Molecular cytogenetic analysis of radiation-induced wheat-rye terminal and intercalary chromosomal translocations and the detection of rye chromatin specifying resistance to Hessian fly

The molecular cytogenetics of Vigna unguiculata (L.) Walp: the physical organization and characterization of 18s-5.8s-25s rRNA genes, 5s rRNA genes, telomere-like sequences, and a family of centromeric repetitive DNA sequences

A knowledge of genome organization is important for understanding how genomes function and evolve, and provide information likely to be useful in plant breeding programmes involving hybridization and genetic manipulation. Molecular techniques, including in situ hybridization, molecular cloning and DNA sequencing, are proving valuable tools to investigate the structure, organization, and diversity of chromosomes in agricultural crops. Heterologous labelled 18 s-5.8 s-25 s (pTa71) and 5 s rDNAs (pTa794) were used for in situ hybridization on Vigna unguiculata (L.) Walp. chromosomes. Hybridization with 18 s-5.8 s-25 s rRNA gene probes occurred at the same chromosomal sites which were positive to the CMA fluorochrome. Silver staining of nucleolar-organizing regions indicated that all the rDNA sites detected using the 18 s-5.8 s-25 s rRNA gene probe possessed active genes. Degenerate telomeric repeats gave hybridization signals at the telomeres of most chromosomes and no intercalary sites were detected at metaphase; the sequences appear to have no preferential distribution in interphase nuclei. A repetitive DraI family from V. unguiculata was cloned (pVuKB1) and characterized. The DraI repeat is 488 nucleotides long, AT rich (74%), and hybridized on all chromosomes in the centromeric areas. The presence of this sequence family was investigated by Southern hybridization in different Vigna species and other Leguminoseae. It was only detected in V. unguiculata, and hence represents a species-specific DNA sequence.

Fluorescent in situ hybridization and C-banding analyses of highly repetitive DNA sequences in the heterochromatin of rye (Secale montanum Guss.) and wheat incorporating S. montanum chromosome segments

The molecular characterization of C-banded regions of Secale montanum Guss. by means of in situ hybridization was performed in order to provide new information about their chromosome structure relative to cultivated rye, Secale cereale L. Accurate identification of individual chromosomes was achieved using simultaneous and (or) successive fluorescent in situ hybridization (FISH) and C-banding. FISH identification was performed using total rye DNA, three highly repetitive rye DNA sequences (pSc119.2, pSc74, and pSc34), and the ribosomal RNA probes pTa71 (18S, 5.8S, and 26S rDNA) and pTa794 (5S rDNA). FISH was also used to identify the chromosome segment involved in two spontaneous translocation lines recovered from a 'Chinese Spring'--S. montanum wheat-rye addition line. FISH analysis revealed the exact translocation breakpoints and allowed the identification of the transferred rye segments. The value of this type of analysis is discussed.

Targeted mapping of rye chromatin in wheat by representational difference analysis

A targeted mapping strategy using representational difference analysis (RDA) was employed to isolate new restriction fragment length polymorphism probes for the long arm of chromosome 6 in rye (6RL), which carries a gene for resistance to Hessian fly larvae. Fragments from the 6RL arm were specifically isolated using a 'Chinese Spring' (CS) wheat – rye ditelosomic addition line (CSDT6RL) as tester, and CS and (or) CS4R as the driver for the genomic subtraction. Three RDA experiments were performed using BamHI amplicons, two of which were successful in producing low-copy clones. All low-copy clones were confirmed to have originated from 6RL, indicating substantial enrichment for target sequences. Two mapping populations, both of which are derived from a cross between two similar wheat–rye translocation lines, were used to map five RDA probes as well as five wheat probes. One of the populations was prescreened for recombinants by C-banding analysis. Fifteen loci, including seven new RDA markers, were placed on a map of the distal half of 6RL. The Hessian fly resistance gene was localized by mapping and C-banding analysis to approximately the terminal 1% of the arm. The utility of RDA as a method of targeted mapping in cereals and prospects for map-based cloning of the resistance gene are discussed.Key words: Secale cereale L., Hessian fly resistance; RDA, RFLP, high density mapping.

