Centre of origin, evolution and diploidization

1. GP 608
Course Title:
Advances in Breeding of Major Field Crops
Assignment:
Centre of origin, evolution and diploidization of wheat
Submitted to:
Dr. (Mrs.) Vijay Rana
Submitted by:
Amit Rana (A-2019-40-013)
DEPARTMENT OF GENETICS & PLANT BREEDING
COLLEGE OF AGRICULTURE, CSK HIMACHAL PRADESH KRISHI
VISHVAVIDYALAYA, PALAMPUR (HP) -176062

2. 1
Wheat: Origin, Evolution and Diploidization
1. Wheat Origin
Wild and cultivated wheat may be classified into three groups on the basis of their genomes (AA,
AABB or AABBDD). The diploid Triticum monococcum (einkorn); the tetraploid; T. turgidum (durum
and emmer wheat) and the hexaploid T. aestivum (bread wheat). The source of the A genome is generally
believed to be T. urartu; the source of B genome is disputed and the source of the D genome of bread
wheat generally understood to be T. tauschii or Aegilops squarrosa.
Origin of Wheat
T. urartu Disputed T. tauschii or Aegilops squarrosa
Diploid (AA) Tetraploid (AABB) DD in Hexaploid (AABBDD)
Triticum monococcum
(einkorn)
T. turgidum
(durum and emmer wheat)
T. aestivum
(bread wheat)
Cultivated einkorn wheat (AA) descended from a wild ssp. through mutation and is a useful
source of disease resistance. The wild and cultivated emmer wheat can be found in the Southern Turkey,
Iraq, Iran, Israel, Syria, Jordan, Ethiopia, India and the Mediterranean countries.
Emmer wheat may have been domesticated earlier than einkorn and was the predominant type
of wheat cultivated though out Europe and Mediterranean for thousands of years before durum wheat
appeared. Durum wheat is a hybridized species that occurred spontaneously between a wild grass and
primitive diploid wheat. The cultivated bread and durum wheat are descendants of hybridized wild grass
species.
Bread wheat is the product of a later spontaneous hybridization between tetraploid wheat and the
diploid wild grass T. tauschii (Ae. squarrosa) known as goat grass and found in the wheat fields of Asia
Minor and its center of origin is South-West Asia.

3. 2
Table 1: Distribution of the species of wheat in the Ancient Mediterranean area of origin of cultivated plants (after Dorofeev et al. 1979)
In literatures, it is evident that since prehistoric time the wheat in being cultivated. The records
met in China showed that it is cultivated there date back to 2700 BC. It was also known to the Egyptian
and Switzerland’s inhabitants till Stone Age. De Condelle’s research provide evidence that wheat
actually originated from the valley of Euphrates and Tigris and spread all together to Egypt, China and
other parts of the world. Some investigators from America believed that the wheat may be originated in
Palestine and Syria.
N.I. Vavilov, after detailed investigation about centre of origin of cultivated wheat plant, reached
the conclusion that origin of the durum wheat may be the region of Abyssinia, whereas soft wheat crop
that included bread wheat, may be originated from the region of Pakistan, South-Western Afghanistan
and Sothern parts of Mountainous Bokhara. Archaeological evidences tell that wheat was cultivated in
United States along the Atlantic coast and moved west world as country established in early 17th century.
In early 1602 captain Gosnold grown wheat along the Elizabeth Island in Buzzards Bay, Massachusetts.

4. 3
The Fertile Crescent is widely accepted to be the centre of origin of our cereal crops (Zohary et
al. 2012), but this is a wide region and the exact location(s) of domestication has been subject to intense
debate. Spread of cultivated wheat started in the Fertile Crescent about 8500 BC, reaching Greece,
Cyprus and India by 6500 BC, Egypt shortly after 6000 BC.
2. Wheat Evolution
The researchers, by analysing the wheat from different geographical zones reported that it had
been continuously modified over a long course of time by nature and man. The archaeological evidences
proved that cultivated wheat had been evolving from about 10,000 years. The archaeological records
indicate the establishment of the wild Emmer (i.e. Triticum dicoccoides) that was obtained by the
hybridization of wild Einkorn (T. monococum) with goat grass (Aegilops speltoides) about 30,000 years
ago. This wild Emmer (Triticum dicoccoides) by spontaneous mutation evolved to cultivate Emmer (T.
dicoccum) about 10,000 years ago. This (T. dicoccum) crossed with another wild goat grass (i.e. Aegilops
tauschii) to give rise spelt wheat (i.e. T. spelta) about 9000 years ago. About 8,500 years ago the natural
mutation occur in spelt wheat and converted it into the bread wheat i.e. T. aestivum. The cultivated
Emmer wheat was naturally evolved into macaroni wheat i.e. T. durum about 7,500 years ago.
The domestication history for wheat is complex. Hexaploid bread wheat (Triticum aestivum L.
ssp. aestivum) derives its genomes (A, B and D) from three diploid wild ancestors: Triticum urartu

