A New Synthesized 6x-Wheats, Derived from Dwarfing Polish Wheat (Triticum polonicum L.) and Aegilops tauschii Cosson
By colchicine treatment of the hybrid
plants between Triticum polonicum L. from Xinjiang in China and
Aegilops tauschii Cosson. from Middle East, a new synthetic hexaploid
wheat (SHW-DPW) was artificially obtained for the fist time. The average
seed set of the intergeneric hybrids was 9.71% (8.27-15.31%) with colchicines
treatment. The morphology of the SHW-DPW plants was comparable to that
of the primary F1 plants and the SHW-DPW plants appeared more
robust. All the SHW-DPW plants were uniform in morphology and had some
obvious traits inherited from T. polonicum and Ae. tauschii.
Especially, the glumes were very soft and the rachis internodia was short,
which was different from other synthetic hexaploid wheat. Their spikes
were morphologically similar to those of Triticum petropavlovskyi
Udacz. et Miguch. The SHW-DPW plants were fertile, with 58.95% selfed
seed set for the euhexaploids and 45.63% for the aneuploids, respectively.
The meiosis of the SHW-DPW plants was quite regular, which showed a pairing
configuration of 0.43 univalents, 20.77 bivalents and 0.01 trivalents.
The rate of chromosome association of the aneuploids was comparatively
lower than that in the euploid individuals. The potential utilization
for wheat improvement and study for the origin of T. petropavlovskyi
are discussed in the present study.
Allopolyploidy has played a major role in the evolution
of crop plants. One of the best-known allopolyploid complexes is Triticum-Aegilops
in Triticeae (Poaceae). Among the roughly 30 species of Triticum
and Aegilops, 75% of them are natural allotetraploids (2n = 4x
= 28) or allohexaploids (2n = 4x = 42) (Sakamoto, 1973; Yen et al.,
2005). Bread wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD),
one of the most remarkable allopolyploids, also called common wheat, has
no direct hexaploid wild progenitor (Morris and Sears, 1967). It possesses
three sets of homologous chromosomes, whose A genome stems from Triticum
urartu Thum., the D genome stems from the wild diploid Aegilops
tauschii Cosson. and the B genome donor is probably Aegilops speltoides
L. or its related species in the section Sitopsis of Aegilops
(Kihara, 1944; Heun et al., 1997; Feldman, 2001). Bread wheat has
undergone two hybridizations and polyploidizations during its origin and
evolution. Tetraploid Triticum wheat (Triticum turgidum
L., 2n = 4x = 28, AABB) was formed in the first intercrossing between
T. urartu and Ae. speltoides or its related species, followed
by chromosome doubling. Then bread wheat was formed by the second polyploidization
after the intercrossing between the cultivated T. turgidum (maternal)
and Ae. tauschii (paternal) in ca. 10000 years ago (Feldman, 2001;
Huang et al., 2002).
Since only a few accessions of tetraploid wheat or Ae.
tauschii genotypes were involved in the evolutionary origin of bread
wheat, the genetic diversity of bread wheat were largely decreased in
comparison with that of its donor species (Miller, 1987; Feldman, 2001).
Numerous genetic variations in the ancestral tetraploid wheat or Ae.
tauschii were not represented at the hexaploid level due to the evolution
bottleneck (Dhaliwal et al., 1993; Ayal et al., 2005; Warburton
et al., 2006). Close evolutionary relationship and extensive genetic
diversity for desirable traits of tetraploid wheat and Ae. tauschii
were especially interesting for bread wheat improvement. The genetic
diversity may be introgressed into bread wheat by the bridge of synthetic
hexaploid or amphidiploids derived from artificial synthesis of hexaploid
wheat (tetraploid wheat x Ae. tauschii), which was a manner analogous
to the evolution of hexaploid wheat. Ever since the reports by McFadden
and Sears (1944) and Kihara and Lilienfeld (1949) on the artificial synthesis
of hexaploid wheat between existing T. turgidum with Ae. tauschii,
many synthetic hexaploids have been obtained from the hybridizations between
Ae. tauschii and the tetraploid wheat, such as T. durum,
T. dicoccon, T. carthlicum, T. paleocolchicum, T.
dicoccoides (Tanaka, 1961; Lange and Jochemsen, 1992; Mujeeb-Kazi
et al., 1996; Lage et al., 2003; Warburton et al.,
2006). Furthermore, there is no report of the successful synthesis of
the amphidiploid between T. polonicum and Ae. tauschii.
