World population is continuously increasing and is expected to double by the
year 2050. To feed this huge population at least in today’s standard health
limits, named "food security", the wheat production necessary is twice as much
as today. According to more optimistic futurists the population will never reach
to these levels even with extended life expectancies, hence Malthus is not
correct in his warning that population will outstrip food production1. Even in
this situation, however, the food supply problem is still not over. Demand is
growing, as developing countries increasingly turn from rice to wheat. As
countries become more industrialised, wheat becomes the preferred cereal, not to
mention, a large agricultural force is altered to industrial workers. It is
claimed that China alone, if keeps growing in the same income rate (indeed, it
is expected to do so), will make necessary an additional 200 million metric tons
per year wheat import by the year 2030. This value is actually the total trade
value of today,2,3. FAO and World Bank worn us on food scarcity much earlier
then expected, after the investigations that have proven a dramatic impairment
in actual world cereal production rates with the projected values which made
authorities announce wheat as strategic material. The accelerated imbalance in
the first half of this decade is accounted in part for the transition state of
Eastern European countries, adverse meteorological conditions and rapid income
increase of some developing countries. Whatever the reason is, today, the world
wheat stocks have declined to a point where only for consumption of 49 days
stocks are present, hence prices are in an increasing trend. This resulted in a
re-evaluation of the "Outlook to 2010" by FAO4. In the 1960s and 1970s the
"green revolution" boosted crop yields, including those of wheat, by as much as
100 per cent. This was largely due to the development of dwarf wheat varieties
which made more effective use of fertilisers and irrigation. The plants were
able to support increased grain yield without collapsing. For the past decade,
however yields have been improving at only 0.5 per cent a year. The huge gains
of green revolution is not expected to be repeated5. The production areas will
not be extended any more and the fertiliser utilisations does not seem to
produce much positive effect on the hostile soil. Thus, the golden rule of
economy seems to apply "Increased demand plus decreased production is equal to
elevated prices". In this respect the only positive thing for Turkey seems to be
that the utilisations of remnant virgin areas with the advance of the GAP
project may increase the wheat production to some extent.
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Wheat Supply
W heat is a major commodity in a number of regions. While China, EU, India,
United States and Russia are the major producers, Turkey is within the first
ten. According to Minnesota Association of Wheat Growers the 1995-96 world wheat
production was 539.57 million metric tons but while the beginning stocks were
114.88 the year was ended in 105.27 million metric tons. Actually this
decreasing trend was seen from the beginning of the decade on. The map below
indicates the most productive area of wheat. While winter wheat is grown in the
yellow region, the blue region is the growing region of spring wheat. In the
intersection where indicated in green there is the only place where both winter
and spring wheat are grown. In other major wheat producing areas like Canada and
Australia the production areas are in rather restricted regions. Mainland China,
EU, India, USA, Russia and Canada each contribute 18.5, 16.2, 11.3, 11.2, 5.9
and 4.4 per cent respectively to the world production. The contribution of
Turkey is 2.9 per cent (1995-96 session). With this structure in the world
supply, no country dominates the international wheat market. Some countries do
have a relatively strong position in the market for particular kinds of wheat.
There is even interst in production of wheat in space
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Wheat's DNA Points to First Farmers
W heat today is synonymous with bread, but in historical times before it became the stuff of life, people grew an ancient form known as einkorn. The cultivation of einkorn, perhaps for eating as gruel, is thought to mark the origin of agriculture in the Old World. "If you know where (einkorn) wheat was domesticated, you know where agriculture originated," says wheat geneticist Jan Dvorck of the University of California, Davis8. Vavilov, back in 1926, proposed three centers of origin in West Asia and North Africa. The Near East center of agricultural origin corresponds geographically to a region known as the Fertile Crescent, an arc that extends from Palestine through Syria and Southern Turkey into Iraq and Western Iran. A new study aims to find those whereabouts is on DNA Fingerprinting of Einkorn Wheat, to Explain Human Evolution from Hunter to Farmer. The DNA fingerprinting has demonstrated the exact location of the domestication of einkorn wheat. One of the "FOUNDER CROPS" along with the chickpea and bitter vetch grown in the fertile crescent of Southwest Asia around 9000 B.C. Where as the sites for the barley, emmer wheat, peas, lentil and flax are still to be located around there. A collaborative work published in Science9 (Heun et al. 1997. Site of Einkorn wheat domestication identified by DNA fingerprinting. Science vol. 278. p 1312.) have used multiple dominant DNA markers-AFLPs (288 polymorphic bands) to find the genetic distance within the species T. monococcum (the einkorn wheat). They analysed 68 lines of cultivated einkorns, plus 261 wild einkorn lines sampled over the crescent’s whole expanse as well as outside the crescent. Among those wild lines, the most distinct genetically proved to be a group of 11 from the Karacadag mountains of Southwest Turkey. Assuming that the progenitor of present cultivated einkorn have not undergone much change during last 10,000 years, those 11 also turned out to be the ones genetically most similar to cultivated einkorn, and so presumably the crop’s immediate ancestors, as it is clearly seen in the aflp gel in figure below. (http://www.mpiz-koeln.mpg.de/~schaefer/mono/ fig14_s.html). This also supporting the previous evidence found by conventional nonmolecular