5. 4
Tumanian ex Gandylian, an unknown relative of Aegilops speltoides Tausch and Ae. tauschii Coss,
respectively. These three ancestors, and other grasses within the Triticeae, are related to one another by
descent and through ancestral hybridization (Marcussen et al. 2014). Further, each of the ancestors has
apparently undergone ancient polyploidization events, followed by subsequent reversion to diploid states
(Pont and Salse 2017; Glemin et al. 2018).
The A genome was contributed by T. urartu (AuAu), although T. urartu itself was never
domesticated (Dvorak et al. 1993) during an amphiploidization event which took place no more than 0.5
million years ago (Chalupska et al. 2008). During this event, the B genome was contributed by a member
of the sitopsis group of Aegilops (Blake et al. 1999). The egg donor in this initial hybridization event
was the donor of the B genome and the pollen donor contributed the A genome; therefore, the genome
formula for tetraploid wheats should be written as BBAA (Ozkan et al. 2011), although some other
authors choose to write the formula alphabetically as AABB. Consequently, the hexaploid formula
should be BBAADD.
The result of the initial hybridization event was tetraploid (emmer) wheat T. turgidum ssp.
dicoccoides (Korn.) Thell (Chalupska et al. 2008). Cultivated tetraploid emmer wheat T. dicoccum (syn:
T. dicoccon) and later, durum wheat (T. turgidum ssp. durum) arose from this wild ancestor. Cultivated
diploid einkorn, T. monococcum ssp. monococcum (AmAm), was derived from T. monococcum ssp.
boeoticum (AbAb), a close relative of T. urartu. Today, einkorn is considered a relic crop since it has
largely been supplanted by agronomically superior wheats (Salamini et al. 2002; Kilian et al. 2007).
About 8,000 years ago, a second hybridization event took place between domesticated emmer
wheat and the donor of the D genome, Ae. tauschii, giving rise to modern bread wheat (Peng et al. 2011;
Wang et al. 2013). It is believed that this event occurred when cultivation of domesticated emmer wheat
spread into the natural range of Ae. tauschii (Salamini et al. 2002). Another species of domesticated
hexaploid wheat, spelt (T. aestivum ssp. spelta) was widely cultivated in Europe until the 20th century
when it was largely replaced by bread wheat due to the agronomic superiority of the latter; however,
spelt is still grown to a limited extent in Central Europe, particularly on marginal land, and may provide
a valuable resource for future bread wheat improvement (Longin and Wurschum 2016 ; Muller et al.
2018 ).
The origin of spelt raises some interesting questions about the domestication process. Since spelt
is not free-threshing, it can be considered to be more primitive than free-threshing bread wheat, at least
from an anthropocentric view. Two genes are important for the free-threshing character: tenacious
glumes (Tg) and the domestication locus Q (which also affects other domestication traits).

6. 5
Figure: A graphical representation of wheat evolution and domestication. Several diploid ancestors have contributed to the genome of modern wheat. A
polyploidization event between a member of the sitopsis group, possibly Aegilops speltoides (BB), and Triticum urartu (AA) resulted in Triticum dicoccoides
(wild emmer; BBAA), Triticum dicoccum (domesticated emmer; BBAA) and Triticum durum (durum; BBAA) are derived from this lineage. Hexaploid
Triticum aestivum ssp. aestivum (bread wheat; BBAADD) arose from a hybridization of domesticated emmer (BBAA) with Aegilops tauschii (goat grass;
DD). The origin of hexaploid Triticum aestivum ssp. spelta (spelt; BBAADD) is still controversial, although it is probably not an ancestor to bread wheat,
but rather the result of hybridization between bread wheat and an emmer species.
It is widely accepted that bread wheat arose from a hybridization event between free-threshing
tetraploid emmer wheat (tg-A1/tg-A1; tg-B1/tg-B1; QQ) and Ae. tauschii (Tg-D1/Tg-D1) (Dvorak et al.
2012). In this scenario, early hexaploid would have been hulled due to the tenacious glumes (Tg-D1/Tg-
D1) contributed by Ae. tauschii. Thus, spelt may be a direct ancestor of bread wheat; however, the
evidence suggests European spelt originated from a secondary hybridization of free-threshing hexaploid
wheat with a tetraploid wheat (Blatter et al. 2004). In contrast to European spelt, Asian spelt may
represent an intermediate stage between free-threshing tetraploid wheat and free-threshing hexaploid
wheat. The presence of Tg-D1 and Q alleles support this hypothesis; however, Tg-B1 also exists in the
Asian spelt gene pool, which contradicts this hypothesis (Dvorak et al. 2012).
Two of the most important traits in the evolution of bread wheat and other cultivated grasses
were an increase in grain size and the development of non-shattering seed. The former has been
associated with successful germination and growth of seedlings in cultivated fields, whereas the latter
trait (a hallmark of domestication) prevents natural seed dispersal and allows humans to harvest and
collect the seed with optimal timing (reviewed in Fuller, 2007; Purugganan and Fuller, 2009).

