Mapping of esterase loci in <Emphasis Type="Italic">Aegilops uniaristata</Emphasis> and homoeologous group 3 chromosomes of wheat

This study was planned to identify the chromosomal location of esterase loci in wheat (Triticum aestivum), in comparison to Aegilops uniaristata, using wheat Ae. uniaristata disomic addition and translocation lines. Two loci (Est-N1 and Est-N8) were identified on 3N chromosome of Ae. uniaristata and their probable homoeoloci were, for the first time, mapped close to three RFLP probes (Xpsr56, Xpsr394, and Xpsr1196) on homoeologous group 3 wheat chromosomes.     

Physical molecular maps of wheat chromosomes

In bread wheat, a set of 527 simple sequence repeats (SSRs) were tried on 164 deletion lines, leading to a successful mapping of 270 SSRs on 313 loci covering all 21 chromosomes. A maximum of 119 loci (38%) were located on B subgenome, and a minimum of 90 loci (29%) mapped on D subgenome. Similarly, homoeologous group 7 carried a maximum of 61 loci (19%), and group 4 carried a minimum of 22 loci (7%). Of the cited 270 SSRs, 39 had multiple loci, but only eight of these detected homoeologous loci. Linear order of loci in physical maps largely corresponded with those in the genetic maps. Apparently, distances between each of only 26 pairs of loci significantly differed from the corresponding distances on genetic maps. Some loci, which were genetically mapped close to the centromere, were physically located distally, while other loci that were mapped distally in the genetic maps were located in the proximal bins in the physical maps. This suggested that although the linear order of the loci was largely conserved, variation does exist between genetic and physical distances.Electronic Supplementary Material Supplementary material is available for this article at .     

Multiple genetic pathways for seed shattering in the grasses

Shattering is an essential seed dispersal mechanism in wild species. It is believed that independent mutations at orthologous loci led to convergent domestication of cereal crops. To investigate genetic relationships of Triticeae shattering genes with those of other grasses, we mapped spike-, barrel- (B-type), and wedge-type (W-type) spikelet disarticulation genes in wheat and its wild relatives. The Br1 gene for W-type disarticulation was mapped to a region delimited by Xpsr598 and Xpsr1196 on the short arm of chromosomes 3A in Triticum timopheevii and 3S in Aegilops speltoides. The spike- and W-type disarticulation genes are allelic at Br1 in Ae. speltoides. The B-type disarticulation gene, designated as Br2, was mapped to an interval of 4.4 cM between Xmwg2013 and Xpsr170 on the long arm of chromosome 3D in Aegilops tauschii, the D-genome donor of common wheat. Therefore, B- and W-type disarticulations are governed by two different orthologous loci on group-3 chromosomes. Based on map position, orthologs of Br1 and Br2 were not detected in barley, maize, rice, and sorghum, indicating multiple genetic pathways for shattering in grasses. The implications of the mapping results are discussed with regard to the evolution of polyploid wheat and domestication of cereals.Supplementary material is available in the online version of this article at     

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.     

Mapping of a BYDV resistance gene fromThinopyrum intermedium in wheat background by molecular markers

The wheat line H960642 is a homozygous wheat-Thinopyrum intermedium translocation line with resistance to BYDV by genomicin situ hybridization (GISH) and RFLP analysis. The genomic DNA ofTh. intermedium was used as a probe, and common wheat genomic DNA as a blocking in GISH experiment. The results showed that the chromosome segments ofTh. intermedium were transferred to the distal end of a pair of wheat chromosomes. RFLP analysis indicated that the translocation line H960642 is a T7DS-7DL-7XL translocation by using 8 probes mapped on the homoeologous group 7 in wheat. The translocation breakpoint is located between Xpsr680 and Xpsr965 about 90–99 cM from the centromere. The RFLP markers psr680 and psr687 were closely linked with the BYDV resistance gene. The gene is located on the distal end of 7XL around Xpsr680 and Xpsr687.     