methods. Archeological findings have shown that, some 10,000 years ago, barley and wheat, lentil, pea, flax and vetch were domesticated in the Fertile Crescent. Contemporary geographical distribution of the wild progenitors of these crops corroborates the archeological evidence. Stretching between present-day Turkey and Iran, the Fertile Crescent is well known from archaelogical evidence as the home of the first farmers, about 10,000 years ago. Wild einkorn coincides perfectly with the limits of the Fertile Crescent. The grain "was a logical choice for neolithic people," says Dvorck. The DNA fingerprinting study, more then just telling us the exact location where cultivated einkorns have originated, it tells us the change that might have occurred in HUNTERS to settle and become FARMERS. The potential of having a cereal, a food of high energy and quality, all year around made possible the first human settlement10. All the great Near East and Mediterranean cultures, from the Sumerians and Egyptians to the Greeks and Romans, were able to develop because of the cereals in the Fertile Crescent and efficient agricultural production which provided enough food for large cities and growing nations. Many other crop progenitors originating from the Near East accompanied barley and wheat during their spread20. The first basic step toward development of cereals was made when humans started sowing seeds that had been gathered the previous season, adopting the first measures for soil preparation and tillage, and discovering the best time for seeding. The availability of special types of grasses, able to grow in large swards, with spikes characterized by relatively large kernels, started the expansion and growth of the western Asian civilization, resulting in the Egyptian and later the European and Western civilizations8. It might have taken not more than a few centuries for it to happen. This accompanies the change the einkorn wheat has gone through genetically while it was domesticated (within such short time) by the hunters. Finding reveals it is very less, just few loci have changed, accounting for few morphological changes feasible it to be cultivated. But these morphological changes were enormous: heavier seeds, denser seed masses, and firm stalk. Making it feasible for FARMERS grow and harvest. HUNTERS might have selected the einkorns for these characters changing the morphology of the wheat, in turn evolving themselves into FARMERS, settling down in one place, instead of roaming around. Barley and lentils, then other crops came that led eventually to the familiar bread wheat. From the Northeast of crescent along the Caspian Sea, Aegilops squarosa is grows which is one of the parent of present day modern bread wheat, emmer wheat being another. Likewise, sheep, goats, and pigs were domesticated in the Fertile Crescent, along with dogs. This also explains the reason for the rapid development of human habitat and so the agriculture around the crescent. The development of agriculture started from that part of the world and spread around: "Other great civilisation started to take shape-dense, sedentary human population-emergence of Kings, bureaucrats, scribes, professional soldiers and metal-workers and other full-time craftspeople. Literacy, metallurgy, stratified societies, advance weapons, and empires rested on food production. In addition, smallpox and other crowd epidemic diseases of Eurasia could evolve only in those dense, sedentary human populations living in close contact with domesticated animals whose own pathogens evolved into those specialised pathogens afflicting us. Thus, a long straight line runs through world history, from first domesticates at the Karacadag mountains and elsewhere in the Fertile Crescent, to the "Guns, germs, and steel" by which European colonies in modern times destroyed so many native societies of other countries."
Some recent archaelogical excavations in Anatolia enlighten the importance of the wheat in more recent historical times. The figure seen below is from Kemerhisar, Nigde (Cappadocia: Land of Beautiful Horses), from the Tuwana/Tyana Kingdom (4 B.C.-1 A.C.). The excavation is carried by German archaeologist, Dr. Dietrrich Berges from Erlangen University. The figure illustrates the King Marpalawa (small figure) presenting wheat to the God of Plants, Tarhunza. The importance of wheat as a gift to God is clear19. A ll wheats belong to the genus Triticum. Cytologic and cytogenetic work showed that wheats fall into three basic natural groups, each one characterized by having 14 chromosomes (seven pairs) or a multiple of 14 chromosomes in each somatic cell; diploid wheats have 14, tetraploid wheats 28, and hexaploid wheats 42. Domestication and cultivation in large areas, often very different from the agroecological point of view, have induced great variability through thousands and thousands of generations. This variability has been increased in tetraploid and hexaploid wheats by their ploidy level and by the survival possibility of mutations, which could permit progress in human utilization of these cereals. Tetraploid wheats are certainly allopolyploid in nature (in other words, they do not have four sets of seven basically similar chromosomes, as autotetraploid organisms do, but two sets each of chromosomes only remotely related to each other by having an ancient, common ancestor). This allopolyploid nature allows a disomic type of chromosome pairing at meiosis. In tetraploid wheats, 14 pairs of chromosomes are then identified. Genetic and cytogenetic analysis conducted mainly at the hexaploid level have demonstrated that the chromosomes of the three basic genomes in hexaploid (ABD) or two basic genomes in tetraploids (AB) can be grouped into seven basic types. One pair of chromosomes of genome A is at least partially able to substitute for a specific pair of the genome B (in tetraploids) or B and D in hexaploids. Therefore, in wheat, the corresponding chromosomes in all three genomes have been identified and a so-called homoelogous series established as follows:
1A 2A 3A 4A 5A 6A 7A
1B 2B 3B 4B 5B 6B 7B
1D 2D 3D 4D 5D 6D 7D
Very often, similar mutations (or variations) can be found in all of the
three types of genomes, in corresponding (homoelogous) chromosomes. This
phenomenon is easily explained by the fact that the three genomes (A, B, and D)
had a common ancestor in the past.
