The launch of satellite: DNA repeats as a cytogenetic tool in discovering the chromosomal universe of wild Triticeae

Fluorescence in situ hybridization is a powerful tool that enables plant researchers to perform systematic, evolutionary, and population studies of wheat wild relatives as well as to characterize alien introgression into the wheat genome. This retrospective review reflects on progress made in the development of methods for creating new chromosomal markers since the launch of this cytogenetic satellite instrument to the present day. DNA probes based on satellite repeats have been widely used for chromosome analysis, especially for "classical" wheat probes (pSc119.2 and Afa family) and "universal" repeats (45S rDNA, 5S rDNA, and microsatellites). The rapid development of new-generation sequencing and bioinformatical tools, and the application of oligo- and multioligonucleotides has resulted in an explosion in the discovery of new genome- and chromosome-specific chromosome markers. Owing to modern technologies, new chromosomal markers are appearing at an unprecedented velocity. The present review describes the specifics of localization when employing commonly used vs. newly developed probes for chromosomes in J, E, V, St, Y, and P genomes and their diploid and polyploid carriers Agropyron, Dasypyrum, Thinopyrum, Pseudoroegneria, Elymus, Roegneria, and Kengyilia. Particular attention is paid to the specificity of probes, which determines their applicability for the detection of alien introgression to enhance the genetic diversity of wheat through wide hybridization. The information from the reviewed articles is summarized into the TRepeT database, which may be useful for studying the cytogenetics of Triticeae. The review describes the trends in the development of technology used in establishing chromosomal markers that can be used for prediction and foresight in the field of molecular biology and in methods of cytogenetic analysis.

A Novel Wheat-Rye 2R (2D) Disomic Substitution Line Pyramids Two Types of Resistance to Powdery Mildew

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is a devastating disease of wheat that seriously affects yield and quality worldwide. Because of the extensive growth of wheat cultivars with homogeneous genetic background, exploring novel resistant resources from wheat relatives has become important for increasing the genetic diversity of wheat. Rye (Secale cereale) is a wheat relative possessing abundant resistance genes because of its high variation. Wheat line AL69, resistant to powdery mildew, was developed by crossing, backcrossing, and self-pollination for multiple generations between hexaploid triticale Zhongsi 237 and common wheat cultivar Zimai 17. Through genomic in situ hybridization (GISH) and multicolor fluorescence in situ hybridization (FISH), nondenaturing FISH, multicolor GISH, and selection with specific molecular markers, AL69 was determined to be a wheat-rye 2R (2D) disomic substitution line. Testing with different B. graminis f. sp. tritici isolates and genetic analysis showed that the all-stage resistance (also called seedling resistance) of AL69 was conferred by the cataloged powdery mildew resistance gene Pm4b derived from Zimai 17, and its adult-plant resistance was derived from the alien chromosome 2R of Zhongsi 237, which was found to be different from the previously reported rye-derived Pm genes, including Pm7 on 2RL. In addition, AL69 showed improved spike number per plant, spike length, fertile spikelet number per spike, kernel number per spike, and grain yield per plant compared with its wheat parent Zimai 17. An elite line S251 combining powdery mildew resistance with excellent agronomic performance was selected from the progenies of AL69 and wheat cultivar Jimai 22. Therefore, AL69 has two types of resistance genes to powdery mildew and improved agronomic traits through pyramiding and thus can be used as a promising genetic stock for wheat breeding.

Molecular Cytogenetic Identification of the Wheat–Dasypyrum villosum T3DL·3V#3S Translocation Line with Resistance against Stripe Rust

