Extension of the Messapia x dicoccoides linkage map of Triticum turgidum (L.) Thell

A set of recombinant inbred lines (RIL) derived from a cross between the cultivar Messapia of durum wheat (Triticum turgidum var. durum) and the accession MG4343 of T. turgidum var. dicoccoides was analysed to increase the number of assigned markers and the resolution of the previously constructed genetic linkage map. An updated map of the durum wheat genome consisting of 458 loci was constructed. These loci include 261 Restriction Fragment Length Polymorphisms (RFLPs), 91 microsatellites (Simple Sequence Repeats, SSRs), 87 Amplified Fragment Length Polymorphisms (AFLPs), two ribosomal genes, and nine biochemical (seven seed storage proteins and two isozymes) and eight morphological markers. The loci were mapped on all 14 chromosomes of the A and B genomes, and covered a total distance of 3038.4 cM with an average distance of 6.7 cM between adjacent markers. The molecular markers were evenly distributed between the A and the B genomes (240 and 218 markers, respectively). An additional forty loci (8.8%) could not be assigned to a specific linkage group. A fraction (16.4%) of the markers significantly deviated from the expected Mendelian ratios; clusters of loci showing distorted segregation were found on the 1B, 2A, 2B, 3A, 4A, 7A and 7B chromosomes. The genetic lengths of the chromosomes range from 148.8 cM (chromosome 6B) to 318.0 cM (chromosome 2B) and approximately concur with their physical lengths. Chromosome 2B has the largest number of markers (47), while the chromosomes with the fewest markers are 3A and 6B (23). There are two gaps larger than 40 cM on chromosomes 2A and 3B. The durum wheat map was compared with the published maps of bread and durum wheats; the order of most common RFLP and SSR markers on the 14 chromosomes of the A and B genomes were nearly identical. A core-map can be extracted from the high-density Messapia x dicoccoides map and a subset of uniformly distributed markers can be used to detect and map quantitative trait loci.     

Retention of D genome chromosomes in pentaploid wheat crosses

The transfer of genes between Triticum aestivum (hexaploid bread wheat) and T. turgidum (tetraploid durum wheat) holds considerable potential for genetic improvement of both these closely related species. Five different T. aestivum/T. turgidum ssp. durum crosses were investigated using Diversity Arrays Technology (DArT) markers to determine the inheritance of parental A, B and D genome material in subsequent generations derived from these crosses. The proportions of A, B and D chromosomal segments inherited from the hexaploid parent were found to vary significantly among individual crosses. F(2) populations retained widely varying quantities of D genome material, ranging from 99% to none. The relative inheritance of bread wheat and durum alleles in the A and B genomes of derived lines also varied among the crosses. Within any one cross, progeny without D chromosomes in general had significantly more A and B genome durum alleles than lines retaining D chromosomes. The ability to select for and manipulate this non-random segregation in bread wheat/durum crosses will assist in efficient backcrossing of selected characters into the recurrent durum or hexaploid genotype of choice. This study illustrates the utility of DArT markers in the study of inter-specific crosses to commercial crop species.     

Identification of QTL and association of a phytoene synthase gene with endosperm colour in durum wheat

The yellow colour of durum wheat (Triticum turgidum L. var durum) semolina is due in part to the presence of carotenoid pigments found in the endosperm and is an important end-use quality trait. We hypothesized that variation in the genes coding for phytoene synthase (Psy), a critical enzyme in carotenoid biosynthesis, may partially explain the phenotypic variation in endosperm colour observed among durum cultivars. Using rice sequence information, primers were designed to PCR clone and sequence the Psy genes from Kofa (high colour) and W9262-260D3 (medium colour) durum cultivars. Sequencing confirmed the presence of four Psy genes in each parent, corresponding to a two member gene family designated as Psy1-1, Psy1-2 and Psy2-1 and Psy2-2. A genetic map was constructed using 155 F1-derived doubled haploid lines from the cross W9262-260D3/Kofa with 194 simple sequence repeat and DArT markers. Using Psy1-1 and Psy2-1 allele-specific markers and chromosome mapping, the Psy1 and Psy2 genes were located to the group 7 and 5 chromosomes, respectively. Four quantitative trait loci (QTL) underlying phenotypic variation in endosperm colour were identified on chromosomes 2A, 4B, 6B, and 7B. The Psy1-1 locus co-segregated with the 7B QTL, demonstrating an association of this gene with phenotypic variation for endosperm colour. This work is the first report of mapping Psy genes and supports the role of Psy1-1 in elevated levels of endosperm colour in durum wheat. This gene is a target for the further development of a molecular marker to enhance selection for endosperm colour in durum wheat breeding programs.     