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Diploid Wheat
All the diploid level, basically only one species, T. monococcum, has been
domesticated and possibly has been used by humans for 8,000-10,000 years. This
type of wheat has a relatively tough rachis of the spike and has seeds
(generally only one per spikelet) almost twise the size of those of the wild T.
boeoticum. This wheat is still cultivated in some remote mountainous areas of
Italy, the Balkans, Turkey, and Transcaucasia, often mixed with other cereals;
however, most likely, it will soon disappear from cultivation. The kernels are
tightly covered by the glumes, and the yield is rather poor. However, the
diploid species is rather resistant to diseases (particularly rusts). The area
of distribution is possibly more restricted than in the past, particularly for
the wild species, which are being pushed away from the cultivated areas.
However, the present area of distribution of these species is fully in line with
the hypothesis of speciation of wheat, as mentioned earlier. The present area of
distribution of a group of Aegilops species (or subspecies) belonging to the
Sitopsis (Platystachys) section of the genus (identified with the genome S),
which are thought by many to be the most probable donors of genome B (in the T.
turgidum group) and of the genome G (or Bt) of T. timopheevi.
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Tetraploid Wheat
Most certainly, wild tetraploid wheats were already largely distributed in
the Near East when humans started harvesting them in nature. Their general size,
and particularly the size of the head and the kernels, made them much more
worthwhile for domestication than diploid wheats. Therefore, the potential of
tetraploid wheats was quite interesting from the beginning. From a morphological
point of view, T. dicoccoides types, mostly found in Palestin, Syria, and
Lebanon, are not clearly distinguishable from T. araraticum populations, now
scattered in the Zagros Mountains, on the border between Iraq and Iran, in
Armenia, on the borders of Iran and Turkey, and in some mountainous ares on the
border between Syria and Turkey. These are probably remnants of more largely
distributed populations of wild tetraploid wheats. It is also likely that even
more than two distinct hybridization and allopolyploidization events (leading to
the present two tetraploid species) occured thousands of years ago. The two wild
types were both domesticated, T. araraticum giving T. timopheevi and T.
dicoccoides first giving domesticated types resembling the present T. dicoccum
types. However, these first two domesticated types had quite different success.
T. timopheevi remained in cultivation in limited areas in Armenia and
Transcaucacia, whereas T. dicoccum spread from the near East to large areas of
the Mediterranean and Middle East, to Egypt, and to Ethiopia. Later, the more
advanced types (characterized by naked kernels and much wider adaptation)
belonging to T. turgidum and T. durum spread to all Europe, the Middle East, and
North Africa. During the Roman Empire, most of the wheat carried to Rome from
the colonies belonged to the T. durum-turgidum-dicoccum group. The basic
differences between the T. turgidum and T. durum types are in the kernel
structure and in the better adaptation of T. durum to semiarid conditions and of
T. turgidum to a more continental and humid climate. Both types were used for
various types of breadmaking or for a number of other uses, some of which are
still common today (bulgur, couscous, etc.). thousands of years of cultivation
and continuous human and natural selection have resulted in tremendous
variability in the tetraploid wheats derived from the T. dicoccoides wild type.
In this group, a number of subspecies have been described: T. paleocolchicum (T.
georgicum), T. carthlicum (T. persicum), T. aetiopicum, T. Turanicum (T.
orientale), and T. polonicum (T. ispahanicum). These differenciations are based
mainly on morphological modifications. All of the subspecies are easily
intercrossed and fertile; they all have the same chromosome structure and
perfect pairing at meiosis; and often, in progeny of crosses, several
intermediate types are found breeding true. All the domesticated and wild types
having the genome AABB have been grouped into only one valid species, T.
turgidum (L.), which in turn is subdivided into several subspecies and botanical
varieties. Among all cultivated tetraploid wheats, T. durum types are by far the
most important, even though they are grown in only 10% of all the
wheat-cultivated area, the remaining 90% being represented by the hexaploid
bread wheat, T. aestivum.
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Hexaploid Wheat
Two species of wheat are described at the hexaploid level: T.zhukovskyi
(genome AAG), derived from tetraploid wheat T. timopheevi, and T. aestivum (L).
The bread or common wheat, derived from the tetraploid wheat T. turgidum (genome
AB) and Ae. Squarrosa L. (genome D). Within T. aestivum, a number of subspecies
have been described (T. spelta, T. vavilovi, T. macha, T. vulgare, T. compactum,
and T. sphaerococcum), T. vulgare and T. compactum representing the most common
types of bread wheat now cultivated.