7. 6
Therefore, bread wheat (Triticum aestivum) evolved through two polyploidization events
between Triticum urartu (AA genome) and an Aegilops speltoides-related species (BB genome) 0.5
million years ago, forming Triticum turgidum ssp. diccocoides, and between Triticum turgidum ssp.
durum (AABB genome) and Aegilops tauschii (DD genome) 10000 years ago, forming the modern
hexaploid bread wheat (AABBDD) genome (Feldman et al., 1995; Huang et al., 2002).
Recently available wheat genomic resources offered the opportunity to gain novel insights into
the origin of wheat with the release of the genome shotgun sequences of hexaploid and tetraploid wheat
(IWGSC, 2014) as well as diploid progenitors (Jia et al., 2013; Ling et al., 2013; Luo et al., 2013). From
these resources, Marcussen et al. (2014), confirmed in Sandve et al. (2015), estimated the phylogenetic
history of the A, B and D sub-genomes from 2269 gene trees involving A, B and D homeologs conserved
between the hexaploid wheat subgenomes, among which 275 trees include orthologous sequences from
five diploid relatives (T. urartu, A. speltoides, A. tauschii, Triticum monococcum and Aegilops
sharonensis).
The authors reported that the two tree typologies A(B/D) and B(A/D) were twice as abundant as
D(A/B). This gene-based phylogenetic approach then revealed that the A and B sub-genomes are more
closely related individually to the D sub-genome than to each other. The authors then proposed that the
D genome originated from a homoploid ancestor derived from the hybridization of the A and B diploid
progenitors 5 Ma.
Li et al. (2015a), confirmed in Li et al. (2015b), re-evaluated the origin of hexaploid bread wheat
based on the phylogenomic investigation of 20 chloroplast genomes, which are maternally inherited in
this species complex. The authors argued that, in Marcussen et al.’s (2014) scenario of a homoploid
origin of the D sub-genome, A. tauschii would be expected to share the chloroplast genome of one (the
maternal) of the two progenitors (either T. urartu or A. speltoides). Instead, the authors reported a nested
topology of the A. taushii chloroplast genome. Taking into account not only the A, B and D progenitor
genomes but also the M, N, T, U and C (referred to as S) diploid relatives within this species complex,
the authors reported that the chloroplast genome of A. taushii (D) is more closely related to other D and
S genomes than to the genomes of A. speltoides (S) and T. urartu (A).

8. 7
Li et al. (2015) then hypothesized that the origin of the D (A. tauschii) genome may be more
complex (additional hybridization events to be considered) than suggested initially by Marcussen et al.
(2014). In addition to previous investigations of the evolutionary history of the hexaploid wheat D sub-
genome, the origin of the B sub-genome has also been the subject of intense debate. Several phylogenetic
studies have tried to identify the progenitor of the B genome of polyploid wheat based on cytology
(Zohary & Feldman, 1962), nuclear and mitochondrial DNA sequences (Dvorak et al., 1989; Dvorak &
Zhang, 1990; Terachi et al., 1990) and chromosome rearrangement studies (Feldman, 1966a, b;
Hutchinson et al., 1982; Gill & Chen, 1987; Naranjo et al., 1987; Naranjo, 1990; Jiang & Gill, 1994;
Devos et al., 1995; Maestra & Naranjo, 1999). Molecular comparisons at the whole genome level using
germplasm collections have shown that the B sub-genome from hexaploid wheat could be related to
several A. speltoides lines but not to other species of the Sitopsis section (Salina et al., 2006; Kilian et
al., 2007). At the Storage Protein Activator (SPA) locus (Salse et al., 2008), close relationships between
A. speltoides and the hexaploid B sub-genome have been reported based on both coding and noncoding
sequence comparisons, but with lower conservation compared with the A sub-genome and its T. urartu
progenitor at the putative ATP binding cassette (ABC ) transporter gene (PSR920) locus (Dvorak &
Akhunov, 2005; Dvorak et al., 2006). Taken together, the findings of these studies suggest two
hypotheses, the first being that the progenitor of the B genome is a unique and ancient Aegilops species
that remains unknown (i.e. monophyletic origin and ancestor closely related to A. speltoides from the
Sitopsis section), and the second being that the B genome resulted from the introgression of several
parental Aegilops species (i.e. polyphyletic origin) from the Sitopsis section that need to be identified.
2.1 Systematic and Evolution
The tribe Triticeae Dumort (Hordeae Benth.), a festucoid tribe of the family Poaceae
(Gramineae) has been of great economic importance to humanity. The tribe contains three of the major
cereals-barley, rye and wheat and the man-made cereal Triticale. Triticale is a synthetic amphiploid
between wheat (Triticum spp) and rye (Secale sp.). Several other members of the tribe are also important
as forage and pasture grasses.
Although more knowledge of intergeneric and interspecific relations is probably available than
for any other group of plants of comparable size, the taxonomy and nomenclature of the Triticeae is still
very confused. At this point of time, the nomenclatural subdivisions of the tribe are of little practical
value for the improvement of wheat. However, a brief summary of the genera is also useful because of
their potential value as a source of useful genetic variation for the improvement of wheat.

