Search

• Organisms that are of major significance to the study of genetics and cytogenetics are characterized. • The reasons for the genetic importance of these species are summarized.

• Aneuploid chromosome stocks in polyploids provide a very efficient procedure for assigning genes and DNA sequences to specific chromosomes. • Nullisomic lines can be scored at the DNA or phenotypic levels for the simple presence or absence of a DNA sequence or a gene. • Monosomic lines usually need to be crossed and the segregation patterns analyzed before a gene can be assigned to a chromosome. • Intervarietal-chromosome substitutions are valuable for assigning genes for quantitative traits to chromosomes.

• DNA transformation has removed most of the barriers that limit the introduction of specific genes from one organism to another. • The use of Agrobacteria and modified Ti plasmids to transform dicotyledons, and the use of microinjection to transfect the nuclei of animal cells, are being superseded by particle-bombardment biolistic procedures, developed from the need to transform monocotyle-donous crops. • Chimeric-gene constructs combine the gene to be transformed with regulator and promoter sequences suitable for the host, in order to improve the stability or rate of formation of the gene product in the transformed host. • Through DNA transformation, certain plants and animals can be regarded as factories for the production of novel gene products. • Transformation by antisense genes can block the formation of product from normal sense genes, if necessary in a specific tissue; the insertion of multiple copies of sense genes can have the same effect. • Successfully transformed organisms must transmit introduced genes in a Mendelian manner.

Distorted segregation ratios of genetic markers are often observed in progeny of inter- and intraspecific hybrids and may result from competition among gametes or from abortion of the gamete or zygote. In this study, 194 markers mapped in an Aegilops tauschii F2 population were surveyed for distorted segregation ratios. Region(s) with skewed segregation ratios were detected on chromosomes 1D, 3D, 4D, and 7D. These distorter loci are designated as QSd.ksu-1D, QSd.ksu-3D, QSd.ksu-4D, and QSd.ksu-7D. Three regions of segregation distortion identified on chromosome 5D were analyzed in two sets of reciprocal backcross populations to analyze the effect of sex and cytoplasm on segregation distortion. Extreme distortion of marker segregation ratios was observed in populations in which the F1 was used as the male parent, and ratios were skewed in favor of TA1691 alleles. There was some evidence of differential transmission caused by nucleo-cytoplasmic interactions. Our results agree with other studies stating that loci affecting gametophyte competition in male gametes are located on 5DL. The distorter loci on 5DL are designated as QSd.ksu-5D.1, QSd.ksu-5D.2, and QSd.ksu-5D.3.

• Genetic maps include a wide range of markers involving morphological traits, chromosome landmarks, disease-resistance genes, biochemical defects, enzyme and protein characters, and different classes of DNA sequences. • Following the assignment of markers to chromosomes using chromosome aberrations, a genetic map is constructed by analyzing the progeny from crosses between distinguishable individuals and estimating recombination frequencies between the genetic loci. • Codominant markers are ideal for genetic mapping because the coupling or repulsion linkage phase of markers with dominant and recessive alleles becomes an additional variable. • Physical and genetic maps are colinear with respect to gene order, but major distortions occur in apparent distances between loci due to uneven levels of crossing over in different parts of the genome. • Bulked F2 segregant analyses and interval mapping can be used to analyze quantitative-trait loci (QTLs). • Genetic maps provide bases for human genetic counseling, and for plant and animal breeding programs.

• The amounts of nuclear DNA vary greatly between organisms even though (80–100) × 106 base pairs of DNA are theoretically sufficient to define a basic set of genes necessary for eukaryotic life-forms. • Under suitable experimental conditions, denatured DNA spontaneously reforms a double helix, based on A-T and G-C base pairing. • Repetitive DNA sequences are a major source of variation in DNA amount and contribute to modulating gene activity. • Many repetitive sequences are dispersed throughout the genome and were transposable or retrotransposable elements at some stage in their evolutionary history. • Some regions of DNA that affect the expression of a gene can be thousands of base pairs away from the respective gene.

• Chromosome deletions or duplications that do not affect vital functions can be maintained in an organism, where they can be characterized by chromosome banding or in situ hybridization techniques, as well as by pairing behavior at meiosis. • Some deletions or duplications have phenotypes that are typical of dominant mutations. Deletions, in particular, are involved in numerous human abnormalities. • Chromosome deletions can be terminal—at least when first produced—or interstitial, and duplications may be adjacent to the original segment or displaced on the same or a different chromosome. • Losses or gains of chromosome segments may arise from unequal crossingover, from other chromosome aberrations such as reciprocal translocations or as the direct effect of a chromosome-breaking agent. • Deletions usually have more severe effects on viability and fertility than duplications.