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Genetic Dublication
T he presence of polyploid series suggests that a genetic factor could be
present up to 3 times, for example, in 3A, 3B, and 3D of common wheat. At the
beginning of this century, this was anticipated by Nilsson-Ehle (1911) who
reported 3 factors for red kernel color in common wheat. Later a genetic factor
for red kernel color was found to be located on each of the chromosomes 3A, 3B,
and 3D. The presence of any one of these 3 genes produces a red kernel color,
and only the complete absence of all 3 produces a white kernel. The multiplicity
of genetic factors governing a trait in the hexaploid wheats has been a problem
in the genetic studies of such wheats. In wheat breeding it has both negative
and positive effects. For example, most flour quality traits are controlled by
multiple factors. Therefore, when a poor quality but otherwise desirable wheat
is crossed with a good quality wheat, large numbers of offspring must be grown
in order to recover some good-quality lines having in addition the other desired
traits. However, on the positive side it should be pointed out that this allows
the accumulation of genetic factors enhancing a desired trait. Further, it
allows for the maintenance of something similar to the heterozygosity of a
cross-pollinated plant in the self-pollinated wheat plant.
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Breeding Methods
W heat breeding is both and art and a science. Before 1900, most plant
breeding was largely an art. Most of the important cultivated plants of today
stem from the selection efforts begun thousands of years ago by those who turned
to plant cultivation from a nomadic existance. Since most early plant
cultivation was done by women, they must be recognized for their selection
influences on many of the crops grown today. Wheat is a self-pollinated and thus
a naturally inbred plant. Cross-pollination is usually less than 1% in most
varieties, but may be considerably higher in some varieties or in certain
locations or in some years. Wheather conditions, such as light frost at
blooming, may cause some anther damage (male sterility) and lead to extensive
cross-pollination.
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Mass & Pure Line Selection from Land
Varieties
Selection was the major procedure before hybridization, and two types of
selection methods were used in "land" varieties. (Land varities are varieties
grown over a long time in certain areas, and usually consists of a mixture
within an overall type). From such "land" varieties early breeders either
selected single plants and evaluated them seperately (pure line selection) or
selected a number of similar plants and composited them to begin a new variety
(mass selection). These techniques are no longer used much. Their success
depended on the range of the variability within the land variety and on the
continued expression of superiority of these selections (heritability).
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Hybridization and Selection
Success of this breeding program is highly dependent on the parental wheat
varieties used in making the crosses and the choise of parental varieties
depends largely on the objectives of making the crosses. All available
information regarding the prospective parental varieties should be used in
deciding what combinations will provide the best chances for creating
recombinations that will exceed the parents in value. Objectives may vary
depending on immidiate crop needs or on long-range goals. For example, if a
currently popular variety is threatened by a new form of a destructive disease,
the choise of at least one of the parents will be quite different from what it
would be if the long-range goal were to increase yielding ability. After the
suitable parental choise,one of the wheat parents is designated as the female
parent. Since wheat is a perfect flowered plant (male and female organs in the
same flower), the male organs (anthers) have to be removed (emasculation) from
the flowers of the parent designated as female, living the pistil (female organ)
intact. This is usually done a day or two after the spike is bagged to keep out
unwanted pollen. About two days later anthers carrying pollen from the parent
designated as male are used to pollinate the emasculated female flowers, and the
cross has been completed. Fertilization normally follows. When only two
varieties are used it is a single cross. When breeders wish to add traits from a
third variety double cross is necessary. The F1 (First filial generation), or
hybrid between two parents, is heterozygous for all traits by which the parents
differ. Wheat , being a self pollinated plant is naturally inbreeding.
Inbreeding leads to eventual homozygosity (identical alleles at the same
chromosome location) during the segregating generations following the cross. For
the whole population, heterozygosity is reduced by ½ per segregating generation;
thus by the F3 generation only 25% of the plants are homozygous for any one gene
pair, but by the F8 generation most plants are homozygous for any one gene pair.
However, some wheat breeders are making selections much earlier in the
segregating generations in order to retain genetic variability in the progeny to
provide buffering against environmental hazards.
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Pedigree Method
A chronological record of plant selection following a cross is maintained.
Plant selection and reselection can be practised in successive generations until
the breeder is satisfied with the lines available for yield testing and a nearly
homozygous progeny is obtained. This is then multiplied to provide sufficient
seed for estimation of yield and other quantitative characters. This method
provides the most information and the breeder gets to know his material
intimately but it is laborious, and fewer crosses can be handled since selection
is on a single-plant basis. In such an example from Plant Breeding Institute,
UK, about 1200 crosses were made each year in a winter-wheat breeding program.
Three-quarters of these were two-parent hybridizations, and most of the
remainder were crosses of an F1 with a third parent or a single back-cross,
particularly when one of the parents of the original F1 was a variety poorly
adapted to the UK environment, but with some desirable feature. Little selection
was exercised in the F1 generation of two-parent crosses. About 2000 plants were
grown from each cross in the F2, giving a total of about 2 million plants in
this generation. From these, about 60 000 plants were selected by eye on the
basis of superior disease resistance or agronomic type. For crosses intended
mainly to improve disease resistance, yield or agronomic characters, one ear was
harvested from each selected F2 plant. Whole plants harvested from material
aimed at improvement in bread-making quality. In the following (F3) year a small
amount of seed from each harvested plant was sown as a 1-m row for
more-intensive selection. Selection in the F3 was considered to be the most
critical in a breeding program; about 6000 rows were harvested, each of which
was then represented in F4 by six or 12 ear rows. This process of plant-to-row
selection was repeated until uniformity was achieved. It is usually possible to
base the pure stock of a new variety on a single F6 or F7 plant. Yield trials
start at F5 and are followed by more-extensive trials in F6 and F7. The seed for
each line in these trials was obtained as a bulk of the pedigree family. Thus,
it is not necessary to grow separate plots to produce seed for trial purposes
and, as the trial stocks are renewed each year they become progressively closer
to the retained pedigree lines in genetic constitution.