9. 8
The goat grasses (Aegilops spp); ryes (Secale spp) and wheats (Triticum spp) deserve special
mention because Aegilops spp are involved in the evolution of polyploidy wheats and ryes are involved
in the synthesis of Triticale.
2.2 Classification of the Triticeae
The tribe triticeae forms a distinct natural group characterized by a compound spike, laterally
compressed spikelets with two legumes, single starch grains and fairly large chromosomes in multiples
of seven (Melderis 1953). It is distributed worldwide in the temperate regions of both hemispheres, and
is found in a wide range of habitats. It contains both annual and perennial forms. The perennial forms
predominate in the higher latitudes and the annuals in the areas with a Mediterranean-style climate of
hot dry summers and cooler wet winters.
The tribe contains the following 25 generally recognized genera: Aegilops L.: ploidy status - 2x,
4x, 6x; annuals; Agropyron Gaertn.: ploidy status - 2x, 4x, 6x, 8x, 10x, 12x; mostly used as forages;
Elytrigia Desvaux; Elymus L. Leymus Hochst; Sitanion Rafin.; Haynaldia Schur.; Critesion Rafinesque;
Hordeum L.; Secale L. and Triticum L.
Several of genera may in reality be merged into larger groups. There have been different studies
regarding classification, nomenclature and merger of different genera in one subdivision based on the
possession of a common genome.
2.2.1. Goat Grasses (Aegilops L.)
Genome relationships:
Although the status of Aegilops as a separate genus has been disputed (Bowden 1959; Morris
and Sears 1967), the incorporation of Aegilops into the genus Triticum has not been universally accepted,
and studies and published literature before 1959 treated them as separate genera. The classification and
nomenclature of Kihara and Tanaka (1970), with the addition of Ae. searsii (Feldman and Kislev 1977),
has been widely accepted the genus Aegilops has been retained as such. Kihara (1954), as a result of
investigations based on interspecific crosses, recognized nine diploid analyser species: Ae. umbellulata;
Ae. comosa, Ae. uniaristata, Ae. mutica, Ae. bicornis, Ae. squarrosa; Ae. longissima and Ae. speltoides.
i) The C genome group: Ae. caudate CC and Ae. umbellulata UU (CU
CU
)
ii) The M genome group: Ae. comosa Mm, Ae uniaristata MU
MU
, Ae. mutica Mt
Mt
and Ae
squarrosa DD
iii) The S genome group: Ae. speltoides (SS); Ae bicornis Sb
Sb
and Ae longissima SI
SI

10. 9
All polyploid Ae. species, are amphiploids resulting from combinations of the nine diploid
analysers. The evolution of polyploidy in the genus Aegilops appears to have confirmed considerable
adaptive change. Many of the polyploids have a wider distribution than that of their diploid progenitors.
2.2.2. Rye (Secale L.)
The genus Secale contains wild perennial diploids, wild and cultivated annual diploids and a few
synthetic autotetraploid cultivars.
S. montanum Guss. (2n=2x=14)
S. dalmaticum Vis.
S. ciliataglume (Boiss.) Gross.
S. daralgesii Thum.
S. kuprijanovii Grossh.
S. anatolicum Boiss. (2n = 2x = 14)
S. africanum Stapf. (2n = 2x = 14)
S. cereale L. (2n = 2x = 14)
S. afghanicum (Vav.) Roshev.
S. dighoricum (Vav.) Roshev.
S. segetale (Zhuk.) Roshev.
S. ancestrale Zhuk.
S. turkestanicum Bensin.
S. vavilovii Grossh. (2n = 2x = 14),
S. silvestre Host. (S. fragile M.B., S. campestre Kit. and Schult.) (2n = 2x = 14)
S. iranicum Kobyl. (2n = 2x = 14)
Several workers have attempted to explain the phylogeny of the genus mostly on the basis of
chromosomal translocation differences. The perennial species, S. montanum, S. anatolicum and S.
africanum differ from S. cereale by two translocations involving three pairs of chromosomes. The
genomes of S. montanum and S. cereale have also been found to be non-homologous. Secale silvestre
also differs from S. cereale by the same translocations as the perennials. It is generally accepted that S.
montanum and S. silvestre are ancient species. Stutz (1972) proposed that S. silvestre had evolved from
the S. montanum complex, S. vavilovi then evolved from S. silvestre with the acquisition of translocations
of S. cereale then arose as a result of polymorphic intercrossing among the annual weedy forms and
possibly the perennial forms.