• The standard light microscope has many variations based on manipulating the path of light through the optical system. • The wavelength of light is a major determinant of resolution in the microscope, and the shorter “wavelength” of electrons provides greater magnification with the electron microscope. • Fluorescence provides a versatile basis for tracing specific features of chromosomes. • Computer capture of microscopic images has enhanced the analytical power of microscopes.

• The replication of DNA is semiconservative, involving a single origin of replication in prokaryotes and multiple origins of replication in eukaryotes. • Synthesis of new DNA occurs by copying an existing template through the addition of 5′-nucleoside monophosphate units to the 3′OH moiety of a primer sequence. • Many proteins are involved in the replication process, and the enzymatic activities are the same in prokaryotes and eukaryotes. • The high fidelity of DNA replication is the result of several different postreplicative editing activities that remove errors. • Any single DNA segment is usually replicated only once per cell cycle, and the assembly of chromatin after DNA replication in eukaryotes is a critical period for competition between transcription-control factors and histones. • Developmentally regulated deviations from the “once only per cell cycle” rule for DNA replication can lead to amplification of regions of the genome.

• Genetic variation is essential for the development of new plant varieties and animal breeds, and this can often be achieved by introducing chromosomes from relatively distant species by hybridization. • In plants, new species have been generated by the formation of amphiploids. • Additions or substitutions of alien chromosomes are important steps in gene transfers to a host genome. • Reciprocal translocations, or mutations that allow homoeologous pairing in polyploids, provide the basis for transferring foreign-chromosome segments to the host chromosomes. • Reliable means for identifying particular chromosome segments are essential for carrying out chromosome manipulations.

• Chromosome breakage can be caused by external factors such as radiation or certain chemicals, and by internal factors such as aging and transposable elements. • The efficient repair of damaged chromosomes is crucial for survival and utilizes enzymatic activities that recognize and remove lesions in DNA, carry out repair synthesis, or transfer homologous DNA from an undamaged chromosome to a damaged one. • Sister-chromatid exchanges involving unrepaired lesions are associated with aging and some disease syndromes in humans. • Certain regions of chromosomes, the fragile sites, are more susceptible to breakage than others. • Transposable elements are segments of DNA that are capable of moving from one place to another in the genome and, in so doing, they can cause extensive disruption to chromosome structure. • Gametocidal chromosomes induce breaks in other chromosomes during the mitotic divisions that occur in the postmeiotic maturation of gametes.

• The three-dimensional, double-helix structure of DNA has numerous variations depending on the sequence of internal base pairs and the presence of interacting external molecules. • Methylation of DNA is a chemical modification of the cytosine and adenosine residues that results in changes in the control of gene expression and DNA imprinting. • Proteins interacting with DNA to control gene expression have characteristic structures and distort the double helix upon binding. • The folding of DNA into chromosomes involves the formation of nucleosomes, which consist of a core of four different histones with DNA wrapped around the outside. • Further folding involves the ordering of nucleosomes into fibers and chromatin domains by the formation of loops, which are attached at their bases to a nuclear scaffold. • Single chromosomes tend to form domains within the nucleus and have nuclear-membrane attachment sites.

• Polyploids have more than two basic chromosome sets and they occur mainly in plants. • A major cause of spontaneous polyploidy in plants is unreduced gametes resulting from meiotic irregularities. • Doubling chromosome numbers in somatic cells by means of colchicine is the universal method for inducing polyploidy in plants. • The phenotypic consequences of polyploidy result from increased dosages of genes in each cell of all or some of the tissues of an organism, and they vary depending on whether the organism is autopolyploid or allopolyploid. • The diploidlike pairing in many natural allopolyploids is due to the presence of genes that prevent pairing between partially homologous chromosomes. • Gene segregations in polyploids depend on the relationships between the constituent genomes, and the meiotic behavior of the homologous or partially homologous chromosomes. • Genome analysis is based on quantitative analyses of meiotic pairing, genetic compensation of substituted chromosomes, and homoeo-logous relationships between genomes.

• Some organisms can survive with half the usual somatic chromosome number. • The smaller size of haploids is due to the reduction in the normal gene dosage of somatic cells. • Haploids arise spontaneously from irregularities in sexual reproduction and are induced by various methods such as chromosome elimination in wide crosses or in vitro anther culture. • Haploids are highly sterile because of the irregular meiotic behavior of unpaired chromosomes. • Doubled-haploid plants are genetically homozygous and provide an important basis for producing new varieties with greater efficiency.

• Eukaryotic somatic cells carry two copies of each chromosome, and meiosis must intervene to produce gametes with only a single copy of each chromosome. • During meiosis, homologous chromosomes pair and exchange segments (crossovers) between chromatids before separating and eventually entering different gametes. • The synaptonemal complex is a structure usually required for the intimate pairing that leads to crossing over between homologous pairs of chromosomes. • Chiasmata are the cytological manifestations of crossing over and are essential for regular chromosome behavior during meiosis. • The enzyme topoisomerase II is responsible for unraveling interlocked chromosome segments. • Many genes are involved in the successful completion of meiosis, and mutations in these genes disrupt specific stages of the process.

