the unit of selection was
a combination of genotypes rather than a single one.
M. Feldman and A.A. Levy