Dwarfing polish wheat (Triticum polonicum L.,
2n = 4x = 28, AABB, DPW) is a tertraploid species that was collected from
Tulufan, Xingjiang, China. It is the only one T. polonicum with
the trait of dwarfing in China, which was characterized as dwarfing, more
tillers, more length of spike, more spikelets per spike and length of
glume (Yang et al., 2001; Liu et al., 2002). In 2005, we
successfully carried out the intergeneric hybridization between T.
polonicum (DPW) and Ae. tauschii (AS60) and obtained the
F1 hybrids without the use of embryo rescue. The production,
morphology, meiotic behavior and fertility of a new synthetic hexaploid
wheat (2n = 6x = 42, AABBDD, SHW-DPW) derived from T. polonicum (DPW)
x Ae. tauschii (AS60) by chromosome doubling are described in the
MATERIALS AND METHODS
Dwarfing polish wheat (Triticum polonicum L., 2n = 4x = 28,
AABB; DPW) was collected in Tulufan, Xingjiang, China by Prof. C. Yen
and J.L. Yang. Aegilops tauschii Cosson. (AS60) was originated
from Middle East. Triticum petropavlovskyi Udacz. et Miguch (2n
= 6x = 42, AABBDD) (AS356 and AS358) was originated from Xingjiang, China.
All the materials were deposited in Triticeae Research Institute, Sichuan
Agricultural University, Sichuan, China. This research was conducted at
the Triticeae Research Institute of Sichuan Agricultural University, China
The Production of Synthetic Hexaploid Wheat
To produce artificially synthetic wheat, T. polonicum (DPW)
as female was crossed to Ae. tauschii (AS60) and F1
seeds were obtained in 2005. The emasculation and pollination techniques
were carried out as described by Liu et al. (2002). No embryo rescue
techniques or gibberellic acid solution were applied to the embryos after
pollination. The F1 seeds were germinated in petri dishes and
the seedlings were then planted in the field. To double the chromosome,
colchicine treatment was carried out as per the following procedure. When
the F1 seedlings grew to the six-seven leaf stage, they were
removed from soil and the roots were washed and cut to a length of 25
mm. These were then placed in 30 mm diameter glass tubes and a solution
of 0.05% colchicine was added such that the crown was submerged to a depth
of 50 mm. The plants (in tubes) were then placed at 20 °C for 22 h.
Then they were removed from the tubes, rinsed with water and returned
to the tubes containing only water. The water in the tubes was changed
every 20 min for 8 h to ensure complete removal of residual colchicines.
After the leaves were trimmed to approximately half their original length
to reduce transpiration, the plants were transplanted into field. To maintain
a humid environment, the plants were covered by clear plastic covers.
After seven days, the plastic covers were removed. Before flowering, spikes
on well developed tillers of the treated plants were bagged to ensure
self-pollination. At maturity, seeds (F2) were harvested from
all the treated plants. The F2 plants were bagged and selfed
and the F3 seeds were obtained. Crossability percentages and
seedset were estimated.
Morphology and Meiosis Analysis
Eight morphological characters including plant height, tillering,
length of spike, number of spikelets per spike, length of glume, length
of flag leaf, length of internodes of spike and length of awn of the amphidiploid
plants (SHW-DPW) obtained from the hybrids T. polonicum
(DPW) x Ae. tauschii (AS60) and the parental species were measured
For mitotic studies, the root tips (1-3 cm long) were
cut and pretreated in ice water at 0-4 °C for 20-24 h and fixed in
Carnoy`s fixative I (a 3:1 mixture of 95% ethanol and glacial acetic acid)
overnight. The root tips were hydrolyzed in 1 M HCl at 60 °C for 10
min, stained following the Feulgen procedure. Somatic chromosomes were
counted. Analyses of meiosis behavior in the synthetic hexaploid wheat
(SHW-DPW) were performed by the conventional aceto-carmine squash method.
