Haplotype diversity of preharvest sprouting QTLs in wheat.

Preharvest sprouting (PHS) is one of the most important factors affecting wheat production worldwide in environments characterized by rainfall and high humidity at harvest. In such environments, the incorporation of seed dormancy of a limited duration is required to minimize losses associated with PHS. A global collection of 28 PHS-resistant and -susceptible wheat germplasm was characterized with microsatellite markers flanking the genomic regions associated with PHS-resistance quantitative trait loci (QTLs), particularly on chromosomes 3D and 4A. The genetic diversity analysis revealed 380 alleles at 54 microsatellite loci, with an average of 7.0 alleles per locus, among the 28 wheat genotypes. Gower's genetic similarity values among all possible pairs of genotypes varied from 0.44 to 0.97, indicating that there is considerable diversity in the PHS germplasm evaluated. Cluster and principal coordinates analysis of genetic similarity estimates differentiated the genotypes into groups, according to their source of PHS resistance. Three major SSR haplotypes were observed on chromosome 4AL, designated RL4137-type allele, Aus1408-type allele, and synthetic-hexaploid-type allele. The RL4137-type allele was prevalent in Canadian cultivars, mostly in cluster 6, followed by the Aus1408-type and its derivatives in clusters 4 and 5. The Syn36 and Syn37 alleles on chromosome 4AL were rare. On chromosome 3DL, the SSRs haplotypes derived from Syn36 and Syn37 were also rare, and proved unique to the Aegilops tauschii - derived synthetic hexaploids. They are therefore likely carrying resistance genes different from those previously reported. Based on genetic relationships, PHS resistance might be improved by selecting parental genotypes from different clusters.     

Mapping quantitative trait loci for preharvest sprouting resistance in white wheat

The premature germination of seeds before harvest, known as preharvest sprouting (PHS), is a serious problem in all wheat growing regions of the world. In order to determine genetic control of PHS resistance in white wheat from the relatively uncharacterized North American germplasm, a doubled haploid population consisting of 209 lines from a cross between the PHS resistant variety Cayuga and the PHS susceptible variety Caledonia was used for QTL mapping. A total of 16 environments were used to detect 15 different PHS QTL including a major QTL, QPhs.cnl-2B.1, that was significant in all environments tested and explained from 5 to 31% of the trait variation in a given environment. Three other QTL QPhs.cnl-2D.1, QPhs.cnl-3D.1, and QPhs.cnl-6D.1 were detected in six, four, and ten environments, respectively. The potentially related traits of heading date (HD), plant height (HT), seed dormancy (DOR), and rate of germination (ROG) were also recorded in a limited number of environments. HD was found to be significantly negatively correlated with PHS score in most environments, likely due to a major HD QTL, QHd.cnl-2B.1, found to be tightly linked to the PHS QTL QPhs.cnl-2B.1. Using greenhouse grown material no overlap was found between seed dormancy and the four most consistent PHS QTL, suggesting that greenhouse environments are not representative of field environments. This study provides valuable information for marker-assisted breeding for PHS resistance, future haplotyping studies, and research into seed dormancy.     

Dissection of genetic components of preharvest sprouting resistance in white wheat

Preharvest sprouting (PHS) in rain-affected wheat (Triticum aestivum) is a major constraint to the production of high-quality wheat, especially in regions where white grain wheat cultivars are preferred. To characterize quantitative trait loci (QTLs) for PHS resistance and seed dormancy (SD), we evaluated 162 recombinant inbred lines developed from the cross between PHS-resistant white wheat landrace Tutoumai A and PHS-susceptible white wheat cultivar ‘Siyang 936’ for PHS resistance and SD in field and greenhouse experiments. Composite interval mapping (CIM) identified four QTLs for PHS resistance and long SD that explained up to 45 and 40.8% of the phenotypic variation in five PHS and four SD experiments, respectively. Qphs.pseru-4A.1 was detected in three of the five PHS experiments, and Qphs.pseru-5B.1, Qphs.pseru-5B.2, and Qphs.pseru-4B.1 were detected in two of the five PHS experiments, respectively. All four QTLs for PHS resistance also affected SD. Qphs.pseru-4A.1 was significant in all four SD experiments; the other three QTLs were detected only in one experiment. Additive and epistatic effects were observed for PHS resistance and SD. Besides three additive QTLs for PHS resistance and two for long SD, an additional 11 and 10 QTLs were detected with epistatic effects on PHS resistance and SD, respectively. The major genetic component of PHS resistance was SD, and other genetic factors may also contribute to PHS resistance in this population.     