Genetic and physical mapping of new EST-derived SSRs on the A and B genome chromosomes of wheat

The availability of genetic maps and phenotypic data of segregating populations allows to localize and map agronomically important genes, and to identify closely associated molecular markers to be used in marker-assisted selection and positional cloning. The objective of the present work was to develop a durum wheat intervarietal genetic and physical map based on genomic microsatellite or genomic simple sequence repeats (gSSR) markers and expressed sequence tag (EST)-derived microsatellite (EST-SSR) markers. A set of 122 new EST-SSR loci amplified by 100 primer pairs was genetically mapped on the wheat A and B genome chromosomes. The whole map also comprises 149 gSSR markers amplified by 120 primer pairs used as anchor chromosome loci, two morphological markers (Black colour, Bla1, and spike glaucousness, Ws) and two seed storage protein loci (Gli-A2 and Gli-B2). The majority of SSR markers tested (182) was chromosome-specific. Out of 275 loci 241 loci assembled in 25 linkage groups assigned to the chromosomes of the A and B genome and 34 remained unlinked. A higher percentage of markers (54.4%), localized on the B genome chromosomes, in comparison to 45.6% distributed on the A genome. The whole map covered 1,605 cM. The B genome accounted for 852.2 cM of genetic distance; the A genome basic map spanned 753.1 cM with a minimum length of 46.6 cM for chromosome 5A and a maximum of 156.2 cM for chromosome 3A and an average value of 114.5 cM. The primer sets that amplified two or more loci mapped to homoeologous as well as to non-homoeologous sites. Out of 241 genetically mapped loci 213 (88.4%) were physically mapped by using the nulli-tetrasomic, ditelosomic and a stock of 58 deletion lines dividing the A and B genome chromosomes in 94 bins. No discrepancies concerning marker order were observed but the cytogenetic maps revealed in some cases small genetic distance covered large physical regions. Putative function for mapped SSRs were assigned by searching against GenBank nonredundant database using TBLASTX algorithms.     

A cytogenetically based physical map of chromosome 1B in common wheat.

We have constructed a cytogenetically based physical map of chromosome 1B in common wheat by utilizing a total of 18 homozygous deletion stocks. It was possible to divide chromosome 1B into 17 subregions. Nineteen genetic markers are physically mapped to nine subregions of chromosome 1B. Comparison of the cytological map of chromosome 1B with an RFLP-based genetic linkage map of Triticum tauschii revealed that the linear order of the genetic markers was maintained between chromosome 1B of hexaploid wheat and 1D of T. tauschii. Striking differences were observed between the physical and genetic maps in relation to the relative distances between the genetic markers. The genetic markers clustered in the middle of the genetic map were physically located in the distal regions of both arms of chromosome 1B. It is unclear whether the increased recombination in the distal regions of chromosome 1B is due to specific regions of increased recombination or a more broadly distributed increase in recombination in the distal regions of Triticeae chromosomes.     

Mapping of a BYDV resistance gene from Thinopyrum intermedium in wheat background by molecular markers

The wheat line H960642 is a homozygous wheat-Thinopyrum intermedium translocation line with resistance to BYDV by genomicin situ hybridization (GISH) and RFLP analysis. The genomic DNA ofTh. intermedium was used as a probe, and common wheat genomic DNA as a blocking in GISH experiment. The results showed that the chromosome segments ofTh. intermedium were transferred to the distal end of a pair of wheat chromosomes. RFLP analysis indicated that the translocation line H960642 is a T7DS-7DL-7XL translocation by using 8 probes mapped on the homoeologous group 7 in wheat. The translocation breakpoint is located between Xpsr680 and Xpsr965 about 90–99 cM from the centromere. The RFLP markers psr680 and psr687 were closely linked with the BYDV resistance gene. The gene is located on the distal end of 7XL around Xpsr680 and Xpsr687. Project supported by the 863 program and the National Natural Science Foundation of China (Grant No. 39680027).     