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Bulk Method
In the bulk method of propagating segregating populations following a cross, the shift towards homozygosity is similar to that occuring in the pedigree system. But since there is no record-keeping by plants and lines with this system, many more crosses and larger populations can be handled. In the simplest of these systems F2 plants are harvested as a bulk population without selection. The population is then grown on as a bulk for a further three or four generations, after which single plants are selected, multiplied and tested in yield trials. On the other hand breeder may wait until the F6-F8 generation to make his initial selections and then proceed to yield testing.
The bulk method has been used successfully by some breeder. Such a program was applied in Australia in which the bulk method was supplemented by selection for earliness of maturity by adjusting the time of harvest. Many breeders combine the pedigree and bulk methods in various ways to take advantage of the best features of each. In the F2 progeny method, for example, single plants are selected by eye judgement in F2 and their progenies grown as small plots in F3. These plots are again assessed by eye, and the more promising harvested for yield trials which are grown without further selection in F4, F5, and F6, after which single plant selections are made within the more promising families. Selections from several crosses can be made in these trials and the better parental combinations identified.
Over the years, there has been considerable interest in the possibility of yield testing early-generation bulks (F1, F2, F3) as a method of predicting the value of crosses and choosing among them. The results and conclusions from various studies have been quite variable, probably due to the reasons:
1. Heterosis in early generations, particularly the F1 and F2, varies from cross to cross and many mask differences in additive genetic variance, which is more valuable to the wheat breeder.
2. Genotype x environment interactions may be large, so that crosses that are favored in one year may be at a disadvantage in another; e.g., early crosses may be favored one year and late crosses may be favored the next, depending on temperature and rainfall patters.
High mean yield may not be the sole measure of the value of a cross-variability may be important as well. A cross with a wide range in yield (i.e., greater genetic variability) may be of greater value than one with a somewhat higher mean yield but less variability.
Even if early-generation yield testing appears to provide useful information,
it will not be used if the value of the information does not justify the extra
work involved in obtaining it. A study was carried out in 1993 to assess the
value of yield testing F2 bulks of durum wheat crosses as a method of
discriminating among crosses. The studied 17 crosses differed significantly in
yield in both the bulk F2 test and the bulk F3 test. However, there was no
significant correlation between the results for the two years or between the
results of the bulk yield tests and the mean yields of lines selected from the
crosses. Thus, there was no evidence that selecting crosses based on bulk yield
tests would improve the probability of obtaining high-yielding lines.
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Back-crossing
W hen it is desired to introduce a simple and easily identified character,
such as a major gene for disease resistance, the back-crossing technique may be
used. In the case of disease resistance this involves crossing a donor parent
with the desired resistance to a susceptible but otherwise desirable variety.
The F1 is then crossed back with the susceptible parent; if the gene is
dominant, then the resulting progeny will segregate equal numbers of
heterozygous resistant and homozygous susceptible plants. The latter are
discarded and the former crossed back again with the susceptible parent. The
process can be repeated until the resistant plants are nearly isogenic with the
susceptible parent in all characters except disease resistance. For this purpose
up to six back-crosses may be needed. However, in a practicle breeeding program
one or two back-crosses are usually considered sufficient, after which the
better plants are selected.
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Single-seed Descent
T he production of nearly homozygous lines in a breeding program may be accelerated by the use of the single-seed descent (SSD) sytem, in which early generation seed is grown in growth rooms at high plant density to produce plants with only a few grains in a much-reduced generation time. Only one or two seeds are harvested from each plant, and the process is repeated as many times as the breeder may consider advantageous, after which the grain is multiplied for field evaluation. It is used mainly for the early generations of the most promising crosses, which are grown either as unselected populations from F1 to F4 or as lines maintained separately after preliminary selection in F2 or F3. In some cases useful selection may be possible during the generations of single seed descent, as when selecting for dwarfing genes, spring types in winter x spring crosses and for some disease resistances.
In a more recent study (1989), conducted in India where a quantum jump in
wheat productivity was achieved, Single Seed Descent (SSD) was compared to
Single Plant Selection (SPS), Bulk Population (BP), and Mechanical Mass
Selection (MMS). The four methods of generation advance were compared in F3 and
F4 generations. In the F3 generation, the SPS and SSD methods of generation
advance proved superior to the BP and MMS methods for grain yield per plant and
for at least one of the yield component traits. The F3 SSD population did not
differ significantly from the F3 SPS for any of the traits. However, the F3 SSD
population retained more range and cv for different traits than with other
methods of generation advance. F4 progenies derived from F3 SSD population were
significantly superior for grain yield than lines derived from the other three
F3 populations.