Cytogenetics of Triticum x Dasypyrum hybrids and derived lines

Genomic in situ hybridization was used to study Triticum x Dasypyrum wide hybrids and derived lines. A cytogenetic investigation was carried out in progenies of (i) amphiploids derived from T. turgidum var. durum (T. durum; 2n = 14; genomes AABB) x D. villosum (2n = 14; genome VV), (ii) three-parental hybrids (T. durum x D. villosum) x T. aestivum (2n = 42, genomes A'A'B'B'D'D'), and (iii) T. aestivum aneuploid lines carrying D. villosum chromosomes or chromatin. The amphiploids derived from T. durum x D. villosum showed a stable chromosomal constitution, made up of 14 V chromosomes, 14 chromosomes carrying the wheat A genome and 14 chromosomes carrying the B genome. High karyological instability was observed in the progenies of three-parental hybrids ([T. durum x D. villosum] x T. aestivum). Plants having the expected 14 A chromosomes, 14 B chromosomes, 7 D chromosomes, and 7 V chromosomes were rather rare (4.5%). Many progeny plants (45.5%) had the hexaploid wheat genome with 42 chromosomes and lacked any detectable D. villosum chromatin. Other plants (50%) had 14 A chromosomes and 14 B chromosomes, plus variable numbers of D and V chromosomes, the former being better retained than the latter in most cases. Some T. aestivum lines carrying D. villosum chromosomes or chromatin, as the result of addition, substitution, or recombination events or even a combination of these karyological events, were found to be stable. Other lines were unstable, and these lines carried 1V, 3V, or 5V chromosomes or their portions. Substitution or recombination events where 1V chromosomes were involved could concern the homeologous counterparts in both the A and B and D genomes of wheat. No line could be recovered where the shorter arm of 3V chromosomes was present. Changes in the morphology and banding pattern of V chromosomes were observed in hybrids that did not carry the entire D. villosum complement. By comparing the results of our cytogenetic analyses with certain phenotypic characteristics of the lines studied, genes for discrete traits could be assigned to specific V chromosomes or V chromosome arms. From the frequency of V chromosomes that were involved in chromatin exchanges with or substituted for one of their homeologous counterparts in the A, B, and D wheat genomes, it was inferred that D. villosum belongs to the same phyletic lineage as T. urartu (donor of the A genome of wheat) and Aegilops speltoides (B genome), and that Ae. squarrosa (D genome) diverged earlier from D. villosum.     

Regulation of carotenoid biosynthesis during tomato development.

Phytoene synthase (Psy) and phytoene desaturase (Pds) are the first dedicated enzymes of the plant carotenoid biosynthesis pathway. We report here the organ-specific and temporal expression of PDS and PSY in tomato plants. Light increases the carotenoid content of seedlings but has little effect on PDS and PSY expression. Expression of both genes is induced in seedlings of the phytoene-accumulating mutant ghost and in wild-type seedlings treated with the Pds inhibitor norflurazon. Roots, which contain the lowest levels of carotenoids in the plant, have also the lowest levels of PDS and PSY expression. In flowers, expression of both genes and carotenoid content are higher in petals and anthers than in sepals and carpels. During flower development, expression of both PDS and PSY increases more than 10-fold immediately before anthesis. During fruit development, PSY expression increases more than 20-fold, but PDS expression increases less than threefold. We concluded that PSY and PDS are differentially regulated by stress and developmental mechanisms that control carotenoid biosynthesis in leaves, flowers, and fruits. We also report that PDS maps to chromosome 3, and thus it does not correspond to the GHOST locus, which maps to chromosome 11.     