Molecular genetic maps of the group 6 chromosomes of hexaploid wheat (Triticum aestivum L. em. Thell.).

Restriction fragment length polymorphism (RFLP) maps of chromosomes 6A, 6B, and 6D of hexaploid wheat (Triticum aestivum L. em. Thell.) have been produced. They were constructed using a population of F7-8 recombinant inbred lines derived from a synthetic wheat x bread wheat cross. The maps consist of 74 markers assigned to map positions at a LOD >= 3 (29 markers assigned to 6A, 24 to 6B, and 21 to 6D) and 2 markers assigned to 6D ordered at a LOD of 2.7. Another 78 markers were assigned to intervals on the maps. The maps of 6A, 6B, and 6D span 178, 132, and 206 cM, respectively. Twenty-one clones detected orthologous loci in two homoeologues and 3 detected an orthologous locus in each chromosome. Orthologous loci are located at intervals of from 1.5 to 26 cM throughout 70% of the length of the linkage maps. Within this portion of the maps, colinearity (homosequentiality) among the three homoeologues is strongly indicated. The remainder of the linkage maps consists of three segments ranging in length from 47 to 60 cM. Colinearity among these chromosomes and other Triticeae homoeologous group 6 chromosomes is indicated and a consensus RFLP map derived from maps of the homoeologous group 6 chromosomes of hexaploid wheat, tetraploid wheat, Triticum tauschii, and barley is presented. Key words : RFLP, wheat, linkage maps, molecular markers.     

Structural rearrangement in chromosome 2M of Aegilops comosa has prevented the utilization of the Compair and related wheat-Ae. comosa translocations in wheat improvement

The genetic constitutions of chromosome 2M of Aegilops comosa and the derived wheat-Ae. comosa translocations were analyzed by molecular cytogenetic techniques. Hybridization of 15 RFLP markers covering the entire length of the group-2 chromosomes revealed that chromosome 2M was structurally rearranged compared to the homoeologous chromosomes of wheat by either a pericentric inversion or a terminal intrachromosomal translocation. The breakpoint of the rearrangement was located in a region between the loci Xpsr131 and Xcdo405, resulting in the translocation of 47% of 2MS to 2ML. This aberrant structure of 2M allowed homoeologous recombination between 2M and its wheat counterpart only in the translocated segment on 2ML. C-banding and genomic in situ hybridization analyses confirmed that all translocation chromosomes consisted of the complete 2MS arm, a large part of 2ML, and very small distal segments derived from 2AS or 2DS, as expected from the aberrant structure of chromosome 2M. Thus, the translocation in the line 2A-2M?4/2 can be described as T2AS-2M?1L???2M?1S and the translocations in the lines Compair and 2D-2M?3/8 as T2DS-2M?1L???2M?1S. RFLP analysis determined the breakpoints in these translocation chromosomes to be within the telomeric 16% of the wheat chromosome arms. The breakpoint of the 2A/2M translocation was between Xbcd348 and Xcdo783, and that of the 2D/2M translocation was between Xcdo783 and Xpsr666. Because the translocation chromosomes retain the structural aberration found in chromosome 2M, further exploitation of the wheat-Ae. comosa translocations for cultivar improvement is questionable.     

Genetics and molecular mapping of a male fertility restoration locus (Rfg1) in rye (Secale cereale L.)