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Doubled Haploids
Selection of desirable genotypes in the early generations of a breeding programme is necessarily complicated by the heterozygosity of the material handled. This problem could be avoided if a system were available for developing homozygous lines by haploidization followed by chromosome doubling of early-generation material. To be useful in the earliest generations of a breeding program, it would be necessary to develop a system for producing such doubled haploids in sufficient numbers to include, with reasonable statistical certainity, selections exhibiting useful genetic advances over the parents involved. According to Snape (1981), additive genetic variation is more readily expressed in doubled haploids than in conventional breeding lines, because variation due to dominance has been eliminated. This advantage, which applied also to SSD, is constant in successive years of assessment because the material is homozygous. This homozygosity also eliminates differences between individuals in breeding plots, thus facilitating choice of the better lines in a breeding nursery. Two methods are available for the production of doubled-haploid lines in wheat; namely, by anther culture, and by crossing with Hordeum bulbosum. In the former, anthers from post-meiotic spikes are incubated on a potato-extract medium. Embryoids appear in 20-40 days and are transferred first to an appropriate growth medium and subsequently to potting compost. When they reach the three-tiller stage they are treated with colchicine solution for chromosome doubling. In the Hordeum bulbosum technique, ears of wheat are fertilized with pollen from H. bulbosum, the chromosomes of which are subsequently eliminated during development, leaving a haploid wheat embryo. This is excised and transferred to an agar medium 2-3 weeks after pollination. The plantlets are later transferred to potting soil and treated with colchicine as in anther culture technique.
Anther culture is possible with many wheat genotypes, though there are differences in the numbers of embryoids produced, and many of these show cytological abnormalities. However, the H. bulbosum technique can only be applied to the wheat genotypes which are cross compatible with this barley species. Cross compaticility is determined by two genes, Kr1 and Kr2, on chromosomes 5B and 5A, respectively, and appropriate alleles of these genes must be introduced before non-compatible genotypes can be hybridized with H. bulbosum. Little difference was found between the two methods in the number of hours of work required by one person per doubled haploid produced or in the total time, about 13 months, to produce them12,15. In more recent times maize is used intead of barley since it is more productive, especially in durum wheat applications. O`Donoughue and Bennett (1994) obtained 14.5-15.5% haploid embryo from the hybridization of durum wheats with maize. More recently, in 1996, Dr. C. Savaskan has obtained similar results from Turkish durum wheat varieties (up to 20% embryo formation). From these haploid embryos plants were generated (up to 3.1%) and graines from these double haploid plants were produced16 (1997). Introduction of alien variation C hromosome substitution provides breeder with a useful tool for studying the effects of individual chromosomes on the expression of any character in which they are interested, and for studying the interactions of the genotype with the environment in the expression of such characters. It is particularly useful in assessing the effects of quantitatively determined characters, especially if these are determined by a number of gene differences carried on a single chromosome or chromosome arm. It has, for example been used to evaluate and compare the effects of the dwarfing genes derived from Norin 10 and other varieties, and to determine whether these genes have deleterious or advantageous effects on yield,in addition to their obvious effects on plant height (Gale et al., 1982). The technique involved, also enable breeders to analyse and exploit the effects of chromosomes or segments introduced into wheat from related species or genera. The techniques have been successfully used to transfer resistance to eyespot from Aegilops ventricosa, and to yellow rust from Aegilops comora (Riley et al., 1968). Synthetic Species S ynthetic wheat-wheat-related materials have been studied for many decades. New cultivated species were created from crosses of tetraploid or hexaploid wheats with rye (Secale cereale L., genome R). great interest has arisen particularly in the hexaploid triticale (genome ABR), not only from a scientific but also from an economic point of view. Researchers are enthusiastic about triticales for two reasons:
1. They show promise of exceeding all small grains in yield, and
2. They may have nutritive value exceeding that of wheat.
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Mutation Breeding
Most early work on mutation breeding was concerned with mutants causing major
changes in the phenotype of the plant in terms of gross morphology or reaction
to disease. Although such changes can be induced in wheat, they occur much less
frequently than in other species, owing to the polyploid nature of the crop.
This is attributed to the buffering effects of ploidy, which enable the plant to
tolerate chromosomal abnormalities, such as deficiencies, deletions and
dublications, which would be lethal in a diploid. However, major gene mutations
of considerable economic importance have beeen induced in wheat (especially in
tetraploid durums).
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Multilines
W hen a certain genotype (an inbred wheat varity) occupies a very large
acreage, there is an opportunity for a previously silent disease or insect pest
to suddenly increase to epidemic proportions if the plant genotype provides a
favorable host (genetic vulnerability). A mutant strain of a pest may also
emerge. In order to prevent such a disaster either growers are provided with a
number of varieties genetically different or the genetic diversity within the
cultivated variety is prepared. A simultaneous back-crossing program for
transferring many different resistance factors separately into the desirable
adapted variety and a bulk of them is suggested.
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Methods for Producing Hybrids
H eterosis or hybrid vigor has been the most evident in cross-pollinating species. The tremendous productivity of corn and sorghum hybrids turned the attention to hybrid wheat. Since wheat is a self pollinated, perfect flowered plant, the first requirement for producing hybrid wheat seed on a large scale is a mechanism for producing male sterility.