The annual species Dasypyrum villosum possesses several potentially valuable genes for the improvement of common wheat. Previously, we identified a new stripe rust-resistant line, the Chinese Spring (CS)–D. villosum 3V#3 (3D) substitution line (named CD-3), and mapped its potential rust resistance gene (designated as YrCD-3) on the 3V#3 chromosome originating from D. villosum. The objective of the present study was to further narrow down the YrCD-3 locus to a physical region and develop wheat-3V#3 introgression lines with strong stripe rust resistance. By treating CD-3 seeds with 60Co γ-irradiation, two CS-3V#3 translocation lines, T3V#3S.3DL and T3DS.3V#3L (termed 22-12 and 24-20, respectively), were identified from the M4 generation through a combination of non-denaturing fluorescence in situ hybridization (ND-FISH) and functional molecular markers. Stripe rust resistance tests showed that the line 22-12 exhibited strong stripe rust resistance similarly to CD-3, whereas 24-20 was susceptible to stripe rust similarly to CS, indicating that YrCD-3 is located on the short arm of 3V#3. The line 22-12 can potentially be used for further wheat improvement. Additionally, to trace 3V#3 in the wheat genetic background, we produced 30 3V#3-specific sequence tag (EST) markers, among which, 11 markers could identify 3V#3S. These markers could be valuable in fine-mapping YrCD-3.

Karyotyping Dasypyrum breviaristatum chromosomes with multiple oligonucleotide probes reveals the genomic divergence in Dasypyrum

The perennial species Dasypyrum breviaristatum (genome V^b) contains many potentially valuable genes for the improvement of common wheat. Construction of a detailed karyotype of D. breviaristatum chromosomes will be useful for the detection of Dasypyrum chromatin in wheat background. We established the standard karyotype of 1V^b-7V^b chromosomes through nondenaturing fluorescence in situ hybridization (ND-FISH) technique using 28 oligonucleotide probes from the wheat - D. breviaristatum partial amphiploid TDH-2 (AABBV^bV^b) and newly identified wheat - D. breviaristatum disomic translocation and addition lines D2138 (6V^bS.2V^bL), D2547 (4V^b), and D2532 (3V^bS.6V^bL) by comparative molecular marker analysis. The ND-FISH with multiple oligo probes was conducted on the durum wheat - D. villosum amphiploid TDV-1 and large karyotype differences between D. breviaristatum and D. villosum was revealed. These ND-FISH probes will be valuable for screening the wheat - Dasypyrum derivative lines for chromosome identification, and the newly developed wheat - D. breviaristatum addition lines may broaden the gene pool of wheat breeding. The differences between D. villosum and D. breviaristatum chromosomes revealed by ND-FISH will help us understand evolutionary divergence of repetitive sequences within the genus Dasypyrum.

Pairing and Exchanging between Daypyrum villosum Chromosomes 6V#2 and 6V#4 in the Hybrids of Two Different Wheat Alien Substitution Lines

Normal pairing and exchanging is an important basis to evaluate the genetic relationship between homologous chromosomes in a wheat background. The pairing behavior between 6V#2 and 6V#4, two chromosomes from different Dasypyrum villosum accessions, is still not clear. In this study, two wheat alien substitution lines, 6V#2 (6A) and 6V#4 (6D), were crossed to obtain the F₁ hybrids and F₂ segregating populations, and the testcross populations were obtained by using the F₁ as a parent crossed with wheat variety Wan7107. The chromosomal behavior at meiosis in pollen mother cells (PMCs) of the F₁ hybrids was observed using a genomic in situ hybridization (GISH) technique. Exchange events of two alien chromosomes were investigated in the F₂ populations using nine polymerase chain reaction (PCR) markers located on the 6V short arm. The results showed that the two alien chromosomes could pair with each other to form ring- or rod-shaped bivalent chromosomes in 79.76% of the total PMCs, and most were pulled to two poles evenly at anaphase I. Investigation of the F₂ populations showed that the segregation ratios of seven markers were consistent with the theoretical values 3:1 or 1:2:1, and recombinants among markers were detected. A genetic linkage map of nine PCR markers for 6VS was accordingly constructed based on the exchange frequencies and compared with the physical maps of wheat and barley based on homologous sequences of the markers, which showed that conservation of sequence order compared to 6V was 6H and 6B > 6A > 6D. In the testcross populations with 482 plants, seven showed susceptibility to powdery mildew (PM) and lacked amplification of alien chromosomal bands. Six other plants had amplification of specific bands of both the alien chromosomes at multiple sites, which suggested that the alien chromosomes had abnormal separation behavior in about 1.5% of the PMCs in F₁, which resulted in some gametes containing two alien chromosomes. In addition, three new types of chromosome substitution were developed. This study lays a foundation for alien allelism tests and further assessment of the genetic relationship among 6V#2, 6V#4, and their wheat homoeologous chromosomes.