 A gene determining the restoration of cytoplasmic genic male sterility (CMS) caused by the Gülzow (G)-type cytoplasm was mapped by analyzing an F₂ and F₃ population comprising 140 and 133 individual plants, respectively. The target gene, designated Rfg1, was mapped on chromosome 4RL distally to three RFLP (Xpsr119, Xpsr167, Xpsr899) and four RAPD (XP01, XAP05, XR11, XS10) loci. Xpsr167 and Xpsr899 are known to be located on the segment of chromosome 4RL which was ancestrally translocated and is homoeologous to the distal end of other Triticeae 6S chromosomes. It is suggested that Rfg1 may be allelic to the gene determining the restoration of rye CMS caused by the Pampa (P) cytoplasm (chromosome 4RL) and to Rfc4 that on rye addition lines of chromosome 4RL restores male fertility of hexaploid wheat with T. timopheevi cytoplasm. Homoeoallelism to two loci for cytoplasmic-male-sterility restoration on chromosomes 6AS and 6BS in hexaploid wheat is also suggested. Received: 1 December 1997 / Accepted: 10 February 1998     

Molecular and physical mapping of a barley gene on chromosome arm 1 HL that causes sterility in hybrids with wheat.

Addition of the long arm of barley chromosome 1H (1HL) to wheat causes severe meiotic abnormalities and complete sterility of the plants. To map the barley gene responsible for the 1H-induced sterility of wheat, a series of addition lines of translocated 1H chromosomes were developed from the crosses between the wheat 'Shinchunaga' and five reciprocal translocation lines derived from the barley line St.13559. Examination of the seed fertility of the addition lines revealed that the sterility gene is located in the interstitial 25% region of the 1HL arm. The genetic location of the sterility gene was also estimated by physically mapping sequence-tagged site (STS) markers and simple-sequence repeat (SSR) markers with known map locations. The sterility gene is designated Shw (sterility in hybrids with wheat). Comparison of the present physical map of 1HL with two previously published genetic maps revealed a paucity of markers in the proximal 30% region and non-random distribution of SSR markers. Two inconsistencies in marker order were found between the present physical map and the consensus genetic map of group 1 chromosomes of Triticeae. On the basis of the effects on meiosis and chromosomal location, the relationship of the present sterility gene with other fertility-related genes of Triticeae is discussed.     

Mapping of three self-fertility mutations in rye (Secale cereale L.) using RFLP, isozyme and morphological markers

 Three mutations determining self-fertility at the S, Z and S5 self-incompatibility loci on chromosomes 1R, 2R and 5R of rye, respectively, were mapped using three different F₂ populations. There was a close linkage of one isozyme and four RFLP markers, and no recombinant plants were detected. These markers are Prx7, Xiag249 and Xpsr634 for the S locus (1R), Xbcd266 for the Z locus (2R) and Xpsr100 for the S5 locus (5R). Linkage data for markers associated to the self-fertility mutations at the S, Z and S5 loci were calculated and compared with genetic maps computed by MAPMAKER multipoint analysis. Received: 8 October 1997 / Acepted: 26 November 1997     

Linkage maps of porcine Chromosomes 3, 6, and 9 based on 31 polymorphic markers

Linkage maps of porcine Chromosomes (Chrs) 3, 6, and 9, based on 31 polymorphic markers, are reported. The markers include 14 microsatellites, 12 RFLPs, three protein polymorphisms, and two blood group loci. The genetic interpretations of 11 RFLPs are documented. The markers were scored in a three-generation Wild Boar/Large White pedigree, and genetic maps were constructed on the basis of two-point and multi-point linkage analysis. Altogether the maps span a genetic distance of 216 cM, and previous physical assignments indicate that the linkage groups cover major parts of the three chromosomes. Significant differences in recombination rates between the sexes were observed for all three chromosomes. The recombination rate on the q arm of Chr 6 was markedly low. Sixteen loci are informative with regard to comparative mapping, that is, they have previously been mapped in the human and/or mouse genomes.     