• The cell cycle is comprised of a chromosome-replication cycle occurring in concert with a cytoplasm-replication cycle. • The centrosomes determine the polarity of cell division, and centromeres (either well-defined chromosome regions or diffuse) play an active role in the movement of chromosomes to the poles. • The cell cycle is regulated by a series of protein phosphorylation and dephosphorylation reactions controlled by cell-division-cycle (cdc) genes. • Mutations in the cdc genes lead to uncontrolled cell growth and are the primary cause of many types of cancer.

• Euchromatin contains a relatively high density of actively transcribed genes, which can be observed cytologically in the bands of polytene chromosomes. • Heterochromatin has a condensed appearance cytologically and contains a low density of transcribed genes in an environment dominated by the presence of long, tandem arrays of DNA sequences. • The nucleolus-organizer region, or secondary constriction, defines the location of tandem arrays of ribosomal RNA genes. • The centromere, or primary constriction, is usually a well-defined region of the chromosome, to which microtubules attach during cell division. • Telomeres define the ends of the chromosomes and often consist of short arrays of tandemly repeated sequences, added postreplicatively to the DNA.

• Aneuploidy can be caused by meiotic irregularities, chromosome aberrations, aging, or environmental stresses. • The viability, fertility, and phenotypic effects of deleting or adding whole chromosomes varies among organisms. • Aneuploidy for chromosome arms results from misdivision of univalent centromeres during meiosis.

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is an important disease of wheat in cooler and humid geographic regions. It can cause significant yield losses. Three international nurseries, namely, 1st Australian Special Nursery (ASN), 22nd Semi-Arid Wheat Screening Nursery (SAWSN) and 12th High Temperature Wheat Yield Trial (HTWYT), were evaluated for seedling and adult plant response variation for stripe rust. A low proportion of these nurseries displayed stripe rust resistance in the seedling stage.Fifty one percent of 1st ASN, 54% 22nd SAWSN and 17% 12th HTWYT entries did not carry any seedling resistance genes. High level of APR to stripe rust was expressed in the field in all three nurseries. Stripe rust resistance genes Yr6,Yr7,Yr9, Yr17 and Yr27 were postulated either singly or in combinations across the three nurseries. Some entries carried these genes in combination with uncharacterised resistance. Based on Lr34 and Sr2 linked markers, Yr18 and Yr30 are inferred to be present in a high proportion of entries in all three nurseries. Due to the lack of robust marker, we were unable to postulate Yr29, this gene is however expected to be present in Pavon 76 derivatives. Results of this study can be used for strategic deployment of stripe rust resistance in new wheat cultivars.

The paradigm of a 1:1 relationship between cytological chiasmata and genetic crossovers is widely accepted (see Jones, 1987, for review). Extensive literature on meiosis will not be reviewed here, but John (1990) provides a good review of its various aspects. It was Darlington (1932, 1934), who compared chiasma frequencies with genetic map lengths in maize, both for individual chromosomes and the whole genome, and found a good correlation between the two estimators of recombination. With the recent explosion in genetic map construction in diverse organisms using DNA restriction fragment length polymorphisms (RFLPs), genetic maps based on RFLPs far exceed the length expected based on chiasma counts. Nilsson et al., (1993) summarized data from different crop plants documenting the discrepancy between the estimated number of crossovers from chiasma counts or genetic maps (Table 17.1). In every case, chiasma counts underestimated crossover frequencies calculated from recombination data.

The distribution of genes and recombination in the wheat genome was studied by comparing physical maps with the genetic linkage maps. The physical maps were generated by mapping 80 DNA and two phenotypic markers on an array of 65 deletion lines for homoeologous group 5 chromosomes. The genetic maps were constructed for chromosome 5B in wheat and 50 in Triticum tauschii. No marker mapped in the proximal 20% chromosome region surrounding the centromere. More than 60% of the long arm markers were present in three major clusters that physically encompassed