Stages of meiosis were determined in aceto-carmine squashes of one of
three anthers per flower. If appropriate stages were present, the remaining
two anthers were fixed in a mixture of absolute ethanol, chloroform and
acetic acid (6:3:1, v/v) and kept in a refrigerator for 24 h, then stored
under refrigeration (4 °C) in 70% alcohol until use. Cytological observations
and documentation were made with an Olympus BX-51 microscope coupled with
a Photometrics SenSys CCD camera.
Pollen fertility was measured for pollen grains sampled
from mature anthers at flowering. The grains were stained in a 2% aceto-carmine
solution with glycerin.
Without using embryo rescue technique, F1
hybrid seeds between T. polonicum (DPW) and Ae. tauschii (AS60)
were obtained and the crossability was 1.67%. The F1 seeds
of the T. polonicum (DPW) x Ae. tauschii were germinated
and 24 plants were obtained. The morphological characteristics of these
F1 plants were similar to those of bread wheat. The F1
hybrid plants have 2n = 21 chromosomes. Three F1 hybrid plants
were used for colchicines treatment, one was killed and the other two
plants showed various degrees of regrowth. After vernalization, the two
plants were grown to flowering and formed four and eight spikes per plant,
respectively. A total number of 11 and 23 seeds were obtained and the
rates of selfed seed set were 15.31 and 8.27%, respectively (Table
1). All the spikes of the other 21 F1 plants were bagged
to avoid unexpected pollination and no seeds were obtained under self-pollination.
|| The seedset and pollen fertility of F1 and
F2 hybrid of T. polonicum (DPW) with Ae. tauschii
||Comparison of morphological characters among T. polonicum
(DPW), Ae. tauschii (AS60) and synthetic hexaploid wheat
||Spikes morphology of T. polonicum (DPW), Ae.
tauschii (AS60) and the synthetic hexaploid wheat (SHW-DPW) (A)
Spike: a. T. polonicum (DPW); b. SHW-DPW; c. Ae. tauschii
(AS60) (B) Spikelets: a. T. polonicum (DPW); b. SHW-DPW; c.
Ae. tauschii (AS60)
Ten well-developed F2 seeds were selected
and germinated. All of the seeds were germinated and the plants grew vigorously.
The morphology of the SHW-DPW plants was similar to that of the F1
plants. But the SHW-DPW plants appeared more robust, such as plant height,
number of spikelets per spike, length of spikes and length of flag leaf.
As expected, all the SHW-DPW plants were uniform in morphology and had
some obvious traits inherited from T. polonicum (DPW), such as
number of tillers, number of spikelets per spike and length of awn and
internodes of spike, while non-waxiness, tough rachis and hairy auricles
resembled Ae. tauschii (Table 2, Fig.
1). Compared with T. polonicum (DPW), the SHW-DPW plants had
short glumes and long length of spikes. It is remarkable that the SHW-DPW
plants have soft glume and short rachis internodes. Furthermore, the spike
morphology of SHW-DPW was very similar to T. petropavlovskyi (Fig.
2). While the powdery mildew disease and budworm occurred heavily
on T. polonicum (DPW), the SHW-DPW plants were immune. The results
of the pollen fertility of F1 hybrids, synthetic hexaploid
wheat (SHW-DPW) and their parents are listed in Table 1.