11. 10
2.2.3. Wheat (Triticum sp.)
The wheats (Triticum sp.) like the genus Aegilops, form a polyploidy series with diploid
(2n=2x=14), tetraploid (2n=4x=28) and hexaploid (2n=6x=42) forms. In common with the rest of the
Triticeae, they suffer from a confusion of nomenclature and classification.
Loss of Ambiguity: Same form frequently found under many different names; totally different forms
given same name. The genus Triticum has been therefore not combined with the genus Aegilops to avoid
any ambiguity. The following nomenclature for the wheats is therefore given as a practical scheme;
Among diploids T. urartu Tum AA
T. boeoticum Boiss.
spp. aegilopoides
spp thaoudar
AA
T. monococcum L. AA
T. sinskajal AA
Tetraploids T. dicoccoides AABB
T. dicoccum AABB
T. paleocolchicum AABB
T. carthlicum AABB
T. turgidum AABB
T. polonicum AABB
T. durum AABB
T. turanicum AABB
T. araraticum AAGG
T. timopheevi AAGG
T. militina AAGG
Hexaploids T. spelta AABBDD
T. vavilovii AABBDD
T. macha AABBDD
T. sphaerococcum AABBDD
T. compactum AABBDD
T. aestivum AABBDD
T. zhukovskyi AAAAGG

12. 11
although not taxonomically based, and undoubtedly not obeying the rules of nomenclature, it goes some
way towards simplifying the problem.
The wild diploid wheats are separated into the species T. urartu and T. boeoticum. The T.
boeoticum diploids form two races or ecotypes: those with a single fertile floret in each spikelet and
those with two fertile florets. It is therefore convenient to have a separate name for each form. The single
seeded form becomes T. boeoticum spp. aegilopoides and the double seeded form T. boeoticum spp.
thaoudar.
i. The origin of A genome
The diploid wheats comprise a single genomic group with the genome formula AA. This basic
genome is also common to all the polyploidy wheats. The wild diploid spp T. boeoticum and T. urartu,
although exist in the same geographical area, are isolated genetically by the sterility of their hybrids and
can be distinguished on isoenzyme and seed storage protein differences. The cultivated, less fragile T.
monococcum probably results from artificial selection of a more suitable form for cultivation. As it
matches T. boeoticum rather than T. urartu in isoenzymes and storage protein and has only a single
fertile floret to each spikelet, its progeniture was found certainly to be single-seeded T. boeoticum ssp.
aegilopoides. The recently recognized cultivated form from Daghestan, T. sinskajae is probably a mutant
form selected for its free-threshing characteristics.
Morphological and geographic distribution of diploid species
T. urartu (2n=2x=14, AA), wild einkorn or small spelt wheat. A wild wheat with fragile, awned ears
with two fertile florets per spikelet. Endemic in eastern Turkey, Syria, Iraq, western Iran and
Transcaucasia.
T. boeoticum Boiss. (2n = 2x = 14), wild einkorn or small spelt wheat (T. monococcum L. ssp. boeoticum
(Boiss.) Mk., T. spontaneum Flaskb., Crithodium aegilopoides Link.). A wild wheat with fragile awned
ears.
ssp. aegilopoides (T. aegilopoides (Link.) Bal.) with a single fertile floret and usually a
single awn to each spikelet.
ssp. thaoudar (T. thaoudar Reut.) with two fertile florets and usually two awns per
spikelet. Endemic in south east Europe, Asia Minor and from the eastern Mediterranean to
Transcaucasia and western Iran.
T. monococcum L. (2n = 2x = 14, AA), cultivated einkorn or small spelt wheat (T. monococcum L. ssp.
monococcum, Nivieria monococcum Ser.). A cultivated wheat resembling the wild diploids, but the ears
are broader and more dense with shorter awns. The ears are generally less fragile and have one fertile
floret and usually a single awn per spikelet, although forms with two fertile florets do occur.