Haplotype diversity at fusarium head blight resistance QTLs in wheat

Fusarium head blight (FHB) reduces grain yield and quality in common and durum wheat. Host FHB resistance is an effective control measure that is achieved by stacking multiple resistance genes into a wheat line. Therefore, breeders would benefit from knowing which resistance sources carry different resistance genes. A diverse collection of FHB-resistant and -susceptible wheat lines was characterized with microsatellite markers linked to FHB resistance quantitative trait loci (QTLs) on chromosomes 2DL, 3BS (distal to the centromere), 3BSc (proximal to the centromere), 4B, 5AS and 6BS identified in wheat lines Maringa, Sumai 3 and Wuhan 1. Putative Sumai 3 QTLs were commonly observed in advanced breeding lines, whereas putative Maringa and Wuhan 1 QTLs were relatively rare. Marker data suggested the 3BS, 3BSc and 5AS QTLs in the Brazilian cv. Maringa were derived from Asian germplasm and not from Frontana or other Brazilian lines. Haplotype diversity was reduced near the 5AS QTL, which might impact the deployment of this QTL. Finally, Brazilian germplasm was not closely related to other resistance sources and might be useful for pyramiding with Asian wheat-derived FHB resistance.Communicated by J. W. Snape     

Relationship between QTLs for preharvest sprouting and alpha-amylase activity in rye grain

Preharvest sprouting (PHS) and high alpha-amylase activity (AA) negatively affect quality of rye grain. The objective of this study was to reveal genetic relationship between PHS and AA by developing a consensus map of QTLs controlling each trait. A method of composite interval mapping (CIM) was used to search for QTLs within the 541 × Ot1-3 and DS2 × RXL10 F₂ mapping populations representing wide variation range of both traits. Sixteen QTLs for AA were detected on chromosomes 1R (3), 2R (2), 3R (2), 4R (3), 5R (3), 6R (2) and 7R (1). Their distribution was not random showing a tendency of QTL location in distal regions of chromosomes. Nine QTLs for AA located on chromosome arms 1RS, 2RL, 3RS, 4RL, 5RS, 5RL, 6RS, 6RL and 7RS coincided with QTLs for PHS. Seven QTLs for AA independent from PHS were detected on chromosome arms 1RL (2), 2RS, 3RL, 4RS, 4RL and 5RL. Four QTLs for PHS not associated with those for AA were identified on chromosomes 1RL, 2RL, 5RL and 7RL. Partial overlapping of the genetic systems controlling AA and PHS suggests that alpha-amylase found in sound grain of rye could be produced through at least three independent mechanisms i.e. PHS at its initial stage, late maturity alpha-amylase (LMA) and/or retained pericarp alpha-amylase (RPAA). Six QTLs co-located on both maps were found on chromosome arms 1RS, 2RS, 5RS, 5RL, 6RS and 6RL. Valuable features of line Ot1-3 i.e. resistance to preharvest sprouting and low alpha-amylase production in ripening grain can be attributed to seven major QTLs from chromosomes 1RL, 2RL, 5RL (2), 6RL and 7R (2). This set of QTLs, identified in line Ot1-3, might be useful in breeding sprouting resistant cultivars of rye.     

Detection of genetic diversity in closely related bread wheat using microsatellite markers

Wheat microsatellites (WMS) were used to estimate the extent of genetic diversity among 40 wheat cultivars and lines, including mainly European elite material. The 23 WMS used were located on 15 different chromosomes, and revealed a total of 142 alleles. The number of alleles ranged from 3 to 16, with an average of 6.2 alleles per WMS. The average dinucleotide repeat number ranged from 13 to 41. The correlation coefficient between the number of alleles and the average number of repeats was only slight (r _s = 0.55). Based on percentage difference a dendrogram is presented, calculated by the WMS-derived data. All but two of the wheat cultivars and lines could be distinguished. Some of the resulting groups are strongly related to the pedigrees of the appropriate cultivars. Values for co-ancestry (f) of 179 pairs of cultivars related by their pedigrees (f0.1) averaged 0.29. Genetic similarity (GS) based on WMS of the same pairs averaged 0.44. The rank correlation for these pairs was slight, with r _s = 0.55, but highly significant (P<0.001). The results suggest that a relatively small number of microsatellites can be used for the estimation of genetic diversity and cultivar identification in elite material of hexaploid bread wheat.     