Diversified Chromosome Rearrangements Detected in a Wheat‒Dasypyrum breviaristatum Substitution Line Induced by Gamma-Ray Irradiation

To determine the composition of chromosome aberrations in a wheat‒Dasypyrum breviaristatum substitution line with seeds treated by a dose of gamma-rays (200 Gy), sequential non-denaturing fluorescence in situ hybridization (ND-FISH) with multiple oligonucleotide probes was used to screen individual plants of the mutagenized progenies. We identified 122 types of chromosome rearrangements, including centromeric, telomeric, and intercalary chromosome translocations from a total of 772 M1 and 872 M2 plants. The frequency of reciprocal translocations between B- and D-chromosomes was higher than that between A- and D-chromosomes. Eight translocations between D. breviaristatum and wheat chromosomes were also detected. The 13 stable plants with multiple chromosome translocations displayed novel agronomic traits. The newly developed materials will enhance wheat breeding programs through wheat‒Dasypyrum introgression and also facilitate future studies on the genetic and epigenetic effects of translocations in wheat genomics.

Characterization of New Wheat-Dasypyrum breviaristatum Introgression Lines with Superior Gene(s) for Spike Length and Stripe Rust Resistance

Dasypyrum breviaristatum (genome VbVb) contains potentially important traits for commercial wheat production. Chromosome 2Vb of D. breviaristatum carries several desirable agronomic characters, including long spike length as well as enhanced resistance to stripe rust, which are expressed in a common wheat background. In this study, wheat-D. breviaristatum 2Vb deletion lines were produced and identified by fluorescence in situ hybridization (FISH), and 74 molecular markers specific to D. breviaristatum chromosome 2Vb were physically localized in 4 distinct chromosomal regions. New wheat-D. breviaristatum 2Vb translocation lines were also characterized by FISH. The breakpoint of the translocation T3AS.3AL-2VbS was determined by physically mapped molecular markers. Field evaluation revealed that genes affecting plant height and spike length are located on fraction length (FL) 0.65-1.00 of 2VbS, while the stripe rust resistance gene(s) are located on FL 0.40-1.00 of D. breviaristatum chromosome 2VbL. The newly characterized wheat-Dasypyrum chromosomal introgressions are of potential value for the improvement of the yield and disease resistance of wheat.

Molecular markers and cytogenetics to characterize a wheat-Dasypyrum villosum 3V (3D) substitution line conferring resistance to stripe rust

Dasypyrum villosum has been used as a valuable gene resource for disease resistances, yield increase and quality improvement in wheat. A novel wheat-D. villosum alien introgression line CD-3 was generated through hybridization between the common wheat Chinese Spring (CS) and a CS- D. villosum 3V addition line having considerably high stripe rust resistance, which enable the characterization of a potential new stripe rust resistance gene (s) derived from D. villosum. The results of non-denaturing fluorescent in situ hybridization (ND-FISH) showed that CD-3 contained 42 chromosomes, including a 3V chromosome pair, and the absence of both of the 3D chromosomes. PCR-based Landmark Unique Gene (PLUG) molecular marker analysis supported results from the FISH analysis, revealing CD-3 was a wheat-D. villosum 3V (3D) disomic substitution line. Resistant test of stripe rust on 52 plants of F2 generation (CD-3/CS), CD-3, CS and D.villosum have been conducted at seedling stage. 7 plants of F2 generation possessing two 3V chromosomes exhibited high resistance to stripe rust as CD-3 and D.villosum, 10 plants carrying one 3V chromosome and 35 plants without 3V chromosome were susceptive to stripe rust as CS. The result implied the high stripe rust resistance of CD-3 should be controlled by recessive gene(s) originating from D.villosum. To rapidly detect chromosome 3V in the genetic background of wheat, we developed a novel Sequence Characterized Amplified Region (SCAR) marker specific for 3V chromosome based on the sequence of a grain size-related gene DvGS5 in D. villosum, an orthologue of TaGS5 from wheat. The SCAR marker was designated DvGS5-1443, which could successfully amplify a unique 3V-specific fragment in CD-3 and D. villosum, suggesting that this SCAR marker could facilitate targeting the chromosome 3V in the genetic background of wheat for wheat improvement.