13. 12
T. sinskajae A. Filat and Kurk. (2n = 2x = 14, AA), cultivated einkorn or small spelt wheat. Very similar
to T. monococcum but free-threshing; cultivated in Daghestan.
The tetraploid emmer wheats are divided into two groups; those with the genome formula AABB
and those with the genome formula AAGG. Within each group the major recognized forms are given
specific names based on names that are widely used and understood. A few minor forms have been
incorporated into these divisions: A form with long narrow glumes classified by Kihara, Yamashita and
Tanaka (1956) as T. polonicum incorporated into T. dicoccum.
Both genomic group (AABB & AAGG) have wild forms, T. dicoccoides AABB and T.
araraticum AAGG. These two wild emmer wheats although morphologically similar and with over
lapping geographical distributions, are genetically isolated by the infertility of hybrids between them.
ii. The origin of B & G genomes:
The origin of the B and G genomes has been the subject of speculation and investigation and still
remains largely unsolved. McFadden & Sears (1946) proposed Agropyron triticeum as the source of B
genome. Later Sears (1956) suggested Ae. bicornis. Pathak (1940), Sakar & Stebbins (1956) and Riley,
Unrau and Chapman (1958) put forward the widely accepted theory that Ae. speltoides was the donor.
The latest candidate proposed by Feldman (1978) is the recently recognized Ae. searsii.
Recently a considerable amount of experimental evidence has accumulated against Ae.
speltoides, in its present form as a candidate for the origin of the B genome. Chromosomes banding, in
studies have failed to match Ae. speltoides to the B genome.
Several possibilities exist for the origin of the B genome: the original donor may now be extinct;
the donor may be a yet-undiscovered diploid species, the genome may be derived from more than one
source or a rearrangement of the DNA may have occurred since its incorporation into the tetraploid.
The AAGG tetraploids had long been thought to have a second genome differing from that of B
genome. Wagenaar (1961) proposed that the chromosome asynapsis observed in hybrids between the
AAGG tetraploids and hexaploid wheat, AABBDD, was due to a genetic mechanism and not a genomic
difference. This asynapsis has been shown to be largely between the B and G genomes and not the A
genomes, which supports the idea of the deferring second genomes. Ae. speltoides has also been
suggested to be the donor of the G genome on the basis of chromosome pairing. G genome has been
found to be very similar to S genome of Ae. speltoides, however the alternatives the apply to the B
genome donor also apply to the donor of the G genome.

14. 13
Electrophoresis studies have shown that the A genomes of the two tetraploid groups are not
identical, especially with regard to isoenzymes. The AABB group show similarities to T. urartu whereas
the AAGG group show similarities to T. boeoticum and T. monococcum.
Current evidences show that wild allotetraploid emmer wheat T. dicoccoides AABB arose by
amphiploidy between the wild diploid wheat T. urartu AA and an unknown diploid or diploids with
genomes similar to those of the present members of the sitopsis section of the Aegilops. Similarly, the
wild allotetraploid emmer wheat T. araraticum AAGG is the result of amphiploidy between the wild
diploid T. boeoticum AA & a diploid or diploids with genomes very close to that of the present Ae
speltoides. The cultivated diploid T. monococcum AA is unlikely to have the A genome donor because
of the archaeological data, show largely no evidence of its existence before the tetraploids.
A major factor in the successful establishment of both groups of tetraploids is due to the
acquisition of the diploidizing mechanism which restricts chromosome pairing to homologous
chromosome within each genome. T. dicoccum was probably the first cultivated wheat, but may have
arisen in the wild and become established as a result of husbandry and artificial selection of naturally
occurring stands of wild wheat. The other cultivated tetraploids have evolved from the initial cultivated
form by selection of a series of modifications to the current free threshing non-fragile forms, such as the
widely cultivated Macaroni wheat, T. durum.
Triticum carthlicum is a free threshing wheat like the hexaploids T. aestivum, T. compactum and
T. sphaerococcum and was thought to have acquired this character by introgression from the hexaploid
T. aestivum.
T. paleocolchicum is a non-free-threshing form similar to T. dicoccum but with a broad compact
ear and is only found mixed with the hexaploid T. macha. The cultivated AAGG tetraploid, T. timopheevi
must have arisen from T. araraticum in a similar way to that by which T. dicoccum arose from T.
dicoccoides.
Morphological and geographic distribution of tetraploid species
T. dicoccoides (Korn) Schweinf. (2n = 4x = 28, AABB), wild emmer wheat (T. turgidum (L.) Theil. ssp.
dicoccoides (Korn) Theil., T. vulgare Vill. var. dicoccoides Korn). A wild wheat with awned laterally
compressed fragile ears. Endemic from Palestine through the Fertile Crescent to western Iran.
T. dicoccum (Schrank.) Schubl. (2n = 4x -= 28, AABB), emmer wheat (T. turgidum (L.) Theil. ssp.
dicoccum (Schrank) Schubl., T. dicoccum Schrank, T. farrum Bayle-Barelle, T. amyleum Seringe, T. zea
Wagini, Spelta amylea Seringe, T. volgense (Flaskb.) Nevski, T. vulgare dicoccum Ale f., T. sativum