Spikes on well developed tillers of SHW-DPW plants were bagged to avoid
unexpected pollination and the average selfed seedset was 58.95% for the
euhexaploids and 45.63% for the aneuploids (Table 1).
||The configuration of meiotic MI in synthetic hexaploid
wheat (SHW-DPW) obtained from the hybrids of T. polonicum (DPW)
x Ae. tauschii (AS60)
||Spikes morphology of the synthetic hexaploid wheat (SHW-DPW)
and T. petropavlovskyi (A) Spike: a. T. petropavlovskyi
(AS356); b. SHW-DPW; c. T. petropavlovskyi (AS358) (B) Spikelets:
a. T. petropavlovskyi (AS356); b. SHW-DPW
Of the ten synthetic hexaploid wheat (SHW-DPW) plants,
seven were euhexaploids with 42 chromosomes, the other three plants were
aneuploids with 40 (one plants) and 41 (two plants) chromosomes, respectively
(Fig. 3a, b). The results of the chromosome pairing
in meiotic metaphase I (MI) of the SHW-DPW plants are summarized in Table
3. The chromosome pairing of the euploid plants was quite regular
with average 0.43 univalents, 20.77 bivalents and 0.01 trivalents per
cell (Table 3, Fig. 3c, d). Little
variation was observed among the plants. Plant SHW-DPW-4 showed the highest
rate of chromosome association, with 21 bivalents per cell, while plant
SHW-DPW-9 was characterized by the lowest rate of chromosome association
(20.44). At anaphase I, lagging chromosome could be observed in some cells
analyzed and some of them undergo precocious
|| Chromosome observation for the synthetic hexaploid
(a) The somatic chromosomes of SHW-DPW with 2n = 42;
(b) The somatic chromosomes of SHW-DPW with 2n = 41;
(c-f) The meiotic chromosomes of SHW-DPW: C. 2n = 42 = 15 II (ring)
+ 6 II (rod); D. 2n = 42 = 2 I + 15 II (ring) + 5 II (rod); E. 2n
= 40 = 20 II (ring); F. 2n = 41 = 4 I + 17 II (ring) + 1 III
disjunction. In the aneuploid individuals, the rate of
chromosome association was comparatively lower than that in euploid individuals,
which was ascribed to the increment of univalents (Table
3, Fig. 3e, f).
Production of Synthetic Hexaploid Wheat (SHW-DPW)
Between T. polonicum and Ae. tauschii
Ae. tauschii (AS60) has a unique genotype which facilitate
hybrid seed development and viability and which is the important donor
material in cross with tetraploid wheat and bread wheat (Liu et al.,
2002). After a few years` efforts, we have successfully obtained the F1
hybrids between T. polonicum (DPW) and Ae. tauschii (AS60)
without using embryo rescue for the first time in 2005. The seed set of
the cross was 1.67%. In the present study, the synthetic hexaploid wheat
(SHW-DPW) between T. polonicum (DPW) and Ae. tauschii (AS60)
were obtained with colchicine treatment for the first time. The average
seed set of the intergeneric hybrids was 9.71% (8.27-15.31%), which was
much lower than 44% (on the basis of plants) observed for the wheat haploids-double
(De Buyser and Henry, 1986). The selfed seed set of SHW-DPW were 58.95%
for the euhexaploids and 45.63% for the aneuploids, respectively, which
were much lower than the rates (87.40-100%) of synthetic hexaploid wheat
between Ae. tauschii and the tetraploid wheat, T. turgidum,
T. durum, T. carthlicum, T. paleocolchicum, T. dicoccon
and T. dicoccoides (McFadden and Sears, 1944; Lange and Jochemsen,
1992; Warburton et al., 2006).
In this study, we successfully obtained the F1
hybrids and synthetic hexaploid wheat (SHW-DPW) by a lot of crosses between
T. polonicum (DPW) and Ae. tauschii (AS60). Therefore, the
genotypes of different the tetraploid wheat and Ae. tauschii, the
process of chromosome doubling with colchicines treatment and the field
management are very important in hybridization and chromosome doubling
of F1 hybrids between the tetraploid wheat and Ae. tauschii.
Morphology and Cytology Assay of Synthetic Hexaploid
Morphologically, the traits of tillers, spikelets per spike and length
of awn and internodes of spike in SHW-DPW plants were similar to those
in T. polonicum (DPW), while non-waxiness, hairy auricles and tough
rachis in SHW-DPW plants were similar to those in Ae. tauschii.