Molecular Cytogenetic Characterization of New Wheat-Dasypyrum breviaristatum Introgression Lines for Improving Grain Quality of Wheat

As an important relative of wheat (Triticum aestivum L), Dasypyrum breviaristatum contains novel high molecular weight glutenin subunits (HMW-GSs) encoded by Glu-1Vb genes. We identified new wheat-D. breviaristatum chromosome introgression lines including chromosomes 1V^b and 1V^bL.5V^bL by fluorescence in situ hybridization (FISH) combined with molecular markers. We found that chromosome changes occurred in the wheat-D. breviaristatum introgression lines and particularly induced the deletion of 5BS terminal repeats and formation of a new type of 5B-7B reciprocal translocation. The results imply that the D. breviaristatum chromosome 1V^b may contain genes which induce chromosomal recombination in wheat background. Ten putative high molecular weight glutenin subunit (HMW-GS) genes from D. breviaristatum and wheat-D. breviaristatum introgression lines were isolated. The lengths of the HMW-GS genes in Dasypyrum were significantly shorter than typical HMW-GS of common wheat. A new y-type HMW-GS gene, named Glu-Vb1y, was characterized in wheat-D. breviaristatum 1V^b introgression lines. The new wheat-D. breviaristatum germplasm displayed reduced plant height, increased tillers and superior grain protein and gluten contents, improved gluten performance index. The results showed considerable potential for utilization of D. breviaristatum chromosome 1V^b segments in future wheat improvement.

Molecular cytogenetic characterization of Dasypyrum breviaristatum chromosomes in wheat background revealing the genomic divergence between Dasypyrum species

BACKGROUND

The uncultivated species Dasypyrum breviaristatum carries novel diseases resistance and agronomically important genes of potential use for wheat improvement. The development of new wheat-D. breviaristatum derivatives lines with disease resistance provides an opportunity for the identification and localization of resistance genes on specific Dasypyrum chromosomes. The comparison of wheat-D. breviaristatum derivatives to the wheat-D. villosum derivatives enables to reveal the genomic divergence between D. breviaristatum and D. villosum.

RESULTS

The mitotic metaphase of the wheat- D. breviaristatum partial amphiploid TDH-2 and durum wheat -D. villosum amphiploid TDV-1 were studied using multicolor fluorescent in situ hybridization (FISH). We found that the distribution of FISH signals of telomeric, subtelomeric and centromeric regions on the D. breviaristatum chromosomes was different from those of D. villosum chromosomes by the probes of Oligo-pSc119.2, Oligo-pTa535, Oligo-(GAA)7 and Oligo-pHv62-1. A wheat line D2139, selected from a cross between wheat lines MY11 and TDH-2, was characterized by FISH and PCR-based molecular markers. FISH analysis demonstrated that D2139 contained 44 chromosomes including a pair of D. breviaristatum chromosomes which had originated from the partial amphiploid TDH-2. Molecular markers confirmed that the introduced D. breviaristatum chromosomes belonged to homoeologous group 7, indicating that D2139 was a 7V(b) disomic addition line. The D2139 displayed high resistance to wheat stripe rust races at adult stage plant, which may be inherited from, D. breviaristatum chromosome 7V(b).

CONCLUSION

The study present here revealed that the large divergence between D. breviaristatum and D. villosum with respected to the organization of different repetitive sequences. The identified wheat- D. breviaristatum chromosome addition line D2139 will be used to produce agronomically desirable germplasm for wheat breeding.

Molecular and Cytogenetic Characterization of New Wheat-Dasypyrum breviaristatum Derivatives with Post-Harvest Re-Growth Habit