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dicoccum Hack., incl. T. ispahanicum Heslot). A cultivated wheat with awned, often laterally
compressed ears. The ears are less fragile than T. dicoccoides.
T. paleocolchicum Men. (2n = 4x = 28, AABB) (T. turgidum (L.) Theil. ssp. paleocolchicum (Men.) k.,
T. dicoccum Schrank var. chvamlicum Supat., T. georgicum Dek.). A monomorphic cultivated wheat
with a compact, laterally compressed awned ear with a zig-zag rachis found as admixture of T. macha
in western Georgia.
T. carthlicum Nevski (2n = 4x = 28, AABB), the Persian wheat (T. turgidum (L.) Theil. ssp. carthlicum
(Nevski) Mk., T. persicum Vav., T. ibericum Men., T. paradoxum Parodii). A free-threshing cultivated
wheat of Southern Transcaucasia, northeastern Turkey, northern Iraq and northwestern Iran. The ear is
awned on both the lemma and the outer glume.
T. turgidum L. (2n = 4x = 28, AABB) rivet or cone wheat (T. turgidum (L.) Theil. ssp. turgidum conv.
turgidum. T. vulgare turgidum Alef., T. sativum turgidum Hackel incl. T. pyramidale Perc.). A cultivated
wheat generally robust with parallel sided square section ears often with four or five fertile florets per
spikelet. The ears are usually awned.
T. polonicum (2n = 4x = 28, AABB) the Polish wheat. (T. turgidum (L.) Theil. ssp. turgidum conv.
polonicum (L.) Mk., T. levissimum Haller, T. glaucum Moench, Gigachilon polonicum Seidl., De ina
polonica Ale f.). According to Percival (1921) not grown in Poland before 1870, but of Mediterranean
origin. A cultivated wheat characterized by large ears with long narrow empty glumes which extend
beyond the rest of the spikelet.
T. durum Desf. (2n = 4x = 28, AABB) the Macaroni wheat (T. turgidum L.) Theil. ssp. turgidum conv.
durum (Desf.) Mk., T. vulgare durum Alef., T. sativum, and durum Pero., T. vulgare durum Alef., T.
sativum durum Hackel., T. tesax, B. II., durum Asch. and Graes., T. alatum Peterm., incl. T. aethiopicum
Jakubz., T. turgidum ssp. abyssinicum Vav., T. abyssinicum Vav.). A widely cultivated wheat with
square-section or laterally compressed ears which are usually awned. The grains generally have a hard,
translucent, flinty endosperm.
T. turanicum Jakubz: (2n = 4x = 28, AABB) the Khorasan wheat (T. turgidum (L.) Thell. ssp. turgidum
conv. turanicum (Jakubz.) Mk., T. orientale Perc., T. percivalii). A cultivated wheat of central Asia and
the northern Middle East. Characterized by a very lax ear with scabrid awns.
T. araraticum Jakubz. (2n = 4x = 28, AAGG) wild emmer wheat (T. timopheevi Zhuk. ssp. araraticum
(Jakubz.) Mk., T. dicoccoides ssp. armeniacum Jakubz., T. armeniacum Mak., T. montanum Mak., T.
chaldicum Men.). A wild wheat distinguished from T. dicoccoides by differences in its second genome.

16. 15
Endemic in northeastern Asia Minor, Transcaucasia, northern Iraq and western Iran, overlapping the
distribution of T. dicoccoides.
T. timopheevi Zhuk. (2n = 4x = 28, AAGG) (T. timopheevi Zhuk. ssp. timopheevi, T. dicoccum
dicoccoides Korn. var. timopheevi Zhuk.). A cultivated wheat of western Georgia and possibly
northeastern Turkey. Genomically the same as T. araraticum. The ears are wide, laterally compressed
and awned.
T. militinae Zhuk. and Migusch. (2n = 4x = 28, AAGG). A single specimen derivative of T. timopheevi,
with short, wide, dense, black, free-threshing ears, with an extra awn on the outer glume.
iii. The origin of the hexaploid wheats
The hexaploid wheats are of two types: the major group with the formula AABBDD & a single
hexaploid, T. zhukovskyi AAAAGG. T. zhukovskyi was isolated from mixed cultivation populations of
T. monococcum and T. timopheevi and is certainly on amphiploid of these two sp. The AABBDD
hexaploids are the result of amphiploidy between an AABB tetraploid and the D genome diploid Ae.
squarrosa. Aegilops squarrosa was first proposed as the donor of the third genome (Pathak 1940) and
this was confirmed by McFadden and Sears (1946) and Riley and Chapman (1960). The hexaploids are
all cultivated wheats – no wild forms exist. It is therefore likely that they arose in cultivation. The present
of Ae. squarrosa as a common weed of wheat fields in Transcaucasia and the middle east supports this
hypothesis. Archaeological evidence indicates that hexaploid wheat was established by 7000 BC.
The principal differences between the hexaploid forms are due to single genes with the possible
exception that T. vavilovi may differ by two genes. These genes are:
i. q (the speltoid gene) and its dominant allele Q (which confers a free-threshing grain and a tough
rachis) on chromosome 5A;
ii. c and its dominant compact ear producing allele C on chromosome 2D;
iii. S and its recessive spherical-grain producing allele s on chromosome. 3D;
and V on chromosome 5A of T. vavilovi.
T. spelta qq cc SS
T. macha qq CC SS
T. compactum QQ CC SS
T. aestivum QQ cc SS
T. sphaerococcum QQ cc ss