Whether the two tetraploid wheat species, the well known Triticum turgidum L. (macaroni wheat, AABB genomes) and the obscure T. timopheevii Zhuk. (A(t)A(t)GG), have monophyletic or diphyletic origin from the same or different diploid species presents an interesting evolutionary problem. Moreover, T. timopheevii and its wild form T. araraticum are an important genetic resource for macaroni and bread-wheat improvement. To study these objectives, the substitution and genetic compensation abilities of individual T. timopheevii chromosomes for missing chromosomes of T. aestivum 'Chinese Spring' (AABBDD) were analyzed. 'Chinese Spring' aneuploids (nullisomic-tetrasomics) were crossed with a T. timopheevii x Aegilops tauschii amphiploid to isolate T. timopheevii chromosomes in a monosomic condition. The F1 hybrids were backcrossed one to four times to Chinese Spring aneuploids without selection for the T. timopheevii chromosome of interest. While spontaneous substitutions involving all A(t)- and G-genome chromosomes were identified, the targeted T. timopheevii chromosome was not always recovered. Lines with spontaneous substitutions from T. timopheevii were chosen for further backcrossing. Six T. timopheevii chromosome substitutions were isolated: 6A(t) (6A), 2G (2B), 3G (3B), 4G (4B), 5G (5B) and 6G (6B). The substitution lines had normal morphology and fertility. The 6A(t) of T. timopheevii was involved in a translocation with chromosome 1G, resulting in the transfer of the group-1 gliadin locus to 6A(t). Chromosome 2G substituted for 2B at a frequency higher than expected and may carry putative homoeoalleles of gametocidal genes present on group-2 chromosomes of several alien species. Our data indicate a common origin for tetraploid wheat species, but from separate hybridization events because of the presence of a different spectrum of intergenomic translocations.

Genomic in situ hybridization to somatic metaphase chromosomes of hexaploid wheat cv. Chinese Spring using biotinylated maize genomic DNA as a probe revealed the existence of amplified maize DNA sequences in five pairs of chromosomes. The in situ hybridization sites were located on chromosomes 1A, 7A, 2B, 3B, and 7B. One pair of in situ hybridization sites was also observed in hexaploid oat. The locations and sizes of in situ hybridization sites varied among progenitor species.Key words: Triticum aestivum, Zea mays, shared DNA sequences, genomic in situ hybridization.

C-banding patterns of Hordeum chilense and of Triticum aestivum 'Chinese Spring' – H. chilense disomic addition lines were analyzed and compared with in situ hybridization patterns using a biotin-labeled highly repetitive Triticum tauschii DNA sequence, pAs1, and a wheat 18S–26S rDNA probe. All seven H. chilense chromosomes pairs and the added H. chilense chromosomes present in the addition lines were identified by their characteristic C-banding pattern. Chromosome morphology and banding patterns were similar to those of the corresponding chromosomes present in the parent H. chilense accession. A C-banded karyotype of the added H. chilense chromosomes was constructed and chromosome lengths, arm ratios, and relative length, as compared with chromosome 3B, were determined. The probe pAs1 was found to hybridize to specific areas on telomeres and interstitial sites along the chromosomes, allowing the identification of all seven pairs of the H. chilense chromosomes. Comparison of the patterns of distribution of the hybridization sites of clone pAs1 in the T. tauschii and H. chilense chromosomes was carried out by in situ hybridization on somatic metaphase chromosomes of the HchHchDD amphiploid. In situ hybridization using the 18S–26S rDNA probe confirmed that the H. chilense chromosomes 5Hchand 6Hchwere carrying nucleolus organizer regions. The results are discussed on the basis of phylogenetic relationships between D and Hchgenomes.Key words: Hordeum, Triticum, C-banding, in situ hybridization, phylogeny.

Fluorescence in situ hybridization (FISH) is a powerful tool for physical mapping in human and other mammalian species. However, application of the FISH technique has been limited in plant species, especially for mapping single- or low-copy DNA sequences, due to inconsistent signal production in plant chromosome preparations. Here we demonstrate that bacterial artificial chromosome (BAC) clones can be mapped readily on rice (Oryza sativa L.) chromosomes by FISH. Repetitive DNA sequences in BAC clones can be suppressed efficiently by using rice genomic DNA as a competitor in the hybridization mixture. BAC clones as small as 40 kb were successfully mapped. To demonstrate the application of the FISH technique in physical mapping of plant genomes, both anonymous BAC clones and clones closely linked to a rice bacterial blight-resistance locus, Xa21, were chosen for analysis. The physical location of Xa21 and the relationships among the linked clones were established, thus demonstrating the utility of FISH in plant genome analysis.

The C-banding technique was used to describe the chromosomes of a relatively recently-discovered Moroccan oat species, Avena agadiriana (2n=4x=28). A substantial amount of polymorphism for arm ratios and C-banding patterns was observed among five accessions of this species. However a common set of ten putatively homologous chromosomes was identifiable among the five accessions. The chromosomes of A. Agadiriana do not closely match those of any of the previously described diploid or tetraploid oat species in terms of their arm ratios and C-banding patterns. However, their overall C-banded appearance generally resembles the A/B/D groups of chromosomes of Avena species, rather than the more hetrochromatic C genomes. Implications of these findings in terms of chromosome evolution in the genus Avena are discussed.