Dissection and fine mapping of a major QTL for preharvest sprouting resistance in white wheat Rio Blanco

Preharvest sprouting (PHS) is a major constraint to white wheat production. Previously, we mapped quantitative trait loci (QTL) for PHS resistance in white wheat by using a recombinant inbred line (RIL) population derived from the cross Rio Blanco/NW97S186. One QTL, QPhs.pseru-3A, showed a major effect on PHS resistance, and three simple sequence repeat (SSR) markers were mapped in the QTL region. To determine the flanking markers for the QTL and narrow down the QTL to a smaller chromosome region, we developed a new fine mapping population of 1,874 secondary segregating F₂ plants by selfing an F6 RIL (RIL25) that was heterozygous in the three SSR marker loci. Segregation of PHS resistance in the population fitted monogenic inheritance. An additive effect of the QTL played a major role on PHS resistance, but a dominant effect was also observed. Fifty-six recombinants among the three SSR markers were identified in the population and selfed to produce homozygous recombinants or QTL near-isogenic lines (NIL). PHS evaluation of the recombinants delineated the QTL in the region close to Xbarc57 flanked by Xbarc321 and Xbarc12. To saturate the QTL region, 11 amplified fragment length polymorphism (AFLP) markers were mapped in the QTL region with 7 AFLP co-segregated with Xbarc57 by using the NIL population. Dissection of the QTL as a Mendelian factor and saturation of the QTL region with additional markers created a solid foundation for positional cloning of the major QTL.     

Assessment of genetic diversity of Czech sweet cherry cultivars using microsatellite markers

We analyzed 24 sweet and wild cherry genotypes collected in Czech Republic to determine genetic variation, using previously described 16 SSR primers to adapt a fast, reliable method for preliminary screening and comparison of sweet cherry germplasm collections. All SSRs were polymorphic and they were able all together to distinguish unambiguously the genotypes. These SSR primers generated 70 alleles; the number of alleles per primer ranged from 2 to 7, with a mean of 4.4 putative alleles per primer combination. The primer UDP-98-412 gave the highest number of polymorphic bands (totally 7), while Empa2 and Empa3 gave the lowest number (2). The allele frequency varied from 2.1% to 87.5%. We observed 10% of unique alleles at different loci. The observed heterozygosity value ranged from 0.25 to 0.96 with an average of 0.72 while expected heterozygosity value varied from 0.22 to 0.75 with an average of 0.59. The PIC value ranged from 0.21 to 0.71 with a mean value of 0.523. Cluster analysis separated the investigated cultivars in two groups. High level of genetic diversity obtained in the collection and proved to be sufficiently genetically diverse and therefore these genotypes would be useful to breeders for the development of new cherry cultivars.     

DNA markers for Fusarium head blight resistance QTLs in two wheat populations

Genetic resistance to Fusarium head blight (FHB), caused by Fusarium graminearum, is necessary to reduce the wheat grain yield and quality losses caused by this disease. Development of resistant cultivars has been slowed by poorly adapted and incomplete resistance sources and confounding environmental effects that make screening of germplasm difficult. DNA markers for FHB resistance QTLs have been identified and may be used to speed the introgression of resistance genes into adapted germplasm. This study was conducted to identify and map additional DNA markers linked to genes controlling FHB resistance in two spring wheat recombinant inbred populations, both segregating for genes from the widely used resistance source ’Sumai 3’. The first population was from the cross of Sumai 3/Stoa in which we previously identified five resistance QTLs. The second population was from the cross of ND2603 (Sumai 3/Wheaton) (resistant)/ Butte 86 (moderately susceptible). Both populations were evaluated for reaction to inoculation with F. graminearum in two greenhouse experiments. A combination of 521 RFLP, AFLP, and SSR markers were mapped in the Sumai 3/Stoa population and all DNA markers associated with resistance were screened on the ND2603/Butte 86 population. Two new QTL on chromosomes 3AL and 6AS wer found in the ND2603/Butte 86 population, and AFLP and SSR markers were identified that explained a greater portion of the phenotypic variation compared to the previous RFLP markers. Both of the Sumai 3-derived QTL regions (on chromosomes 3BS, and 6BS) from the Sumai 3/Stoa population were associated with FHB resistance in the ND2603/Butte 86 population. Markers in the 3BS QTL region (Qfhs.ndsu-3BS) alone explain 41.6 and 24.8% of the resistance to FHB in the Sumai 3/Stoa and ND2603/Butte 86 populations, respectively. This region contains a major QTL for resistance to FHB and should be useful in marker-assisted selection. Received: 17 August 2000 / Accepted: 16 October 2000     

