Detection of intergenomic translocations with centromeric and noncentromeric breakpoints in Triticum araraticum: mechanism of origin and adaptive significance.

Triticum araraticum Jakubz. (2n = 4x = 28, AtAtGG), a wild progenitor of the polyploid cultivated wheat T. timopheevii, shows extensive chromosome translocation polymorphism in natural populations from the Middle East and Transcaucasia. From an extensive survey, eight intergenomic translocation types were observed and their breakpoints analyzed by genomic in situ hybridization. The previously reported species-specific 6At-1G-4G cyclic translocation was found in all accessions studied. In four translocation types, the breakpoints were in interstitial regions of chromosomes and the other four arose via centric-breakage-fusion. A model is presented on the mechanism of origin and the adaptive significance of translocations with centromeric and noncentromeric breakpoints.     

Standard karyotypes ofAegilops uniaristata,Ae. mutica,Ae. comosa subspeciescomosa andheldreichii (Poaceae)

C-banding patterns and polymorphisms were analyzed in several accessions of the diploidAegilops speciesAe. uniaristata, Ae. mutica, andAe. comosa subsp.comosa and subsp.heldreichii, and standard karyotypes of these species were established. Variation in C-band size and location was observed between different accessions, but did not prevent chromosome identification. One accession ofAe. uniaristata was homozygous for whole-arm translocations involving chromosomes 1N and 5N. The homoeologous relationships of these chromosomes were established by comparison of chromosome morphologies and C-banding patterns to other diploidAegilops species with known chromosome homoeology. In addition, in situ hybridization analysis with a 5S rDNA probe was used to identify homoeologous groups 1 and 5 chromosomes. The present analysis permitted the assignment of allAe. mutica, comosa subsp.comosa, andAe. comosa subsp.heldreichii chromosomes, and three of the sevenAe. uniaristata chromosomes according to their homoeologous groups. The data presented will be useful analyzing genome differentiation in polyploidAegilops species.     

Comparative study ofTriticum aestivum andT. timopheevi genomes using C-banding technique

The somatic chromosomes ofTriticum timopheevi and those of two varieties ofT. aestivum, Chinese Spring and Bezostaya-1, have been identified by a Giemsa staining technique. The data suggest thatT. timopheevi and tetraploid wheats had a common ancestor from which their genomes differentiated due to chromosomal aberrations and the increase of heterochromatin in the chromosomes of theT. timopheevi G-genome. The differences between the chromosomes of the AB and AG genomes result in substitutions and large translocations between these chromosomes in interspecific hybrids.     

General features of chromosome substitutions in Triticum aestivum x T. timopheevii hybrids

Summary Tissue culture of the Zea mays inbred line A188 resulted in the regeneration of plants having a high level of phenotypic variation compared to seed-grown control plants. To determine how such variation was induced and whether this could be related to specific in vitro culture methods, callus cultures were established and maintained on different, commonly used culture media. Plants were regenerated and the genomic DNA of callus cultures and regenerants analysed for RFLP differences. The results show that regardless of the gene probe used, callus formation resulted in significant deviations from the DNA pattern normally found in seed-grown control plants. Alterations in gene copy number also occurred. As differentiation and organogenesis began, the level of DNA variation fell, and most of the regenerated plants showed a genetic similarity to the controls; those with RFLP differences were the somaclonal variants.     

A cytogenetic study of the blue-grain line of the common wheat cultivar Saratovskaya 29

The chromosome composition of the blue-grain line i:S29Ba of the cultivar Saratovskaya 29 was identified by cytological, GISH, and microsatellite analyses and C-banding. It was found that common wheat chromosome 4B of the cultivar Saratovskaya 29 was substituted with the Agropyron elongatum Host. chromosome carrying the gene for blue grain (s:S294Ag(4B)) during the construction of this nearly isogenic line. The blue-grain line was tested for productivity. The substitution of total chromosome 4B of the cultivar Saratovskaya 29 by Ag. elongatum chromosome 4 did not significantly affect the spike productivity parameters and grain quality with the exception of spike length (plus effect), spike density, and vitreousness (minus effects). The blue-grain line with s:S294Ag(4B) can be used in further studies associated with chromosome engineering in cereals and wheat breeding.     