Group 1 chromosomes of the Triticeae tribe have been studied extensively because many important genes have been assigned to them. In this paper, chromosome 1 linkage maps of Triticum aestivum, T. tauschii, and T. monococcum are compared with existing barley and rye maps to develop a consensus map for Triticeae species and thus facilitate the mapping of agronomic genes in this tribe. The consensus map that was developed consists of 14 agronomically important genes, 17 DNA markers that were derived from known-function clones, and 76 DNA markers derived from anonymous clones. There are 12 inconsistencies in the order of markers among seven wheat, four barley, and two rye maps. A comparison of the Triticeae group 1 chromosome consensus map with linkage maps of homoeologous chromosomes in rice indicates that the linkage maps for the long arm and the proximal portion of the short arm of group 1 chromosomes are conserved among these species. Similarly, gene order is conserved between Triticeae chromosome 1 and its homoeologous chromosome in oat. The location of the centromere in rice and oat chromosomes is estimated from its position in homoeologous group 1 chromosomes of Triticeae.Key words: Triticeae, RFLP, consensus, comparative.

Drought and high temperature often occurs simultaneously, causing significant yield losses in wheat (Triticum aestivum L.). The objectives of this study were to: (i) quantify independent and combined effects of drought and high temperature stress on synthetic hexaploid wheat genotypes at anthesis and at 21 days after anthesis; and (ii) determine whether responses to stress varied among genotypes. Four synthetic hexaploid and two spring wheat genotypes were grown from emergence to anthesis (Experiment I) and emergence to 21 days after anthesis (Experiment II), with full irrigation and 21/15°C day/night temperature. Thereafter, four treatments were imposed for 16 days as (a) optimum condition: irrigation + 21/15°C, (b) drought stress: withhold irrigation + 21/15°C, (c) high temperature stress: irrigation + 36/30°C and (d) combined stress: withhold irrigation + 36/30°C. Results indicated a decrease in leaf chlorophyll, individual grain weight and grain yield in an increasing magnitude of drought

We present a high density physical map of homoeologous group 7 chromosomes from Triticum aestivum L. using a series of 54 deletion lines, 6 random amplified polymorphic DNA (RAPD) markers and 91 cDNA or genomic DNA clones from wheat, barley and oat. So far, 51 chromosome segments have been distinguished by molecular markers, and 54 homoeoloci have been allocated among chromosomes 7A, 7B and 7D. The linear order of molecular markers along the chromosomes is almost identical in the A- B- and D-genome of wheat. In addition, there is colinearity between the physical and genetic maps of chromosomes 7A, 7B and 7D from T. aestivum, indicating gene synteny among the Triticeae. However, comparison of the physical map of chromosome 7D from T. aestivum with the genetic map from Triticum tauschii some markers have been shown to be physically allocated with distortion in more distal chromosome regions. The integration of genetic and physical maps could assist in estimating the frequency and distribution of recombination in defined regions along the chromosome. Physical distance did not correlate with genetic distance. A dense map facilitates the detection of multiple rearrangements. We present the first evidence for an interstitial inversion either on chromosome arm 7AS or 7DS of Chinese Spring. Molecularly tagged chromosome regions (MTCRs) provide landmarks for long-range mapping of DNA fragments.

Comparative genetic maps among the Triticeae or Gramineae provide the possibility for combining the genetics, mapping information and molecular-marker resources between different species. Dense genetic linkage maps of wheat and barley, which have a common array of molecular markers, along with deletion-based chromosome maps of Triticum aestivum L. will facilitate the construction of an integrated molecular marker-based map for the Triticeae. A set of 21 cDNA and genomic DNA clones, which had previously been used to map barley chromosome 1 (7H), were used to physically map wheat chromosomes 7A, 7B and 7D. A comparative map was constructed to estimate the degree of linkage conservation and synteny of chromosome segments between the group 7 chromosomes of the two species. The results reveal extensive homoeologies between these chromosomes, and the first evidence for an interstitial inversion on the short arm of a barley chromosome compared to the wheat homoeologue has been obtained. In a cytogenetically-based physical map of group 7 chromosomes that contain restriction-fragment-length polymorphic DNA (RFLP) and random amplified polymorphic DNA (RAPD) markers, the marker density in the most distal third of the chromosome arms was two-times higher than in the proximal region. The recombination rate in the distal third of each arm appears to be 8-15 times greater than in the proximal third of each arm where recombination of wheat chromosomes is suppressed.