All the hybrid plants were resistant to powdery mildew and budworm. In
the previous studies, the synthetic hexaploid wheats between Ae. tauschii
and T. turgidum, T. durum, T. dicoccon and T.
dicoccoides were reported to have a relatively lax spikes, which resulted
from the long rachis internodes (Lange and Jochemsen, 1992; Lage et
al., 2003; Yoshihiro and Shuhei, 2004). In the present study, the
SHW-DPW plants have a comparatively short rachis internode, which in general
was similar to that in the F1 hybrids of T. polonicum (DPW)
x Ae. tauschii (AS60) and T. petropavlovskyi. Compared with
the other synthetic hexaploid wheat, SHW-DPW has a longer and softer glume.
Chromosome association at metaphase I of meiosis of SHW-DPW
was quite regular, with an average of 0.43 univalents, 20.77 bivalents
and 0.01 trivalents per cell. The trivalent configurations only appeared
in two SHW-DPW plants. Three out of 10 of SHW-DPW plants were aneuploids.
The differences of chromosome pairing among the plants were small and
the average values of chromosome association were similar to those of
synthetic hexaploid wheat between T. turgidum, T. durum,
T. carthlicum, T. dicoccon and T. dicoccoides and Ae.
tauschii (Tanaka, 1961; Lange and Jochemsen, 1992; Warburton et
al., 2006). The chromosome behaviour suggests that the SHW-DPW was
cytologically stable and regular.
The Potential Value of Synthetic Hexaploid Wheat (SHW-DPW)
for Studying the Origin of Triticum petropavlovskyi
The origin of Triticum petropavlovskyi Udacz. et Migusch.
(2n = 6x = 42, AABBDD) has remained an issue (Chen et al., 1985;
Yang et al., 1992; Akond et al., 2007). Based on previous
studies, there were three hypotheses: (1) a single mutation in T. aestivum;
(2) independent allopolyploidization between T. polonicum and Ae.
tauschii; (3) introgression from T. polonicum to T. aestivum.
In this study, we successfully obtained the synthetic hexaploid wheat
(SHW-DPW) between T. polonicum (DPW) and Ae. tauschii through
chromosome doubling. The spike morphology of SHW-DPW was similar to T.
petropavlovskyi, especially the long and soft glume (Fig.
2). Meanwhile, the hybrids of bread wheat x T. polonicum
and a set of T. aestivum-T. polonicum introgression lines
with long glume have also been obtained. These materials will be significant
to the study of the origin of T. petropavlovskyi. Further studies
are in progress in our laboratory.
Implications of Synthetic Hexaploid Wheat (SHW-DPW)
for the Wheat Breeding
It is advantageous for wheat improvement through the production of
synthetic hexaploid wheat. Since it allows not only the Ae. tauschii
resistance to be exploited but also incorporates the genetic diversity
of the A and B genomes of the respective tetraploid wheat. In the present
study, a new synthetic hexaploid wheat (SHW-DPW) between T. polonicum
(DPW) and Ae. tauschii (AS60) were obtained successfully. As
expected, the SHW-DPW plants inherited the traits of T. polonicum (DPW),
such as dwarfing (about 75 cm), more tillers (about 10), more length of
spike (about 19 cm) and more spikelets per spike (about 23) and the characters
of resistance to powdery mildew and budworm of Ae. tauschii were
also expressed. Meanwhile, we successfully obtained a set of hybrids and
many derivatives between SHW-DPW and bread wheat. These hybrids, SHW-DPW
and the derivatives could be important germplasms for the genetic improvement
of bread wheat.
The authors thank Profs. You-Liang Zheng, Deng-Cai Liu
and Xiu-Jin Lan of Triticeae Research Institute, Sichuan Agricultural
University for several useful suggestions. This work was supported by
the National Natural Science Foundation of China (No. 30470135, 30670150),
the Program for Changjiang Scholars and Innovative Research Teams in University
of China (IRT0453) and the Education Bureau and Science and Technology
Bureau of Sichuan Province, China.
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