Molecular characterization of a diagnostic DNA marker for domesticated tetraploid wheat provides evidence for gene flow from wild tetraploid wheat to hexaploid wheat

All forms of domesticated tetraploid wheat (Triticum turgidum, genomes AABB) are nearly monomorphic for restriction fragment length polymorphism (RFLP) haplotype a at the Xpsr920 locus on chromosome 4A (Xpsr920-A1a), and wild tetraploid wheat is monomorphic for haplotype b. The Xpsr920-A1a/b dimorphism provides a molecular marker for domesticated and wild tetraploid wheat, respectively. Hexaploid wheat (Triticum aestivum, genomes AABBDD) is polymorphic for the 2 haplotypes. Bacterial artificial chromosome (BAC) clones hybridizing with PSR920 were isolated from Triticum urartu (genomes AA), Triticum monococcum (genomes AmAm), and T. turgidum ssp. durum (genomes AABB) and sequenced. PSR920 is a fragment of a putative ATP binding cassette (ABC) transporter gene (designated ABCT-1). The wheat ABCT-1 gene is more similar to the T. urartu gene than to the T. monococcum gene and diverged from the T. urartu gene about 0.7 MYA. The comparison of the sequence of the wheat A genome BAC clone with that of the T. urartu BAC clone provides the first insight into the microsynteny of the wheat A genome with that of T. urartu. Within 103 kb of orthologous intergenic space, 37 kb of new DNA has been inserted and 36 kb deleted leaving 49.7% of the region syntenic between the clones. The nucleotide substitution rate in the syntenic intergenic space has been 1.6 x 10(-8) nt(-1) year(-1), which is, respectively, 4 and 3 times as great as nucleotide substitution rates in the introns and the third codon positions of the juxtaposed gene. The RFLP is caused by a miniature inverted transposable element (MITE) insertion into intron 18 of the ABCT-A1 gene. Polymerase chain reaction primers were developed for the amplification of the MITE insertion site and its sequencing. The T. aestivum ABCT-A1a haplotype is identical to the haplotype of domesticated tetraploid wheat, and the ABCT-A1b haplotype is identical to that of wild tetraploid wheat. This finding shows for the first time that wild tetraploid wheat participated in the evolution of hexaploid wheat. A cline of the 2 haplotype frequencies exists across Euro-Asia in T. aestivum. It is suggested that T. aestivum in eastern Asia conserved the gene pool of the original T. aestivum more than wheat elsewhere.     

Construction and characterization of a half million clone BAC library of durum wheat (<Emphasis Type="Italic">Triticum turgidum</Emphasis> ssp. <Emphasis Type="Italic">durum</Emphasis>)

Durum wheat (Triticum turgidum ssp. durum, 2n = 4x = 28, genomes AB) is an economically important cereal used as the raw material to make pasta and semolina. In this paper we present the construction and characterization of a bacterial artificial chromosome (BAC) library of tetraploid durum wheat cv. Langdon. This variety was selected because of the availability of substitution lines that facilitate the assignment of BACs to the A and B genome. The selected Langdon line has a 30-cM segment of chromosome 6BS from T. turgidum ssp. dicoccoides carrying a gene for high grain protein content, the target of a positional cloning effort in our laboratory. A total of 516,096 clones were organized in 1,344 384-well plates and blotted on 28 high-density filters. Ninety-eight percent of these clones had wheat DNA inserts (0.3% chloroplast DNA, 1.4% empty clones and 0.3% empty wells). The average insert size of 500 randomly selected BAC clones was 131 kb, resulting in a coverage of 5.1-fold genome equivalents for each of the two genomes, and a 99.4% probability of recovering any gene from each of the two genomes of durum wheat. Six known copy-number probes were used to validate this theoretical coverage and gave an estimated coverage of 5.8-fold genome equivalents. Screening of the library with 11 probes related to grain storage proteins and starch biosynthesis showed that the library contains several clones for each of these genes, confirming the value of the library in characterizing the organization of these important gene families. In addition, characterization of fingerprints from colinear BACs from the A and B genomes showed a large differentiation between the A and B genomes. This library will be a useful tool for evolutionary studies in one of the best characterized polyploid systems and a source of valuable genes for wheat. Clones and high-density filters can be requested at Communicated by P. LangridgeThe first two authors contributed equally to the investigation     