Biochemical and molecular diagnostics of Thinopyrum bessarabicum chromosomes in Triticum aestivum germ plasm

Thinopyrum bessarabicum (2n=2x=14, JJ) is a self-fertile salt-tolerant grass species, and its hybridization with Triticum aestivum to achieve the transfer of this attributes has been promoted. For the detection of alien introgression, development of diagnostic markers of Th. bessarabicum chromosomes in the wheat background has emerged as an important aspect in our intergeneric hybridization program. Six proteins/isozymes-high-molecular-weight glutenins, superoxide dismutase, grain esterase, β-amylase, glutamate oxaloacetate transaminase and α-amylase -were identified as positive markers for detecting the presence of Th. bessarabicum chromosomes in the advanced backcross derivatives of T. aestivum/Th. bessarabicum//n(*) T. aestivum. Fluorescent in situ hybridization further enabled the detection of complete and translocated arms of Th. bessarabicum chromosomes in the T. aestivum background. These diagnostic markers served for tentatively characterizing a distinct set of Th. bessarabicum disomic additions to wheat (2n=44) and have facilitated establishing the homoeology of these added chromosomes.

Inheritance and chromosomal locations of male fertility restoring gene transferred from Aegilops umbellulata Zhuk. to Triticum aestivum L

Restriction fragment length polymorphism (RFLP) markers were used to map male fertility restoring gene that was transferred from chromosome 6U of Aegilops umbellulata Zhuk. to wheat. Segments of chromosome 6U bearing the gene that restore fertility to T. timopheevi Zhuk. male sterile cytoplasm were identified in all four translocation lines by two probes, BCD21 and BCD342. Lines 040-5, 061-1 and 061-4 are T6BL.6BS-6U translocations, while line 2114 is a T6AL.6AS-6U translocation. Line 2114 has a much larger 6U chromosomal segment and lower frequency of transmission of male gametes with the alien segment than the other three lines. The restoring gene carried by the 6U segment in 2114 showed high expressivity and complete penetrance. This restoring gene is designated Rf6. A homoeologous chromosome recombination mechanism is discussed for the alien gene transfer.

Visualization of Secale cereale DNA in wheat germ plasm by fluorescent in situ hybridization

Homozygous wheat/rye (1BL/1RS or 1AS/ 1RL) translocation lines have significantly contributed to wheat production, and several other wheat/rye translocation lines show a potential promise against biotic and abiotic stresses. Detecting the presence of rye at the chromosome level is feasible by C-banding and isozyme protocols, but the diagnostic strength of genomic in situ hybridization for eventually analyzing smaller DNA introgressions has greater significance. As a first step we have applied the genomic in situ hybridization technique to detect rye chromosomes in a wheat background using germ plasm of agricultural significance. By this method rye contributions to the translocations 1BL/1RS, 1AL/1RS, 5AS/5RL and 6BS/6RL could be identified. Differential labelling has further enabled the detection of rye and Thinopyrum bessarabicum chromosomes in a trigeneric hybrid of Triticum aestivum/Th. bessarabicum//Secale cereale.

Evidence for somatic translocation during potato dihaploid induction

Potato dihaploid PDH55 (Solanum tuberosum) is exclusively euploid (2n = 24) but apparently contains and expresses DNA from dihaploid inducer IVP48 (S. phureja). Genomic in situ hybridization (GISH) suggested IVP48 DNA incorporated stably into PDH55 by somatic translocation. This finding has two important implications. Firstly, the long-held implicit assumption that euploid dihaploids produced by dihaploid inducers are pure S. tuberosum seems incorrect. This may complicate meiotic, genetical and molecular studies involving potato dihaploids. Secondly, if such translocations are not rare, the phenomenon may offer a novel way to introduce useful traits directly from wild dihaploid-inducing species into S. tuberosum.

Localization of translocation breakpoints in somatic metaphase chromosomes of barley

Karyotype analyses based on staining by acetocarmine followed by Giemsa N-banding of somatic metaphase chromosomes of Hordeum vulgare L. were carried out on 61 reciprocal translocations induced by X-irradiation. By means of computer-based karyotype analyses all of the 122 breakpoints could be localized to defined sites or segments distributed over the seven barley chromosomes. The pre-definition of translocations with respect to their rearranged chromosome arms from other studies rendered it possible to define the break positions even in translocations having exchanged segments equal in size and the breakpoints located distally to any Giemsa band or other cytological marker. The breakpoints were found to be non-randomly spaced along the chromosomes and their arms. All breaks but one occurred in interband regions of the chromosomes, and none of the breaks was located directly within a centromere. However, short and long chromosome arms recombined at random. An improved tester set of translocations depicting the known break positions of most distal location is presented.