To date, Cytoplasmic male sterility, Nuclear male sterility, and Chemically
induced male sterility, has been defined in wheat. However, all the present
systems have serious handicaps in restoration or in obtaining, under different
environmental conditions, a high degree of male sterility in all parts of the
female parent. Complete sterility is of primer importance, because incomplete
sterility leads to self-fertilization of the female parent which results in an
unacceptable contamination of the F1-hybrid seed. In a very recent study
(1997,TAG, 95: 125-131), the development of a nuclear male sterile (NMS) system
in wheat by means of genetic engineering techniques is presented. The study is
of the type of pure molecular breeding, combining the classical breeding
procedures with the advanced molecular biology techniques. Genes containing the
barnase gene, under the control of tapetum-specific promoters derived from corn
and rice, were introduced into wheat and their expression in succeding
generations were studied. This procedure was successfully applied to tobacco,
rapeseed, and corn to produce NMS plants previously. Expression of the barnase
gene at specific stages of anther development destroys the tapetum, and thus
prevents normal pollen development. Depending on the time the barnase gene is
expressed, e.i., early or late microspore stage, either no pollen or else only
sterile pollen is produced. To achieve NMS in wheat, a transformation system
which regenerates with a high efficiency transformants with "elite" alleles
would be desirable. This means that each transformant would have a simple
integration pattern of the delivered DNA, a low-copy number and no
rearrangements. Ideally, insertion of DNA would be at sites in the genome which
do not interfere with the tapetum-specific expression of the barnase gene in
non-target tissues, such as leaf or root, will interfere with the fitness of the
transgenic plant. The progeny analysis of the transgenic plants showed that,
under greenhouse conditions, the expression of the tapetum specific promoter:
barnase constructs did not affect the vegetative phenotype. The transgenic
plants were vigorous, and had a normal height, leaf size, tillering. Also, no
delay in flowering compared with control plants was observed. All the barnase
constructs were seen on the male-sterile flowers. Microscopic studies confirmed
premature degeneration of the tapetum. Since there were no abnormalities in the
other parts of the flower the rice and corn promoters were confirmed
tapetum-specific expression. Because no self-fertilization occured in
male-starile flowers, the unfertilized florets opened to allow
cross-fertilization. No instability of the male sterility was observed. A few
lines (female) were back-crossed with a male spring wheat variety and the male
sterility was still stable.
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Molecular Methods to Wheat Quality Improvement
Wheat, with an anual production of 540Mt, is a staple product for much of the
world’s population. Moshe Feldman and co-workers, Weizmann Institude, Israel,
estimate the presence of up to 25 000 different cultivars. In Europe, T.
aestivum, bread wheat, is grown throughout the continent and used for livestock
feed as well as food processing. T. durum, pasta or macaroni wheat, is
restricted to the drier parts of Southern Europe and is used almost exclusively
for pasta production. With the dramatic increase in its production in Europe,
new end-uses and markets developed, e.g., as a source of specialized starch to
replace imported maize starch. Genetic engineering is crucial to future wheat
usage, as it will allow the development of a range of specialised types with
optimised properties for different end uses, whether traditional, as in the food
industry, or novel applications requiring wholly new properties
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The current technology
T he first fertile transgenic wheat plants were reported in 1992 by Indra
Vasil et al., Florida Un., after transformation of embryogenic cultures by
particle bombardment. There is progress, however, in the Agrobacterium mediated
transformation procedures. Some difficulties in wheat transformation are: · In
most varieties regeneration from tissue cultures is inefficient · High level of
resistance to most of the selection agents used for plant transformation · Lack
of promoters giving sufficiently high level of expression of resistance genes
for efficient selection. In the past few years however, these difficulties have
been overcome to a certain extent and reproducible transformation of wheats are
performed in numerous locations. While embryo cultures are mostly utilized,
transformation of immature floral tissues are also possible. Selectable markers
are mostly bar or pat genes that confer resistance to the herbicide
phosphinothricin (Bialapos) and transformed cells are selected and the
nontransformed cells killed by culture on a medium containing the herbicide.
Systems such as, nptII (neomycin phosphotransferase) gene with geneticin or
paramomycin selection, the hpt gene with hygromycin selection, GUS gene
selection etc. are also used. In numerous studies model varieties with high
transformation efficiencies (0.5-2.5%), and good response in tissue culture are
utilized. For the genetic manipulation of some characteristics in wheat, e.g.
inserting a herbicide resistance trait, it may be feasible to transform a model
variety and then cross the transgene into the commercially relevant germplasm,
followed by back-crossing to remove deleterious characteristics. However, for
manipulating many quality characteristics, the effect of the transgene will be
strongly influenced by the background of the recipient line. It is therefore
preferable to transform the target variety directly so that the effect of the
transgene on a quality trait can be assessed.