Chromosome sorting in tetraploid wheat and its potential for genome analysis

This study evaluates the potential of flow cytometry for chromosome sorting in durum wheat (Triticum turgidum Desf. var. durum, 2n = 4x = 28). Histograms of fluorescence intensity (flow karyotypes) obtained after the analysis of DAPI-stained chromosomes consisted of three peaks. Of these, one represented chromosome 3B, a small peak corresponded to chromosomes 1A and 6A, and a large peak represented the remaining 11 chromosomes. Chromosomes sorted onto microscope slides were identified after fluorescence in situ hybridization (FISH) with probes for GAA microsatellite, pSc119.2, and Afa repeats. Genomic distribution of these sequences was determined for the first time in durum wheat and a molecular karyotype has been developed for this crop. Flow karyotyping in double-ditelosomic lines of durum wheat revealed that the lines facilitated sorting of any arm of the wheat A- and B-genome chromosomes. Compared to hexaploid wheat, flow karyotype of durum wheat is less complex. This property results in better discrimination of telosomes and high purities in sorted fractions, ranging from 90 to 98%. We have demonstrated that large insert libraries can be created from DNA purified using flow cytometry. This study considerably expands the potential of flow cytogenetics for use in wheat genomics and opens the possibility of sequencing the genome of this important crop one chromosome arm at a time.     

Improving the yellow pigment content of bread wheat flour by selecting the three homoeologous copies of Psy1

The yellow pigment content (YPC) of endosperm affects the quality and nutritional value of wheat grain products. Major quantitative trait loci (QTL) for endosperm YPC have been repeatedly mapped on chromosomes 7A and 7B in durum and bread wheats. The genes coding for phytoene synthase (PSY1), which is involved in the biosynthesis of carotenoids, generally co-segregate with these QTL, indicating their role in determining YPC. Here, to study the genetic factors underlying endosperm YPC in bread wheat, the sequence polymorphism of the homoeologous A, B and D copies of genes coding for PSY1, Psy-A1, Psy-B1, and Psy-D1, was studied in a worldwide core collection, which was also phenotyped for flour YPC. Seven novel alleles of Psy-A1 and two novel alleles of Psy-B1 were detected, which confirms the high level of polymorphism of these genes. Two major QTL with respective candidate genes Psy-A1 and Psy-B1 were identified in the distal region of chromosomes 7A and 7B using progeny of a cross between Apache and Ornicar, high and low YPC cultivars, respectively. Association mapping confirms the role of these genes in YPC and shows that the D copy also significantly influences this trait. These results indicate that breeders need to consider all three Psy1 copies when seeking to improve the YPC of wheat endosperm.     

Maize phytoene desaturase and zeta-carotene desaturase catalyse a poly-Z desaturation pathway: implications for genetic engineering of carotenoid content among cereal crops

Carotene desaturation, an essential step in the biosynthesis of coloured carotenoids, has received much attention (1) as a target of bleaching herbicide action, (2) as a determinant of geometric isomer states of carotenoids and their metabolites, and (3) as a key modulator of accumulation and structural variability of carotenoids. Having previously isolated and functionally characterized the cDNA encoding the first enzyme in maize carotene desaturation, phytoene desaturase (PDS), the isolation and functional characterization of the second desaturase, a maize endosperm cDNA (2265 bp) encoding zetacarotene (zeta-carotene) desaturase (ZDS) is reported here. Functional analysis of the concerted actions of maize PDS and ZDS ex situ showed these enzymes to mediate a poly-Z desaturation pathway to the predominate geometric isomer 7,9,7',9'-tetra-Z-lycopene (poly-Z-lycopene or prolycopene), and not the all-trans substrate required of the downstream lycopene cyclase enzymes. This finding suggests a rate-controlling isomerase associated with the carotene desaturases as a corollary of a default poly-Z carotenoid biosynthetic pathway active in planta for maize. Comparative gene analysis between maize and rice revealed that genes encoding PDS and ZDS are single copy; the Zds cDNA characterized here was mapped to maize chromosome 7S and vp9 is suggested as a candidate locus for the structural gene while y9 is ruled out. Classical genetic resources were used to dissect the desaturation steps further and hydroxyphenylpyruvate dioxygenase was linked to the vp2 locus, narrowing candidate loci for an obligate isomerase in maize to only a few. Since the first functional analysis of the paired carotene desaturases for a cereal crop is reported here, the implications for the genetic modification of the pro-vitamin A content in cereal crops such as rice and maize, are discussed.     