A novel Dasypyrum species, Dasypyrum breviaristatum, serves as a valuable source of useful genes for wheat improvement. The development and characterization of new wheat—D. breviaristatum introgression lines is important to determine the novel gene(s) on specific chromosome(s). We first used multi-color fluorescence in situ hybridization (FISH) to identify the individual D. breviaristatum V^b chromosomes in a common wheat—D. breviaristatum partial amphiploid, TDH-2. The FISH patterns of D. breviaristatum chromosomes were different from those of D. villosum chromosomes. Lines D2146 and D2150 were selected from a cross between wheat line MY11 and wheat—D. breviaristatum partial amphiploid TDH-2, and they were characterized by FISH and PCR-based molecular markers. We found that D2150 was a monosomic addition line for chromosome 5V^b of D. breviaristatum, while D2146 had the 5V^bL chromosome arm translocated with wheat chromosome 5AS. Molecular marker analysis confirmed that the introduced D. breviaristatum chromosome 5V^bL translocation possessed a duplicated region homoeologous to 5AS, revealing that the 5AS.5V^bL translocation may not functionally compensate well. The dwarfing and the pre-harvest re-growth habits observed in the wheat—D. breviaristatum chromosome 5V^b derivatives may be useful for future development of perennial growth wheat lines.

Development and Molecular Cytogenetic Identification of a Novel Wheat-Leymus mollis Lm#7Ns (7D) Disomic Substitution Line with Stripe Rust Resistance

Leymus mollis (2n = 4x = 28, NsNsXmXm) possesses novel and important genes for resistance against multi-fungal diseases. The development of new wheat—L. mollis introgression lines is of great significance for wheat disease resistance breeding. M11003-3-1-15-8, a novel disomic substitution line of common wheat cv. 7182 –L. mollis, developed and selected from the BC₁F₅ progeny between wheat cv. 7182 and octoploid Tritileymus M47 (2n = 8x = 56, AABBDDNsNs), was characterized by morphological and cytogenetic identification, analysis of functional molecular markers, genomic in situ hybridization (GISH), sequential fluorescence in situ hybridization (FISH)—genomic in situ hybridization (GISH) and disease resistance evaluation. Cytological observations suggested that M11003-3-1-15-8 contained 42 chromosomes and formed 21 bivalents at meiotic metaphase I. The GISH investigations showed that line contained 40 wheat chromosomes and a pair of L. mollis chromosomes. EST-STS multiple loci markers and PLUG (PCR-based Landmark Unique Gene) markers confirmed that the introduced L. mollis chromosomes belonged to homoeologous group 7, it was designated as Lm#7Ns. While nulli-tetrasomic and sequential FISH-GISH analysis using the oligonucleotide Oligo-pSc119.2 and Oligo-pTa535 as probes revealed that the wheat 7D chromosomes were absent in M11003-3-1-15-8. Therefore, it was deduced that M11003-3-1-15-8 was a wheat–L. mollis Lm#7Ns (7D) disomic substitution line. Field disease resistance demonstrated that the introduced L. mollis chromosomes Lm#7Ns were responsible for the stripe rust resistance at the adult stage. Moreover, M11003-3-1-15-8 had a superior numbers of florets. The novel disomic substitution line M11003-3-1-15-8, could be exploited as an important genetic material in wheat resistance breeding programs and genetic resources.

Molecular characterization of a new wheat-Thinopyrum intermedium translocation line with resistance to powdery mildew and stripe rust

A new wheat-Thinopyrum translocation line CH13-21 was selected from the progenies derived from a cross between wheat-Th. intermedium partial amphiploid TAI7047 and wheat line Mianyang11. CH13-21 was characterized by using genomic in situ hybridization (GISH), multicolor-GISH (mc-GISH), multicolor-fluorescence in situ hybridization (mc-FISH) and chromosome-specific molecular markers. When inoculated with stripe rust and powdery mildew isolates, CH13-21 displayed novel resistance to powdery mildew and stripe rust which inherited from its Thinopyrum parent. The chromosomal counting analyses indicated that CH13-21 has 42 chromosomes, with normal bivalent pairing at metaphase I of meiosis. GISH probed by Th. intermedium genomic DNA showed that CH13-21 contained a pair of wheat-Th. intermedium translocated chromosomes. Sequential mc-FISH analyses probed by pSc119.2 and pAs1 clearly revealed that chromosome arm 6BS of CH13-21 was replaced by Thinopyrum chromatin in the translocation chromosome. The molecular markers analysis further confirmed that the introduced Th. intermedium chromatin in CH13-21 belonged to the long arm of homoeologous group 6 chromosome. Therefore, CH13-21 was a new T6BS.6Ai#1L compensating Robertsonian translocation line. It concludes that CH13-21 is a new genetic resource for wheat breeding programs providing novel variation for disease resistances.