Characterizing the composition and evolution of homoeologous genomes in hexaploid wheat through BAC-end sequencing on chromosome 3B

Bread wheat (Triticum aestivum) is one of the most important crops worldwide. However, because of its large, hexaploid, highly repetitive genome it is a challenge to develop efficient means for molecular analysis and genetic improvement in wheat. To better understand the composition and molecular evolution of the hexaploid wheat homoeologous genomes and to evaluate the potential of BAC-end sequences (BES) for marker development, we have followed a chromosome-specific strategy and generated 11 Mb of random BES from chromosome 3B, the largest chromosome of bread wheat. The sequence consisted of about 86% of repetitive elements, 1.2% of coding regions, and 13% remained unknown. With 1.2% of the sequence length corresponding to coding sequences, 6000 genes were estimated for chromosome 3B. New repetitive sequences were identified, including a Triticineae-specific tandem repeat (Fat) that represents 0.6% of the B-genome and has been differentially amplified in the homoeologous genomes before polyploidization. About 10% of the BES contained junctions between nested transposable elements that were used to develop chromosome-specific markers for physical and genetic mapping. Finally, sequence comparison with 2.9 Mb of random sequences from the D-genome of Aegilops tauschii suggested that the larger size of the B-genome is due to a higher content in repetitive elements. It also indicated which families of transposable elements are mostly responsible for differential expansion of the homoeologous wheat genomes during evolution. Our data demonstrate that BAC-end sequencing from flow-sorted chromosomes is a powerful tool for analysing the structure and evolution of polyploid and highly repetitive genomes.     

Polymorphism and phylogeny of D-genome species of <Emphasis Type="Italic">Aegilops</Emphasis> L. Based on the results of analysis of the microsatellite loci of the chloroplast genome

A total of 14 chloroplast microsatellite markers were used for analysis of the variability and phylogeny of the cytoplasmic genome in 71 members of six species of goat grass with D-genomes. Greater species diversity was found in diploid species of Aegilops tauschii. Greater species variability among polyploid species was demonstrated for A. ventricosa. The haplotype of A. ventricosa considerably differs from haplotypes of A. tauschii. Analyzed samples of A. cylindrica have haplotypes similar to the haplotypes of A. tauschii.     

Cytogenetic analysis of alloplasmic recombinant lines (H. vulgare)—T. aestivum unstable in fertility and viability

Comparative cytogenetic analysis was performed with four alloplasmic recombinant (Hordeum vulgare)—Triticum aestivum lines differing in morphological traits, number of seeds per spike, and seed plumpness. None of the lines displayed introgression of the barley genetic material: the karyotypes included only common wheat chromosomes. Two lines, 79(B) and 79(D), were cytogenetically stable. Plants of lines 79(A) and 79(C) displayed a high frequency of unbalanced chromosome aberrations, including dicentric and polycentric chromosomes, terminal deletions varying in size, acentric fragments, and multiple unidentifiable translocations. Previous studies of the mitochondrial (mt) genome showed that the two cytologically unstable lines 79(C) and 79(A), which were also unstable in fertility and viability, are characterized by heteroplasmy at the mitochondrial 18S-5S locus (simultaneous presence of barley and wheat mt-fragments). Stable lines 79(B) and 79(D) with normal fertility contained only wheat mitochondrial markers. It was assumed that the substantial instability of the nuclear genome in lines 79(C) and 79(A) was a result of nuclear-cytoplasmic incompatibility and was associated with heteroplasmy, while elimination or considerable reduction of barley material in the mitochondrial genome stabilized the nuclear genome of lines 79(B) and 79(D). In turn, the instability of the nuclear genome was responsible for a decrease in viability and fertility of plants.