J. P. Gustafson

Translocations and modifications of chromosomes in triticale × wheat hybrids

SummarySeveral generations of four triticale × wheat populations were cytologically analyzed on a plant-by-plant basis using C-banding. Among 785 karyotyped plants, 195 wheat/rye and 64 rye/rye translocated chromosomes were found, as well as 15 rye chromosomes that were modified by deletion or amplification of telomeric heterochromatin. Most of the translocations involved complete chromosome arms; only a few involved smaller segments of chromosomes. Out of 39 identified wheat/rye translocations, 10 occurred between homoeologous and 29 between non-homoelogous chromosomes, five involved A-genome chromosomes, six B-genome chromosomes and the remaining 28 involved D-genome chromosomes. The study indicated that wheat/rye translocations can be produced in sufficient numbers to allow the use of this method for the introduction of alien variation into wheat research programs. Changes in the C-banding technique used are discussed in detail.

Genome evolution of allopolyploids: a process of cytological and genetic diploidization

Allopolyploidy is a prominent mode of speciation in higher plants. Due to the coexistence of closely related genomes, a successful allopolyploid must have the ability to invoke and maintain diploid-like behavior, both cytologically and genetically. Recent studies on natural and synthetic allopolyploids have raised many discrepancies. Most species have displayed non-Mendelian behavior in the allopolyploids, but others have not. Some species have demonstrated rapid genome changes following allopolyploid formation, while others have conserved progenitor genomes. Some have displayed directed, non-random genome changes, whereas others have shown random changes. Some of the genomic changes have appeared in the F1 hybrids, which have been attributed to the union of gametes from different progenitors, while other changes have occurred during or after genome doubling. Although these observations provide significant novel insights into the evolution of allopolyploids, the overall mechanisms of the event are still elusive. It appears that both genetic and epigenetic operations are involved in the diploidization process of allopolyploids. Overall, genetic and epigenetic variations are often associated with the activities of repetitive sequences and transposon elements. Specifically, genomic sequence elimination and chromosome rearrangement are probably the major forces guiding cytological diploidization. Gene non-functionalization, sub-functionalization, neo-functionalization, as well as other kinds of epigenetic modifications, are likely the leading factors promoting genetic diploidization.

An ALMT1 gene cluster controlling aluminum tolerance at the Alt4 locus of rye (Secale cereale L).

Aluminum toxicity is a major problem in agriculture worldwide. Among the cultivated Triticeae, rye (Secale cereale L.) is one of the most Al tolerant and represents an important potential source of Al tolerance for improvement of wheat. The Alt4 Al-tolerance locus of rye contains a cluster of genes homologous to the single-copy Al-activated malate transporter (TaALMT1) Al-tolerance gene of wheat. Tolerant (M39A-1-6) and intolerant (M77A-1) rye haplotypes contain five and two genes, respectively, of which two (ScALMT1-M39.1 and ScALMT1-M39.2) and one (ScALMT1-M77.1) are highly expressed in the root tip, typically the main site of plant Al tolerance/susceptibility. All three transcripts are upregulated by exposure to Al. High-resolution genetic mapping identified two resistant lines resulting from recombination within the gene cluster. These recombinants exclude all genes flanking the gene cluster as candidates for controlling Alt4 tolerance, including a homolog of the barley HvMATE Al-tolerance gene. In the recombinants, one hybrid gene containing a chimeric open reading frame and the ScALMT1-M39.1 gene each appeared to be sufficient to provide full tolerance. mRNA splice variation was observed for two of the rye ALMT1 genes and in one case, was correlated with a ∼400-bp insertion in an intron.

Variation at the Nor loci in triticale derived from tissue culture.

SummaryPlants derived from tissue cultures of six triticale genotypes were the subject of an analysis for changes in the rRNA genes located at the site of nucleolar organizer regions (the Nor loci) on chromosomes 1B, 6B and 1R. In addition whole plant phenotypes and the chromosomal constitutions of their progenies were examined for alterations. Following treatment of DNA with the restriction endonuclease Taq1, it was possible to assign electrophoretic bands representing rDNA spacer sequences to each of the chromosomes known to carry a major Nor locus. In general, the rRNA genes were found to be stable except in one family where a marked reduction in the number of rDNA units was observed. This reduction in 1R rDNA spacer sequences was heritable and correlated with reduced C-banding at the position of Nor-R1 on chromosome 1R. The change was clearly a consequence of tissue culture since six other plants regenerated from the same culture, and the original parent, did not carry the alteration.

AFLP markers tightly linked to the aluminum-tolerance gene Alt3 in rye (Secale cereale L.)

Abstract Rye (Secale cereale L.) is considered to be the most aluminum (Al)-tolerant species among the Triticeae. It has been suggested that aluminum tolerance in rye is controlled by three major genes (Alt genes) located on rye chromosome arms 3RL, 4RL, and 6RS, respectively. Screening of an F6 rye recombinant inbred line (RIL) population derived from the cross between an Al-tolerant rye (M39A-1–6) and an Al-sensitive rye (M77A-1) showed that a single gene controls aluminum tolerance in the population analyzed. In order to identify molecular markers tightly linked to the gene, we used a combination of amplified fragment length polymorphism (AFLP) and bulked segregant analysis techniques to evaluate the F6 rye RIL population. We analyzed approximately 22,500 selectively amplified DNA fragments using 204 primer combinations and identified three AFLP markers tightly linked to the Alt gene. Two of these markers flanked the Alt locus at distance of 0.4 and 0.7 cM. Chromosomal localization using cloned AFLP and a restriction fragment length polymorphism (RFLP) marker indicated that the gene was on the long arm of rye chromosome 4R. The RFLP marker (BCD1230) co-segregated with the Alt gene. Since the gene is on chromosome 4R, the gene was designated as Alt3. These markers are being used as a starting point in the construction of a high resolution map of the Alt3 region in rye.