The fate of recombinant chromosomes and genome interaction in Nicotiana asymmetric somatic hybrids and their sexual progeny

Genomic in-situ hybridization (GISH) was used to monitor the behaviour of parental genomes, and the fate of intergenomic chromosome translocations, through meiosis of plants regenerated from asymmetric somatic hybrids between Nicotiana sylvestris and N. plumbaginifolia. Meiotic pairing in the regenerants was exclusively between chromosomes or chromosome segments derived from the same species. Translocation (recombinant) chromosomes contained chromosome segments from both parental species, and were detected at all stages of meiosis. They occasionally paired with respectively homologous segments of N. sylvestris or N. plumbaginifolia chromosomes. Within hybrid nuclei, the meiotic division of N. plumbaginifolia lagged behind that of N. sylvestris. However, normal and recombinant chromosomes were eventually incorporated into dyads and tetrads, and the regenerants were partially pollen fertile. Recombinant chromosomes were transmitted through either male or female gametes, and were detected by GISH in sexual progeny obtained on selfing or backcrossing the regenerants to N. sylvestris. A new recombinant chromosome in one plant of the first backcross generation provided evidence of further chromosome rearrangements occurring at, or following, meiosis in the original regenerants. This study demonstrates the stable incorporation of chromosome segments from one parental genome of an asymmetric somatic hybrid into another, via intergenomic translocation, and reveals their transmission to subsequent sexual progeny.

The use of fluorochromes in the cytogenetics of the small-grained cereals (Triticeae)

This paper describes some of the major advances that have been made in the cytogenetics of the small-grained cereals (Triticeae) using fluorochromes to detect nucleic acids in situ. The method, widely known as fluorescence in situ hybridization, has made a contribution in several areas including (i) chromosome mapping programmes, and (ii) cereal breeding programmes. Flow cytometry of cereal chromosomes has now been developed for the generation of chromosome enriched libraries; these libraries will ultimately be of use in both the cereal mapping and breeding programmes. Fluorescence in situ hybridization has also made a major contribution to the understanding of cereal genome structure by elucidating the distribution of different classes of DNA sequence. By using suitable nucleic acid probes whole chromosomes can now be identified in interphase nuclei. The labelling patterns have revealed a structured arrangement of chromosomes at interphase. Not only are chromosomes organized but the ribosomal RNA genes also show structured patterns of condensation and expression. Progress in each of these areas has been rapid in recent years and this progress is described.

Transfer of Ph (I) genes promoting homoeologous pairing from Triticum speltoides to common wheat

Diploid-like chromosome pairing in polyploid wheat is controlled by several Ph (pairing homoeologous) genes with major and minor effects. Homoeologous pairing occurs in either the absence of these genes or their inhibition by genes from other species (Ph (I) genes). We transferred Ph (I) genes from Triticum speltoides (syn Aegilops speltoides) to T. aestivum, and on the basis of further analysis it appears that two duplicate and independent Ph (I) genes were transferred. Since Ph (I) genes are epistatic to the Ph genes of wheat, homoeologous pairing between the wheat and alien chromosomes occurs in the F1 hybrids. Using the Ph (I) gene stock, we could demonstrate homoeologous pairing between the wheat and Haynaldia villosa chromosomes. Since homoeologous pairing occurs in F1 hybrids and no cytogenetic manipulation is needed, the Ph (I) gene stock may be a versatile tool for effecting rapid and efficient alien genetic transfers to wheat.

Radiation-induced nonhomoeologous wheat-Agropyron intermedium chromosomal translocations conferring resistance to leaf rust

The Agropyron intermedium chromosome 7Ai #2 is the source of the leaf rust resistance gene Lr38 which was transferred to wheat by irradiation. The chromosomal constitutions of eight radiation-induced rust-resistant wheat-Agropyron intermedium derivatives were analyzed by C-banding and genomic in-situ hybridization (GISH). Five lines were identified as wheat Ag. intermedium chromosome translocation lines with the translocation chromosomes T2AS·2AL-7Ai#2L, T5AL · 5AS-7Ai # 2L, T1DS · 1DL-7Ai # 2L, T3DL · 3DS-7Ai#2L, and T6DS · 6DL-7Ai#2L. The sizes of the 7Ai#2L segments in mitotic metaphases of these translocations are 2.42 μm, 4.20 μm, 2.55 μm, 2.78 μm, and 4.19 μm, respectively. One line was identified as a wheat-Ag. intermedium chromosome addition line. The added Ag. intermedium chromosome in this line is different from 7Ai # 2. This line has resistance to leaf rust and stem rust. Based on the rust reactions, and the C-banding and GISH results, the remaining two lines do not contain any Ag. intermedium-derived chromatin.