A genetic linkage map of durum wheat

 A genetic linkage map of tetraploid wheat [Triticum turgidum (L.) Thell.] was constructed using segregation data from a population of 65 recombinant inbred lines (RILs) derived from a cross between the durum wheat cultivar Messapia and accession MG4343 of T. turgidum (L.) Thell. ssp dicoccoides (Korn.) Thell. A total of 259 loci were analysed, including 244 restriction fragment length polymorphisms (RFLPs), one PCR (polymerase chain reaction) marker (a sequence coding for a LMW (low-molecular-weight) glutenin subunit gene located at the Glu-B3 locus), seven biochemical (six seed-storage protein loci and one isozyme locus) and seven morphological markers. A total of 213 loci were mapped at a LOD≥3 on all 14 chromosomes of the A and B genomes. The total length of the map is 1352 cM and the average distance between adjacent markers is 6.3 cM. Forty six loci could not be mapped at a LOD≥3. A fraction (18.6%) of the markers deviated significantly from the expected Mendelian ratios; clusters of loci showing distorted segregation were found on chromosomes 1B, 3AL, 4AL, 6AL and 7AL. The durum wheat map was compared with the published maps of bread wheat using several common RFLP markers and general features are discussed. The markers detected the known structural rearrangements involving chromosomes 4A, 5A and 7B as well as the translocation between 2B-6B, but not the deletion on 2BS. This map provides a useful tool for analysing and breeding economically important quantitative traits and for marker-assisted selection, as well as for studies of genome organisation in small grain cereal species. Received: 5 January 1998 / Accepted: 31 March 1998     

The development of PCR-based markers for the selection of eyespot resistance genes Pch1 and Pch2

Two eyespot resistance genes (Pch1 and Pch2) have been characterised in wheat. The potent resistance gene Pch1, transferred from Aegilops ventricosa, is located on the distal end of the long arm of chromosome 7D (7DL). Pch2 derives from the variety Cappelle Desprez and is located at the distal end of chromosome 7AL. The RFLP marker Xpsr121 and the endopeptidase isozyme allele Ep-D1b, are very closely linked to Pch1, probably due to reduced recombination in the region of the introgressed A. ventricosa segment. Pch2 is less closely linked to these markers but is thought to be closer to Xpsr121 than to Ep-A1b. In the present study simple sequence repeat (SSR) markers were integrated into the genetic map of a single chromosome (7D) recombinant (RVPM) population segregating for Pch1. Sequence-tagged-site (STS)-based assays were developed for Xpsp121 and a 7DL wheat EST containing a SSR. SSR markers Xwmc14 and Xbarc97 and the Xpsr121-derived marker co-segregated with Pch1 in the RVPM population. A single chromosome (7A) recombinant population segregating for Pch2 was screened for eyespot resistance and mapped using SSRs. QTL interval mapping closely associated Pch2 with the SSR marker Xwmc525.     

Use of RAPD- and STS-analysis for marking genes of a homeologic group from chromosome 5 of common wheat

The search for STS (sequence-tagged site) and RAPD (random amplified polymorphic DNA) markers tightly linked to some genes of homeologous group 5 chromosomes of common wheat Triticum aestivum L., more specifically, awns inhibitor genes (B1), vernalization response gene (Vrn1), and homeologous chromosome pairing gene (Ph1), was conducted. To estimate the linkage of the gene with the marker, wheat lines marked with recessive alleles b1 and vrn1 were used. RELP (restriction fragment length polymorphism) and SSR (simple sequence repeat) analyses of isogenic wheat lines were conducted to characterize the chromosomal region transferred to the isogenic line from the donor parent. In RAPD analysis of isogenic wheat lines marked with recessive alleles b1 and vrn1, 95 arbitrary primers were used. To develop STS markers, analysis of the primary structure of RELP markers Xpsr426 and Xcdo504, tightly linked to the Vrn1 gene, and the Xpsr1201 marker, located at the Ph1 locus, was carried out. Two markers that are tightly linked to the Vrn1 gene (5AL)--RAPD marker Xr405 and STS marker Xsts426--were obtained in this work. In addition, there is every reason to believe that Xsts426 can be used as a PCR marker of genes Vrn2 (5BL) and Vrn3 (5DL), while Xsts1201, of the gene Ph1 (5BL).     