Molecular linkage mapping in rye (Secale cereale L.)

Abstract A rye linkage map containing clones from rye, wheat, barley, oat and rice genomic and cDNA libraries, known-function genes and microsatellite markers, was created using an F2 population consisting of 110 F2-derived F3 families. Both co-dominant and dominant markers were added to the map. Of all probes screened, 30.8% were polymorphic, and of those polymorphic 79.3% were mapped. The current map contains 184 markers present in all seven linkage groups covering only 727.3 cM. This places a marker about every 3.96 cM on average throughout the map; however, large gaps are still present. The map contains 60 markers that have been integrated from previous rye maps. Surprisingly, no markers were placed between the centromere and C1–1RS in the short arm of 1R. The short arm of chromosome 4 also lacked an adequate number of polymorphic markers. The population showed a remarkable degree of segregation distortion (72.8%). In addition, the genetic distance observed in rye was found to be very different among the maps created by different mapping populations.

Analysis of phylogenetic relationships in the triticeae tribe using RFLPs

The use of restriction fragment length polymorphisms in combination with other approaches is very useful for the reconstruction of evolutionary events revealing phylogenetic relationships. A set of 21 cDNA probes hybridizing to different chromosome arms in hexaploid wheat was used in a series of experiments designed to estimate the phylogenetic relationships among and within 16 species of the Triticeae tribe. A high degree of polymorphism was found both between and within the species examined. The RFLP data were used to generate a cladogram and a phenogram in order to compare the two different methods of constructing phylogenetic trees. The results of both methods were consistent with each other and with the general taxonomic information provided by earlier morphological studies, meiotic pairing analysis, isozyme tests, and sequence alignment in theTer,NOR, and5s DNA loci. In addition, several correlations were found between the geographical origin of accessions from the same species and their phylogenetic relationships as shown by the cladogram and phenogram.

Direct amplification of minisatellite-region DNA with VNTR core sequences in the genus Oryza

Abstract A polymerase chain reaction (PCR) application, involving the directed amplification of minisatellite-region DNA (DAMD) with several minisatellite core sequences as primers, was used to detect genetic variation in 17 species of the genus Oryza and several rice cultivars (O. sativa L.). The electrophoretic analysis of DAMD-PCR products showed high levels of variation between different species and little variation between different cultivars of O. sativa. Polymorphisms were also found between accessions within a species, and between individual plants within an accession of several wild species. The DAMD-PCR yielded genome-specific banding patterns for the species studied. Several DAMD-PCR-generated DNA fragments were cloned and characterized. One clone was capable of detecting multiple fragments and revealed individual-specific hybridization banding patterns using genomic DNA from wild species as well as rice cultivars. A second clone detected only a single polymorphic locus, while a third clone expressed a strong genome specificity by Southern analysis. The results demonstrated that DAMD-PCR is potentially useful for species and genome identification in Oryza. The DAMD-PCR technique also allows for the isolation of informative molecular probes to be utilized in DNA fingerprinting and genome identification in rice.

Structural variation in the heterochromatin of rye chromosomes in triticales

SummaryAlthough Giemsa C-banding techniques have been used extensively for assaying cereal heterochromatin, a more specific technique for analyzing cereal heterochromatin has been developed recently with the isolation of DNA sequences present in heterochromatin and their employment in in situ hybridization to cereal chromosomes. A number of triticales were examined for the occurrence of modified rye chromosomes using the in situ hybridization technique. With a heterogeneous sequence probe the amount of rye heterochromatin appears to be relatively constant in wheat backgrounds but when a specific sequence probe was employed variation was observed. Whether this variation reflects polymorphism in rye or whether it is a result of adaption of the rye genome to coexistence with the wheat genome in triticales is discussed. — The triticale Rosner was examined in detail and it was established that the rye chromosome 2R had been replaced by the wheat chromosome 2D.

Repetitive, genome-specific probes in wheat (Triticum aestivum L. em Thell) amplified with minisatellite core sequences

The detection and analysis of DNA polymorphisms in crops is an essential component of marker-assisted selection and cultivar identification in plant breeding. We have explored the direct amplification of minisatellite DNA by PCR (DAMD-PCR) as a means for generating DNA probes that are useful for detecting DNA polymorphisms and DNA fingerprinting in wheat. This technique was facilitated by high-stringency PCR with known plant and animal minisatellite core sequences as primers on wheat genomic DNA. The products of DAMD-PCR from Triticum aestivum, T. durum, T. monococcum, T. speltoides and T. tauschii showed a high degree of polymorphism and the various genomes could be identified. Cloning of the DAMD-PCR products and subsequent Southern hybridization frequently revealed polymorphic probes showing a good degree of genome specificity. In addition, polymorphic, single locus, and moderately dispersed PCR products were cloned that may have a potential for DNA fingerprinting. Our experiments were limited primarily to diploid wheats and the results indicated that DAMD-PCR may isolate genome-specific probes from wild diploid wheat species that could be used to monitor genome introgression into hexaploid wheat.