Rapid genome evolution revealed by comparative sequence analysis of orthologous regions from four triticeae genomes

Bread wheat (Triticum aestivum) is an allohexaploid species, consisting of three subgenomes (A, B, and D). To study the molecular evolution of these closely related genomes, we compared the sequence of a 307-kb physical contig covering the high molecular weight (HMW)-glutenin locus from the A genome of durum wheat (Triticum turgidum, AABB) with the orthologous regions from the B genome of the same wheat and the D genome of the diploid wheat Aegilops tauschii (Anderson et al., 2003; Kong et al., 2004). Although gene colinearity appears to be retained, four out of six genes including the two paralogous HMW-glutenin genes are disrupted in the orthologous region of the A genome. Mechanisms involved in gene disruption in the A genome include retroelement insertions, sequence deletions, and mutations causing in-frame stop codons in the coding sequences. Comparative sequence analysis also revealed that sequences in the colinear intergenic regions of these different genomes were generally not conserved. The rapid genome evolution in these regions is attributable mainly to the large number of retrotransposon insertions that occurred after the divergence of the three wheat genomes. Our comparative studies indicate that the B genome diverged prior to the separation of the A and D genomes. Furthermore, sequence comparison of two distinct types of allelic variations at the HMW-glutenin loci in the A genomes of different hexaploid wheat cultivars with the A genome locus of durum wheat indicates that hexaploid wheat may have more than one tetraploid ancestor.     

Molecular linkage map for an intraspecific recombinant inbred population of durum wheat (Triticum turgidum L. var. durum)

Durum wheat (Triticum turgidum L. var. durum) is an economically and nutritionally important cereal crop in the Mediterranean region. To further our understanding of durum genome organization we constructed a durum linkage map using restriction fragment length polymorphisms (RFLPs), simple sequence repeats (SSRs) known as Gatersleben wheat microsatellites (GWMs), amplified fragment length polymorphisms (AFLPs), and seed storage proteins (SSPs: gliadins and glutenins). A population of 110 F₉ recombinant inbred lines (RILs) was derived from an intraspecific cross between two durum cultivars, Jennah Khetifa and Cham 1. The two parents exhibit contrasting traits for resistance to biotic and abiotic stresses and for grain quality. In total, 306 markers have been placed on the linkage map – 138 RFLPs, 26 SSRs, 134 AFLPs, five SSPs, and three known genes (one pyruvate decarboxylase and two lipoxygenases). The map is 3598 cM long, with an average distance between markers of 11.8 cM, and 12.1% of the markers deviated significantly from the expected Mendelian ratio 1:1. The molecular markers were evenly distributed between the A and B genomes. The chromosome with the most markers is 1B (41 markers), followed by 3B and 7B, with 25 markers each. The chromosomes with the fewest markers are 2A (11 markers), 5A (12 markers), and 4B (15 markers). In general, there is a good agreement between the map obtained and the Triticeae linkage consensus maps. This intraspecific map provides a useful tool for marker-assisted selection and map-based breeding for resistance to biotic and abiotic stresses and for improvement of grain quality. Received: 14 February 2000 / Accepted: 28 April 2000     

A new reduced height gene found in the tetraploid semi-dwarf wheat landrace Aiganfanmai