Assessment of genetic diversity of wheat genotypes by resistance gene analog-EST markers

Resistance gene analog-expressed sequence tag (RGA-EST)-based markers have been used for variety discrimination and studies of genetic diversity in wheat. Our aim is to increase the competitiveness of public wheat breeding programs through intensive use of modern selection technologies, mainly marker-assisted selection. The genetic diversity of 77 wheat nucleotide binding site (NBS)-containing RGA-ESTs was assessed. Resistant and susceptible bread wheat (Triticum aestivum) genotypes were used as sources of DNA for PCR amplifications. In our previous studies, the F? individuals derived from the combinations PI178383 x Harmankaya99, Izgi2001 x ES14, and Sonmez2001 x Aytin98 were evaluated for yellow rust resistance at both seedling and adult stages to identify DNA markers. We have now examined the genetic variability among the resistant and susceptible Turkish wheat cultivars for yellow rust disease and the mean genetic distance between the cultivars. The highest similarity was 0.500 between Harmankaya99 and Sonmez2001. The lowest similarity was 0.286 between Aytin98, PI178383 and Aytin98, ES14. A relatively high level (49.5%) of polymorphism was observed with 77 RGA-EST primers across the six wheat genotypes, despite the fact that all of them were local cultivars from geographically close locations. RGA-EST sequences were compared by BlastX algorithms for amino acid sequences to determine the polymorphic categories among the combinations. BlastX analyses of six RGA-ESTs that gave polymorphic patterns for all combinations were NBS-LRR class RGA, NB-ARC domain containing protein, NBS-type resistance protein RGC5, NBS-LRR-S/ TPK stem rust resistance protein, and putative MLA1 proteins, while 38 RGA-EST gave a monomorphic pattern.     

Evaluation of Pakistan wheat germplasms for stripe rust resistance using molecular markers

Wheat production in Pakistan is seriously constrained due to rust diseases and stripe rust (yellow) caused by Puccinia striiformis f. sp. tritici, which could limit yields. Thus development and cultivation of genetically diverse and resistant varieties is the most sustainable solution to overcome these diseases. The first objective of the present study was to evaluate 100 Pakistan wheat cultivars that have been grown over the past 60 years. These cultivars were inoculated at the seedling stage with two virulent stripe rust isolates from the United States and two from Pakistan. None of the wheat cultivars were resistant to all tested stripe rust isolates, and 16% of cultivars were susceptible to the four isolates at the seedling stage. The data indicated that none of the Pakistan wheat cultivars contained either Yr5 or Yr15 genes that were considered to be effective against most P. striiformis f. sp. tritici isolates from around the world. Several Pakistan wheat cultivars may have gene Yr10, which is effective against isolate PST-127 but ineffective against PST-116. It is also possible that these cultivars may have other previously unidentified genes or gene combinations. The second objective was to evaluate the 100 Pakistan wheat cultivars for stripe rust resistance during natural epidemics in Pakistan and Washington State, USA. It was found that a higher frequency of resistance was present under field conditions compared with greenhouse conditions. Thirty genotypes (30% of germplasms) were found to have a potentially high temperature adult plant (HTAP) resistance. The third objective was to determine the genetic diversity in Pakistan wheat germplasms using molecular markers. This study was based on DNA fingerprinting using resistance gene analog polymorphism (RGAP) marker analysis. The highest polymorphism detected with RGAP primer pairs was 40%, 50% and 57% with a mean polymorphism of 36%. A total of 22 RGAP markers were obtained in this study. RGAP, simple sequence repeat (SSR) and sequence tagged site (STS) markers were used to determine the presence and absence of some important stripe rust resistance genes, such as Yr5, Yr8, Yr9, Yr15 and Yr18. Of the 60 cultivars analyzed, 17% of cultivars showed a RGAP marker band for Yr9 and 12% of cultivars exhibited the Yr18 marker band. No marker band was detected for Yr5, Yr8 and Yr15, indicating a likely absence of these genes in the tested Pakistan wheat cultivars. Cluster analysis based on molecular and stripe rust reaction data is useful in identifying considerable genetic diversity among Pakistan wheat cultivars. The resistant germplasms identified with 22 RGAP markers and from the resistance evaluations should be useful in developing new wheat cultivars with stripe rust resistance.     