An Aegilops cylindrica chromosome induces terminal deletions of chromosomes in wheat as identified by C-banding. We are constructing high-density physical maps of wheat chromosomes and have detected additional chromosome rearrangements. Among 63 lines with chromosomal subarm deletions in group 7 chromosomes, 7 lines (11.1%) were shown to harbor additional chromosome rearrangements. Two other lines were also omitted from the physical mapping because of the nature of the breakpoint calculations. The presence or absence of chromosome-specific restriction fragment length polymorphism (RFLP) or random amplified polymorphic DNA (RAPD) markers indicated that additional interstitial deletions are present in 3 lines (4.8%) with deletions in the short chromosome arms and in 4 lines (6.3%) with deletions in the long chromosome arms. We also used chromosome pairing analysis of F1 plants of deletion lines with double ditelosomic lines of ‘Chinese Spring’ wheat to detect small terminal deletions. The deletion of the most distal 1% of chromosome arm 7AL was associated with a pairing reduction of 60%.

Nonisotopic in situ hybridization (ISH) was introduced in plants in 1985. Since then the technique has been widely used in various areas of plant genome mapping. ISH has become a routine method for physical mapping of repetitive DNA sequences and multicopy gene families. ISH patterns on somatic metaphase chromosomes using tandemly repeated sequences provide excellent physical markers for chromosome identification. Detection of low or single copy sequences were also reported. Genomic in situ hybridization (GISH) was successfully used to analyze the chromosome structure and evolution of allopolyploid species. GISH also provides a powerful technique for monitoring chromatin introgession during interspecific hybridization. A sequential chromosome banding and ISH technique was developed. The sequential technique is very useful for more precise and efficient mapping as well as cytogenetic determination of genomic affinities of individual chromosomes in allopolyploid species. A critical review is made on the present resolution of the ISH technique and the future outlook of ISH research is discussed.Key words: in situ hybridization, physical mapping, genome mapping, molecular cytogenetics.

We have constructed cytologically based physical maps (CBPMs), depicting the chromosomal distribution of RFLP markers, of the group-2 chromosomes of common wheat (Triticum aestivum L. em Thell). Twenty-one homozygous deletion lines for 2A, 2B, and 2D were used to allocate RFLP loci to 19 deletion-interval regions. A consensus CBPM was colinearily aligned with a consensus genetic map of group-2 chromosomes. The comparison revealed greater frequency of recombination in the distal regions. Several molecularly tagged chromosome regions were identified which may be within the resolving power of pulsed-field gel electrophoresis. The CBPMs show that the available probes completely mark the group-2 chromosomes, and landmark loci for sub-arm regions were identified for targeted-mapping.

C-banding polymorphism was analyzed in 14 accessions of Triticum searsii from Israel, and a generalized idiogram of the species was established. One accession was homozygous for whole arm translocations T1S(s)S·4S(s)S and T1S(s)L·4S(s)L. C-banding analysis was also used to identify 7 T. aestivum cv 'Chinese Spring'-T. searsii disomic chromosome addition lines, 14 ditelosomic chromosome addition lines, 21 disomic whole chromosome, and 31 ditelosomic chromosome substitution lines. The identity of these lines was further confirmed by meiotic pairing analysis. Sporophytic and gametophytic compensation tests were used to determine the homoeologous relationships of the T. searsii chromosomes. The results show that the T. searsii chromosomes do not compensate well for their wheat homoeologues. The C-banding patterns of T. searsii chromosomes are distinct from those of other S-genome species and from the B-genome chromosomes of wheat, indicating that T. searsii is not a direct B-genome donor species of T. turgidum and T. aestivum.

Three new 18S.26S rRNA gene loci were identified in common wheat by sequential N-banding and in situ hybridization (ISH) analysis. Locus Nor-A7 is located at the terminal area of the long arm of 5A in both diploid and polyploid wheats. Locus Nor-B6 is located in N-band 1BL2.5 of the long arm of chromosome 1B in Triticum turgidum and Triticum aestivum. ISH sites, similar to Nor-B6, were also detected on the long arms of chromosomes 1G in Triticum timopheevii and 1S in Aegilops speltoides, but their locations on the chromosomes were different from that of Nor-B6, indicating possible chromosome rearrangements in 1GL and 1BL during evolution. The third new locus, Nor-D8, was only found on the short arm of chromosome 3D in the common wheat Wichita. The loss of rRNA gene locus Nor-A3 and gain of repetitive DNA sequence pSc119 on the terminal part of 5AS suggest a structural modification of 5AS. Comparative studies of the location of the 18S.26S rRNA gene loci in polyploid wheats and putative A and B (G) genome progenitor species support the idea that: (1) Triticum monococcum subsp. urartu is the donor of both the A and A(t) genome of polyploid wheats. (2) Ae. speltoides is closer to the B and G genome of polyploid wheats than Aegilops longissima and is the most probable progenitor of these two genomes.