Aiganfanmai is a dwarf tetraploid wheat landrace (Triticum turgidum var. turgidum) that stably produces the semi-dwarf trait. Plant height varies from 80-105 cm under cultivation. Compared with tall durum wheat (T. turgidum var. durum) variety Langdon, we found it to have short spikes and seeds, besides a semi-dwarf character. We crossed Aiganfanmai with Langdon to analyze the genetic basis of the semi-dwarf trait. The F(2) population segregated at a 1:3 ratio for the short trait to the normal, which demonstrates that Aiganfanmai carries a recessive reduced height (Rht) gene. This gene was found to be located between the molecular markers Xgwm471 and Xgwm350 on chromosome arm 7AS by microsatellite analysis. No Rht gene had been reported from this chromosome; we designated it as Rht22. Rht 22, unlike other previously reported Rht genes, does not reduce internodal cell length. Reduced cell numbers might explain the short stem trait.     

Dynamics and differential proliferation of transposable elements during the evolution of the B and A genomes of wheat

Transposable elements (TEs) constitute >80% of the wheat genome but their dynamics and contribution to size variation and evolution of wheat genomes (Triticum and Aegilops species) remain unexplored. In this study, 10 genomic regions have been sequenced from wheat chromosome 3B and used to constitute, along with all publicly available genomic sequences of wheat, 1.98 Mb of sequence (from 13 BAC clones) of the wheat B genome and 3.63 Mb of sequence (from 19 BAC clones) of the wheat A genome. Analysis of TE sequence proportions (as percentages), ratios of complete to truncated copies, and estimation of insertion dates of class I retrotransposons showed that specific types of TEs have undergone waves of differential proliferation in the B and A genomes of wheat. While both genomes show similar rates and relatively ancient proliferation periods for the Athila retrotransposons, the Copia retrotransposons proliferated more recently in the A genome whereas Gypsy retrotransposon proliferation is more recent in the B genome. It was possible to estimate for the first time the proliferation periods of the abundant CACTA class II DNA transposons, relative to that of the three main retrotransposon superfamilies. Proliferation of these TEs started prior to and overlapped with that of the Athila retrotransposons in both genomes. However, they also proliferated during the same periods as Gypsy and Copia retrotransposons in the A genome, but not in the B genome. As estimated from their insertion dates and confirmed by PCR-based tracing analysis, the majority of differential proliferation of TEs in B and A genomes of wheat (87 and 83%, respectively), leading to rapid sequence divergence, occurred prior to the allotetraploidization event that brought them together in Triticum turgidum and Triticum aestivum, <0.5 million years ago. More importantly, the allotetraploidization event appears to have neither enhanced nor repressed retrotranspositions. We discuss the apparent proliferation of TEs as resulting from their insertion, removal, and/or combinations of both evolutionary forces.     

Genetic studies of leaf and stem rust resistance in six accessions of Triticum turgidum var. dicoccoides.

Six accessions of Triticum turgidum var. dicoccoides L. (4x, AABB) of diverse origin were tested with 10 races of leaf rust (Puccinia recondita f.sp. tritici Rob. ex Desm.) and 10 races of stem rust (P. graminis f.sp. tritici Eriks. &Henn.). Their infection type patterns were all different from those of lines carrying the Lr or Sr genes on the A or B genome chromosomes with the same races. The unique reaction patterns are probably controlled by genes for leaf rust or stem rust resistance that have not been previously identified. The six dicoccoides accessions were crossed with leaf rust susceptible RL6089 durum wheat and stem rust susceptible 'Kubanka' durum wheat to determine the inheritance of resistance. They were also crossed in diallel to see whether they carried common genes. Seedlings of F1, F2, and BC1F2 generations from the crosses of the dicoccoides accessions with RL6089 were tested with leaf rust race 15 and those from the crosses with 'Kubanka' were tested with stem rust race 15B-1. The F2 populations from the diallel crosses were tested with both races. The data from the crosses with the susceptible durum wheats showed that resistance to leaf rust race 15 and stem rust race 15B-1 in each of the six dicoccoides accessions is conferred by a single dominant or partially dominant gene. In the diallel crosses, the dominance of resistance appeared to be affected by different genetic backgrounds. With one exception, the accessions carry different resistance genes: CI7181 and PI 197483 carry a common gene for resistance to leaf rust race 15. Thus, wild emmer wheat has considerable genetic diversity for rust resistance and is a promising source of new rust resistance genes for cultivated wheats.     