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Targets for trasformation: gluten quality
The ability to process wheat doughs into bread and other food products is
largely determined by the properties of the wheat’s gluten proteins. A precise
balance of viscoelasticity is needed for different food uses, with highly
elastic glutens being required for breadmaking and more extensible glutens for
cakes and biscuites. Similarly, a high level of elasticity is needed for pasta
production from durum wheat. Gluten is made up of over 50 individual proteins
and variation in quality is associated with differences in the amounts,
proportions and properties of these. However, one specific group of proteins,
called the high molecular weight (HMW) subunits of glutenin, are of particular
importance. Variation in their amount and composition is associated with
differences in gluten elasticity and hence breadmaking quality. Since each
subunit is encoded by a single gene it implies that increasing the number of
expressed HMW subunit genes by transformation could have a direct effect on
quality. Several laboratories are working on this idea. While this is a
promising approach, the most exciting opportunity is probably in the non-food
area, for example, producing protein films and packaging for food products or
proteins for biomedical applications
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Starch content and composition
C ereal starch is a major commodity in the food industry. At present the
dominating source of starch if of maize origin, with mutant lines providing
valuable variations in properties. Although wheat starch is produced and
utilized in EU, it can not replace maize starch in many applications due to the
differences in their properties. Probably due to its hexaploid nature bread
wheat has no mutant lines. Starch is a mixture of two polymers,amylose and
amylopectin, both made up of glucose units in a ratio 1:3. Two types of starch
synthase are responsible for amylose synthesis (granule bound) and amylopectin
synthesis (soluble), and all three enzymes exists in several related forms. The
granule bound starch synthase is often called the "waxy" protein as mutations in
this enzyme lead to waxy starch consisting almost solely of amylopectin. Waxy
starch is a valuable commodity and attempts are being made to produce waxy
wheats by classical plant breeding, combining mutant genes present in related
wild grass species. An alternative approach is to reduce the activity of the
granule bound starch synthase by genetic engineering methods, e.g., by antisense
expression of a cloned DNA from wheat or another cereal.
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Novel Products in Wheat
W heat is a suitable system for novel compounds production. It has high yield
and good storage properties, and is backed up with a well developed and
efficient industry for processing and fractionation. The number of potential
products is immense, ranging from high value low volume compounds such as
biologically active proteins and peptides for biomedical use to high volume low
cost raw materials for packaging and building.
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References
1. Berder W. H., 1997, "How much food will we need in the 21st century". Environment 39(2): 7-11,27-28.
2. Anderson K.,Dimaranan B., Hertel T., and Martin W.,1997, "Asia-Pasific food markets and trade in 2005: a global, economy-wide pespective". The Australian J. Agric. and Res. Econ. 41(1): 19-44.
3. Johnson D. G.,1997, "On the resurgent population and food debate". The Australian J. Agric. and Res. Econ. 41(1): 1-17.
4. Alexandratos N.,1995, "The FAO Study World Agriculture: Towards 2010".
5. Kleiner K., 1996, "Superwheat to feed the world". New Scientist. 26 Oct. 1996: 8.
6. Minnesota Ass. Wheat Growes Webpage.
7. "Space wheat". New Scientist. 21/28 Dec. 1996: 12.
8. "Wheat’s DNA points to first farms". Science News. Nov. 15, 1997: 1-2.
9. Heun M., Schafer-Pregl R., Klawan D., Castagna R., Accerbi M., Borghi B., and Salamini F., 1997."Site of Einkorn Wheat Domestication Identified by DNA Fingerprinting". Science. 278:1312-1314.
10. Inglett G. E., 1974. "Wheat: Production and Utilization". The Publishing Company, Inc., Westport, Connecticut, ISBN-087055-154-X., USA.
11. Fabriani G., and Lintas C.,1988. "Durum Wheat: Chemistry and Technology". Amer. Assoc. Cereal Chemists, Inc., St. Paul, Minnesota, USA. ISBN-0-913250-50-3.
12. Lupton F.G.H., 1987,"Wheat Breeding: Its scientific basis". Chapman and Hall Ltd., University Press, Cambridge, UK. ISBN-0-412-24470-5.
13. Knott D.R., 1994,"The use of bulk F2 and F3 yield tests to predict the performance of durum wheat crosses". Can.J. Plant Science., Accepted 27 Oct. 1993., pp. 241-245.
14. Srivastava R.B., Paroda R.S., Sharma S.C., and Yunus Md.,1989. "Genetic variability and advance under four selection procedures in wheat pedigree breeding programme". Theor.Appl.Genet., 77: 516-520.
15. Kasha K.J., and Kao K.N., 1970, "High Frequency Haploid Production in Barley (Hordeum vulgare L.)". Nature., 225: 874-876.
16. Savaskan C., Ellerbrook C., Fish L.J., and Snape J.W., 1997, '' Doubled haploid production in Turkish durum wheats using crosses with maize". Plant Breeding., 116: 1-3.
17. De Block M., Debrouwer D., Moens T., 1997, "The development of a nuclear male sterility system in wheat. Expression of the barnase gene under the control of tapetum specific promoters". Theor. Appl. Genet., 95: 125-131.
18. Shewry P. R., and Lazzeri P., 1997, "Molecular approaches to wheat quality improvement". Chemistry & Industry., 21 July: 559-562.
19. "Alman tarihci ve arkeologlar Turkiye’deki antik Tiyana kentini gun isigina cikariyorlar"., Deutschland, Almanya: Magazine on Politics, Culture, Business and Science., 1997, No.3, Bilim, June: 36-37.
20.Diamond J., 1997, "Location, location, location: the first farmers" Science. 278:1243-1244.
This page was prepared by Erdogan E. HAKKI, Middle East Technical University, Biotechnology Department, Ankara, September 18th, 1999.
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HAKKI