Chromosome painting using multicolor fluorescence in situ hybridization showed that, in addition to the T1AL·1RS translocation derived from rye, a segment from chromosome 3Ae#1 of Agropyron elongatum (2n=10x =70), is present in Amigo wheat. The Agropyron chromosome segment is located on the satellite of chromosome 1B and the translocation chromosome is designated as T1BL·1BS-3Ae#1L. T1BL·1BS-3Ae#1L was inherited from Teewon wheat and carries resistance genes to stem rust (Sr24) and leaf rust (Lr24). The Agropyron chromosome segments in different Sr24/Lr24 carrier wheat lines, including Agent, TAP 48, TAP 67, Teewon, and Amigo, showed a diagnostic C-band, and were derived from the same chromosome, 3Ae#1.

A standard karyotype and a generalized idiogram of Triticum umbellulatum (syn. Aegilops umbellulata, 2n = 2x = 14) was established based on C-banding analysis of ten accessions of different geographic origin and individual T. umbellulatum chromosomes in T. aestivum - T. umbellulatum chromosome addition lines. Monosomic (MA) and disomic (DA) T. aestivum - T. umbellulatum chromosome addition lines (DA1U = B, DA2U = D, MA4U = F, DA5U = C, DA6U = A, DA7U = E = G) and telosomic addition lines (DA1US, DA1UL, DA2US, DA2UL, DA4UL, MA5US, (+ iso 5US), DA5UL, DA7US, DA7UL) were analyzed. Line H was established as a disomic addition line for the translocated wheat - T. umbellulatum chromosome T2DS·4US. Radiation-induced wheat - T. umbellulatum translocation lines resistant to leaf rust (Lr9) were identified as T40 = T6BL·6BS-6UL, T41 = T4BL·4BS-6UL, T44 = T2DS·2DL-6UL, T47 = 'Transfer' = T6BS·6BL-6UL and T52 = T7BL·7BS-6UL. Breakpoints and sizes of the transferred T. umbellulatum segments in these translocations were determined by in situ hybridization analysis using total genomic T. umbellulatum DNA as a probe.

The diploid-like chromosome pairing in polyploid wheat is controlled by the Ph1 (pairing homoeologous) gene that is located on chromosome arm 5BL. By using a combination of cytogenetic and molecular techniques, we report the physical location of the Ph1 gene to a submicroscopic chromosome region (Ph1 gene region) that is flanked by the breakpoints of two deletions (5BL-1 and ph1c) and is marked by a DNA probe (XksuS1). The Ph1 gene region is present distal to the breakpoint of deletion 5BL-1 but proximal to the C-band 5BL2.1. Two other DNA probes (Xpsr128 and Xksu75) flank the region-Xpsr128 being proximal and Xksu75 being distal. The estimated size of the region is less than 3 Mb. The chromosome region around the Ph1 gene is high in recombination as the genetic distance of the region between 5BL-1 breakpoint and C-band 5BL2.1 (not resolved by the microscope) is at least 9.3 cM.

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.

Three lines derived from wheat (6x) x Agropyron elongatum (10x) that are resistant to wheat streak mosaic virus (WSMV) were analyzed by chromosome pairing, banding, and in situ hybridization. Line CI15321 was identified as a disomic substitution line where wheat chromosome 1D is replaced by Ag. elongatum chromosome 1Ae-1. Line 87-94-1 is a wheat-Ag. elongatum ditelosomic addition 1Ae-1L. Line CI15322 contains an Ag. elongatum chromosome, 1Ae-2, that substitutes for chromosome 1D. The short arm of 1Ae-2 paired with the short arm of 1Ae-1 at metaphase I (MI) in 82% of the pollen mother cells (PMCs). However, the long arms of these two chromosomes did not pair with each other. In CI15322, the long arm of chromosome 4D has an Agropyron chromosome segment which was derived from the distal part of 1Ae-1L. This translocation chromosome is designated as T4DS·4DL-1L. T4DS·4DL-1Ae-1L has a 0.73 μm distal part of the long arm of 4D replaced by a 1.31 μm distal segment from 1Ae-1L. The major WSMV resistance gene(s) in these lines is located on the distal part of 1Ae-1L.

Wild relatives of wheat are useful sources of alien resistance genes for wheat breeding. The objective of this review is to document research on the evaluation, transfer, and molecular analysis of alien resistance to wheat leaf rust especially in Aegilops tauschii, the diploid D-genome donor of common wheat. Nine named resistance genes (Lr1, Lr2, Lr15, Lr21, Lr22, Lr32, Lr34, Lr39, and Lr42) occur in the D genome. Twelve new leaf rust resistance genes have been documented in Ae. tauschii. The south-west Caspian Sea region is the centre of genetic diversity for seedling resistance. Adult-plant resistance is widespread in all geographic regions and should be exploited more in the future. Lr1 and Lr21 have been cloned and are typical NBS-LRR genes. The recent documentation of cryptic introgressions of Lr57/Yr40 from Ae. geniculata and Lr58 from Ae. triuncialis offers exciting possibilities for transferring alien genes without linkage drag. Both Lr21 and Lr34 presumably arose during or following the origin of common wheat ~8000 years ago. Leaf rust resistance genes often are located towards the physical ends of wheat chromosomes. These regions are known to be high in recombination, and this may explain their rapid rate of evolution.