Synthesis and cytological characterization of trigeneric hybrids of durum wheat with and without Ph1.

Wild grasses in the tribe Triticeae, some in the primary or secondary gene pool of wheat, are excellent reservoirs of genes for superior agronomic traits, including resistance to various diseases. Thus, the diploid wheatgrasses Thinopyrum bessarabicum (Savul. and Rayss) A. Love (2n = 2x = 14; JJ genome) and Lophopyrum elongatum (Host) A. Love (2n = 2x = 14; EE genome) are important sources of genes for disease resistance, e.g., Fusarium head blight resistance that may be transferred to wheat. By crossing fertile amphidiploids (2n = 4x = 28; JJEE) developed from F1 hybrids of the 2 diploid species with appropriate genetic stocks of durum wheat, we synthesized trigeneric hybrids (2n = 4x = 28; ABJE) incorporating both the J and E genomes of the grass species with the durum genomes A and B. Trigeneric hybrids with and without the homoeologous-pairing suppressor gene, Ph1, were produced. In the absence of Ph1, the chances of genetic recombination between chromosomes of the 2 useful grass genomes (JE) and those of the durum genomes (AB) would be enhanced. Meiotic chromosome pairing was studied using both conventional staining and fluorescent genomic in situ hybridization (fl-GISH). As expected, the Ph1-intergeneric hybrids showed low chromosome pairing (23.86% of the complement), whereas the trigenerics with ph1b (49.49%) and those with their chromosome 5B replaced by 5D (49.09%) showed much higher pairing. The absence of Ph1 allowed pairing and, hence, genetic recombination between homoeologous chromosomes. Fl-GISH analysis afforded an excellent tool for studying the specificity of chromosome pairing: wheat with grass, wheat with wheat, or grass with grass. In the trigeneric hybrids that lacked chromosome 5B, and hence lacked the Ph1 gene, the wheat-grass pairing was elevated, i.e., 2.6 chiasmata per cell, a welcome feature from the breeding standpoint. Using Langdon 5D(5B) disomic substitution for making trigeneric hybrids should promote homoeologous pairing between durum and grass chromosomes and hence accelerate alien gene transfer into the durum genomes.     

The effects of chromosomes 3A and 3B on resistance to Fusarium head blight in tetraploid wheat

Fusarium head blight (FHB) caused by Fusarium graminearum is one of the most destructive diseases of wheat in areas where the weather is warm and humid after heading. Previous studies indicate that the level of resistance to FHB varies not only among wheat cultivars but also among some of their wild relatives. No accession, however, has yet been identified to be completely immune to FHB among the Gramineae. It is known that durum wheat (Triticum turgidum L. conv. durum) is consistently more susceptible to FHB than common wheat (T. aestivum L.). The importance of the D genome in conferring resistance to FHB has been emphasized. Meanwhile, recent studies using molecular markers report effective QTLs on chromosome 3BS in a hexaploid population and on 3A in tetraploid recombinant inbred chromosome lines. In this study, we performed an evaluation of the effects of homoeologous group 3 chromosomes of T. turgidum ssp. dicoccoides on resistance to FHB using a set of chromosome substitution lines of a durum wheat cultivar 'Langdon'. The accession of T. turgidum ssp. dicoccoides examined in this study was more susceptible for Type II resistance (resistance to spread of FHB in the head) than 'Langdon'. Both of the chromosome substitution lines of 3A and 3B showed the same level of resistance with 'Langdon', but bleaching of the heads was completely prevented in the substitution lines of chromosome 3A without relationship to rachis fragility. It was concluded that the chromosome 3A of T. turgidum ssp. dicoccoides carries resistance gene(s) to head bleaching caused by FHB.