Identification and High-Density Mapping of Gene-Rich Regions in Chromosome Group 1 of Wheat

We studied the distribution of genes and recombination in wheat (Triticum aestivum) group 1 chromosomes by comparing high-density physical and genetic maps. Physical maps of chromosomes 1A, 1B, and 1D were generated by mapping 50 DNA markers on 56 single-break deletion lines. A consensus physical map was compared with the 1D genetic map of Triticum tauschii (68 markers) and a Triticeae group 1 consensus map (288 markers) to generate a cytogenetic ladder map (CLM). Most group 1 markers (86%) were present in five clusters that encompassed only 10% of the group 1 chromosome. This distribution may reflect that of genes because more than half of the probes were cDNA clones and 30% were PstI genomic. All 14 agronomically important genes in group 1 chromosomes were present in these clusters. Most recombination occurred in gene-cluster regions. Markers fell at an average distance of 244 kb in these regions. The CLM involving the Triticeae consensus genetic map revealed that the above distribution of genes and recombination is the same in other Triticeae species. Because of a significant number of common markers, our CLM can be used for comparative mapping and to estimate physical distances among markers in many Poaceae species including rice and maize.     

Distribution of genes and recombination on wheat homoeologous group <Emphasis Type="Italic">6</Emphasis> chromosomes: a synthesis of available information

Presence of genes in gene-rich regions on wheat chromosomes has been widely reported. However, there is a lack of information on the precise characterization of these regions with respect to the distribution of genes and recombination. We attempted to critically analyze the available data to characterize gene-rich regions and to study the distribution of genes and recombination on wheat homoeologous group 6 chromosomes which are a reservoir of several useful genes controlling traits of economic importance. Consensus physical and genetic linkage maps were constructed for homoeologous group 6 using physical and genetic mapping data. Five major gene-rich regions were identified on homoeologous group 6 chromosomes, with two on the short and three on long arm. More than 90% of marker or gene loci were present in these five gene-rich regions, which comprise about 30% of the total physical chromosomal length. The gene-rich regions were mainly present in the distal 60% regions of the chromosomes. About 61% of the total loci map in the most distal regions which span only about 4% of the physical length of the chromosome. A range of sub-microscopic regions within each gene-rich region were also identified. Comparisons of the consensus physical and genetic linkage maps revealed that recombination occurred mainly in the gene-rich regions. Seventy percent of the total recombination occurred in the two most distally located regions that span only 4% of the physical length of the chromosomes. The relationship of recombination to the gene-rich region is not linear with distance from the centromere, especially on the long arm. The kb/cM estimates for group 6 chromosomes ranged from 146 kb in the gene-rich to about 10 Mb in the gene-poor region. The information obtained here is vital in understanding wheat genome structure and organization, which may lead in developing better strategies for positional cloning in wheat and related cereals.This revised version was pubished online in April 2005 with corrections to the page numbering.     

A cytogenetic ladder-map of the wheat homoeologous group-4 chromosomes

We report the results of chromosome maps of wheat homoeologous chromosomes 4A, 4B, and 4D using 40 RFLP markers and 39 homozygous deletion lines. Deletion breakpoints divide the chromosomes into 45 subarm intervals with 32 intervals distinguished by molecular markers. The chromosome maps confirm the homoeology of arms 4AS to 4BL and 4DL, and 4AL to 4BS and 4DS. The chromosome map of 4A reveals novel information concerning the 4AL-5AL-7BS cyclical translocation. The presence of homoeologous group-4 long-arm markers, Xksu G10 and Xpsr 1051, intervening between the translocated 5AL and 7BS chromosome segments in 4AL suggests that the translocation events are more complex than was earlier believed. Chromosome maps confirm a pericentric inversion in Chinese Spring chromosome 4B. The consensus chromosome map is compared to the genetic map of wheat to construct a cytogenetic ladder-map (CLM). The CLM reveals an unequal distribution of recombination along the length of the chromosome arms. Recombination is highest in the distal half, and low in the proximal half, of the chromosome arms.