17. 16
As neither ‘C’ nor ‘s’ has been found in Ae. squarrosa, neither T. compactum nor T.
sphaerococcum can have been the first hexaploid. T. aestivum can also be excluded, as none of the
tetraploid wheats cultivated by the ancient farmers has the phenotype produced by QQ. The evolution
of T. zhukovskyi appears to be very simple. According to Upadhyay & Swaminath (1965), this species
is the result of amphidiploidy between T. timopheevi & T. monococcum. It would thus see that hexaploid
wheat arose under cultivated conditions as a result of amphiploidy between T. dicoccum AABB and Ae.
squarrosa DD. The free threshing, compact-eared and spherical-grained forms arose later as a result of
mutation.
Morphology and geographical distribution of hexaploid species
T. zhukovskyi Men. and Er. (2n = 6x = 42, AAAAGG). A cultivated wheat of the area of T. timopheevi
cultivation of western Georgia. Distinguished from T. timopheevi by slightly longer and less compact
ears.
T. spelta L. (2n = 6x = 42, AABBDD) spelt of Dinkel wheat (T. aestivum (L.) Thell. ssp. spelta (L.)
Thell., T. vulgare spelta Alef., T. sativum spelta Hackel, T. zea Host, Spelta vulgaris Seringe). A
cultivated wheat with long lax fragile awned or awnless ears with tightly invested grains.
T. vavilovi (Tum.) Jakubz. (2n = 6x = 42, AABBDD) (T. aestivum (L.) Thell ssp. vavilovi (Tum.) Sears).
A cultivated spelt wheat of Armenia with branched (elongated rachilla) ears carrying short awns.
T. macha Dek. and Men. (2n = 6x = 42, AABBDD) (T. aestivum (L.) Thell. ssp. macha (Dek. and Men.)
Mk., T. tuballicum Dek., T. imereticum Dek.). A polymorphic cultivated spelt wheat of western Georgia,
with laterally compressed wide fragile ears, which may be awned or awnless.
T. sphaerococcum Perc. (2n = 6x = 42, AABBDD). Indian dwarf or shot wheat. (T. aestivum (L.) Thell.
ssp. sphaerococcum (Perc.) Mk.). A cultivated wheat of northwest India and parts of Iran. Characterized
by short dense awned or awnless ears and small near hemispherical grains.
T. compactum Host. (2n = 6x = 42, AABBDD) club wheat (T. aestivum (L.) Thell. ssp. compactum
(Host.) Mk., T. vulgare compactum Alef., T. sativum compactum Hackel, T. tenax, A II., compactum
Asch. and Graeb.). A free-threshing cultivated wheat with short uniformly dense, oblong or oval awned
or awnless ears.
T. aestivum L. (2n = 6x = 42, AABBDD) bread or common wheat (T. aestivum (L.) Theil ssp. vulgare
(Vull.) Mk., T. vulgare Vill., T. vulgare Host., T. hybericum L., T. sativum Lamk., T. sativum Pers.). The
most widely cultivated wheat today.

18. 17
3. Diploidization of wheat
• The acquisition of diploidizing mechanism was the major factor in the successful establishment
of the polyploid wheats. This mechanism does not allow pairing between homeologous
chromosomes and thus restricts chromosome pairing within each genome but no pairing between
genomes. Therefore, because of this mechanism, polyploids behave like diploids as regards
pairing between chromosomes.
• However, Okamoto (1957) and Riley and Chapman (1958) found that the diploidizing
mechanism in hexaploid wheat is genetically controlled. According to them, the presence of
some gene or genes on the long arm of chromosome 5B considerably affects the meiotic pairing
in hexaploid wheat by restricting chromosome pairing between the homeologous chromosomes
only.
• The absence of 5B chromosomes causes considerable pairing between homeologous
chromosomes. The discovery indicated that hexaploid wheat is more autopolyploid than an
allopolyploid. However, there should be no controversy about the kind of polyploidy in tetraploid
and hexaploid wheats. In fact, the polyploid wheats are segmental polyploids and these wheats
should be neither called as autopolyploids nor as allopolyploids.
• The gene responsible for the suppression of chromosome pairing between homeologous in wheat
was identified by Wall et al. (1971) and designated as Ph (later modified to Ph 1).
• In addition to 5B suppressor system, other suppressing system with minor suppressing effect
have been found by Upadhya and Swaminathan (1967) and Driscoll (1973) on short arm of 3A
and called as 3AS; by Upadhya and Swaminathan (1967), Mello-Sampayo (1968, 1971) and
Mello- Sampayo et al. (1973) on the short arm of 3D and called as 3DS; and by Driscoll (1973)
on 4D.
• On the other hand, some chromosome pairing promoter systems have also found by Feldman
(1968) on the long arm of 5A called as 5AL; by Feldman and Mello-Sampayo (1967) and Riley
and Chapman (1967) on the short arm of 5B called as 5BS; by Feldman (1966) and Riley et al.
(1966) on the long arm of 5D called as 5DL and by Feldman (1968) on short arm of 5D called
as 5DS.

19. 18
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