A new Hessian fly (Mayetiola destructor) resistance gene derived from ‘Balbo’ rye and its transfer to hexaploid wheat via radiation-induced terminal and intercalary chromosomal translocations are described. Crosses between resistant ‘Balbo’ rye and susceptible ‘Suwon 92’ wheat and between the F1 amphidiploids and susceptible ‘TAM 106’ and ‘Amigo’ wheats produced resistant BC2F3 lines that were identified by C-banding analysis as being 6RL telocentric addition lines. Comparative chromosomal analyses and resistance tests revealed that the resistance gene is located on the 6RL telocentric chromosome. X-irradiated pollen of 6RL addition plants was used to fertilize plants of susceptible wheats ‘TAM 106,’ ‘TAM 101,’ and ‘Vona.’ After several generations of selection for resistance, new sublines were obtained that were homogeneous for resistance. Thirteen of these lines were analyzed by C-banding, and three different wheat-6RL chromosomal translocations (T) were identified. Wheat chromosomes involved in the translocations were 6B, 4B, and 4A. Almost the complete 6RL arm is present in T6BS · 6BL-6RL. Only the distal half of 6RL is present in T4BS · 4BL-6RL, which locates the resistance gene in the distal half of 6RL. Only a very small segment (ca 1.0 μm) of the distal region of 6RL is present in an intercalary translocation (Ti) Ti4AS · 4AL-6RL-4AL. The 6RL segment is inserted in the intercalary region between the centromere of chromosome 4A and the large proximal C-band of 4AL. The break-points of the translocations are outside the region of the centromere, indicating that they were induced by the X-ray treatment. All three translocations are cytologically stable and can be used directly in wheat breeding programs.

From the beginning of the 20th Century, we have seen tremendous advances in knowledge and understanding in almost all biological disciplines, including genetics, molecular biology, structural and functional genomics, and biochemistry. Among these advances, cytogenetics has played an important role. This paper details some of the important milestones of modern cytogenetics. Included are the historical role of cytogenetics in genetic studies in general and the genetics stocks produced using cytogenetic techniques. The basic biological questions cytogenetics can address and the important role and practical applications of cytogenetics in applied sciences, such as in agriculture and in breeding for disease resistance in cereals, are also discussed. The goal of this paper is to show that cytogenetics remains important in the age of molecular genetics, because it is inseparable from overall genome analysis. Cytogenetics complements studies in other disciplines within the field of biology and provides the basis for linking genetics, molecular biology and genomics research.

Genome sequencing and the associated bioinformatics is now a widely accepted research tool for accelerating genetic research and the analysis of genome structure and function of wheat because it leverages similar work from other crops and plants. The International Wheat Genome Sequencing Consortium addresses the challenge of wheat genome structure and function and builds on the research efforts of Professor Bob McIntosh in the genetics of wheat. Currently, expressed sequence tags (ESTs; similar to 500 000 to date) are the largest sequence resource for wheat genome analyses. It is estimated that the gene coverage of the wheat EST collection is similar to 60%, close to that of Arabidopsis, indicating that similar to 40% of wheat genes are not represented in EST collections. The physical map of the D-genome donor species Aegilops tauschii is under construction ( http:// wheat. pw. usda. gov/ PhysicalMapping). The technologies developed in this analysis of the D genome provide a good model for the approach to the entire wheat genome, namely compiling BAC contigs, assigning these BAC contigs to addresses in a high resolution genetic map, filling in gaps to obtain the entire physical length of a chromosome, and then large-scale sequencing.

A total of 259 accessions of wild Triticum species originating from different countries, along with 91 triticale (6×)× bread wheat true-breeding derivatives, two bread wheat, and three triticale cultivars were screened for resistance to the Russian wheat aphid, a serious insect pest of the wheat crop. Twenty-four entries with low damage ratings on the basis of amount of leaf rolling and leaf chlorosis were retested along with resistant and susceptible controls. On the basis of leaf roll damage ratings, eight entries including four Triticum monococcum var. boeoticum (T. boeoticum), one T. monococcum var. monococcum (T. monococcum), two T. timopheevii var. araraticum (T. araraticum), and one triticale cultivar were significantly superior to ‘Karl’ (susceptible control) wheat. Among these, four accessions — three T. boeoticum and one T. araraticum— were significantly superior to all other entries and were equal to the resistant control (PI 372129) in resistance rating based on leaf rolling and leaf chlorosis (except T. boeoticum TA 202). The leaf chlorosis damage rating of all accessions were significantly lower than that of the susceptible check.