61Joppa and colleagues (Joppa and Cantrell 1990 ; Joppa et al. 1991 ; Joppa 1993 )
found that genes on chromosomes 6B, 2A, 5B, and 6A of wild emmer increased
grain protein percentage (GPP) in durum wheat. Genotypes with wild emmer chro-
mosomes 6B had the highest GPP (Cantrell and Joppa 1991 ). Olmos et al. ( 2003 )
mapped the high protein gene of chromosome 6B as a Mendelian locus and named
it Gpc-B1. This gene was isolated and found to be responsible for earlier senescence
and shorter grain-fi lling period in wild emmer (Uauy et al. 2006 ). In domesticated
free-threshing durum and common wheat, this gene was mutated to a nonfunctional
allele resulting in longer grain-fi lling period (more than 3 weeks longer) and conse-
quently, a higher percentage of starch and lower percentage of proteins and minerals
in the grains (Uauy et al. 2006 ). Since the wild type allele exists in most analyzed
domesticated emmer (subsp. dicoccon) lines, it is assumed that the mutation
occurred in free-threshing tetraploid wheat (Uauy et al. 2006 ).
The increased yield in cultivated wheat stems to a large extent from delayed fl ag-
leaf senescence, which prolongs the duration of post-anthesis photosynthesis. This
trait can also be achieved by accelerating spike development, which advances anthesis
by a few days. In semi-arid regions, where the available amount of water is often
a limiting factor during grain development, senescence is delayed for only a short
period while in mesic regions senescence and, consequently, maturity in general are
delayed for longer periods.
The recent development of new arsenal of genomic tools may facilitate the iden-
tifi cation of additional loci that control domestication-related traits in wheat. There
has been remarkable progress in the amount of datasets and tools for wheat genom-
ics. Whole genome sequences are also available for the A (Ling et al. 2013 ) and D
(Jia et al. 2013 ) genomes, yet a good assembly of contigs is still missing. SNP map-
ping in a broad collection of wheat landraces and modern varieties has indicated the
genomic regions that underwent selection (selective sweep) during post-
domestication wheat bree ding (Cavanagh et al. 2013 ). Recently, a major advance in
durum transcriptome analysis was the development of tools for the discrimination
of homeologues from the A and B genomes from expression sequence data such as
RNA-Seq (Krasileva et al. 2013 ). Data sets from small RNAs are also becoming
available (Kenan-Eichler et al. 2011 ; Yao and Sun 2012 ).
A comparison of wild and domesticated wheat may reveal changes that have
occurred under domestication. Ben-Abu et al. ( 2014 ) focused on the analysis of
genomic changes that are correlated with the process of domestication and evolu-
tion of modern durum by comparing four genetic groups: wild emmer, domestic
emmer, durum landraces and modern durum varieties. Changes in gene expression
and copy number variation of genes and transposons were analyzed in these four
groups. Genes were clustered based on their pattern of change in expression during
durum evolution, e.g. gradual increase, or decrease, or increase at the onset of
domestication and plateauing later on. There were not many genes that changed >2
fold in copy number. However, interestingly, the copy number of transposons
increased with domestication, possibly refl ecting the genomic plasticity that was
required for adaptation under cultivation. Extensive changes in gene expression
were seen in developing grains. For example, there was an enrichment for certain
functions: genes involved in vesicle traffi cking in the endosperm showed a gradual
2 Origin and Evolution of Wheat and Related Triticeae Species