Analysis of tetraploid Elymus species using wheat microsatellite markers and RAPD markers.

An analysis of Amplification fragment polymorphism of DNA from 27 accessions of 19 tetraploid Elymus species was carried out using 18 wheat microsatellite (WMS) primer pairs and 10 decamer primers. Ten WMS primer pairs produced multiple polymorphism on all accessions tested. Two independent phenograms, one based on WMS-PCR and one on RAPDs, separated the 19 tetraploid species into two main groups, viz., the SH genome species group and the SY genome species group. The results coincide with the genomic classification of these species and hence support previous studies showing that Elymus is not a monophyletic genus. The assays indicated that accessions within a species cluster together, which concurs with the morphological classification. Interspecific and intraspecific polymorphisms were detected by the WMS-PCR and RAPD analyses. Variation was observed among accessions of Elymus caninus. The WMS-PCR detected a much higher level of polymorphism than the RAPD analysis. WMSs seem to be more efficient markers than RAPD markers for studying the population diversity of Elymus species. The potential of cross-species amplification of microsatellite markers as an additional source for genetic analysis and applications in Elymus is discussed in the context of these results.     

Genetic diversity in Chinese modern wheat varieties revealed by microsatellite markers

Genetic diversity of 1680 modern varieties in Chinese candidate core collections was analyzed at 78 SSR loci by fluorescence detection system. A total of 1336 alleles were detected, of which 1253 alleles could be annotated into 71 loci. For these 71 loci, the alleles ranged from 4 to 44 with an average of 17.6, and the PIC values changed from 0.19 to 0.89 with an average of 0.69. (1) In the three genomes of wheat, the average genetic richness was B>A>D, and the genetic diversity indexes were B>D>A. (2) Among the seven homoeologous groups, the average genetic richness was 2=7>3>4>6>5>1, and the genetic diversity indexes were 7>3>2>4>6>5>1. As a whole, group 7 possessed the highest genetic diversity, while groups 1 and 5 were the lowest. (3) In the 21 wheat chromosomes, 7A, 3B and 2D possessed much higher genetic diversity, while 2A, 1B, 4D, 5D and 1D were the lowest. (4) The highest average genetic diversity index existed in varieties bred in the 1950s, and then it declined continually. However, the change tendency of genetic diversity among decades was not greatly sharp. This was further illustrated by changes of the average genetic distance between varieties. In the 1950s it was the largest (0.731). Since the 1960s, it has decreased gradually (0.711, 0.706, 0.696, 0.695). The genetic base of modern varieties is becoming narrower and narrower. This should be given enough attention by breeders and policy makers.     

Genetic and QTL analyses of seed dormancy and preharvest sprouting resistance in the wheat germplasm CN10955

The inheritance and genetic linkage analysis for seed dormancy and preharvest sprouting (PHS) resistance were carried out in an F₈ recombinant inbred lines (RILs) derived from the cross between “CN19055” (white-grained, PHS-resistant) with locally adapted Australian cultivar “Annuello” (white-grained, PHS-susceptible). Seed dormancy was assessed as germination index (GI7) while assessment for preharvest sprouting resistance was based on whole head assay (sprouting index, SI) and visibly sprouted seeds (VI). Segregation analysis of the F₂, F₃ data from the glasshouse and the RIL population in 2004 and 2005 field data sets indicated that seed dormancy and PHS resistance in CN19055 is controlled by at least two genes. Heritabilities for GI7 and VI were high and moderate for SI. The most accurate method for assessing PHS resistance was achieved using VI and GI7 while SI exhibited large genotype by environment interaction. Two quantitative trait loci (QTLs) QPhs.dpivic.4A.1 and QPhs.dpivic.4A.2 were identified. On pooled data across four environments, the major QTL, QPhs.dpivic.4A.2, explained 45% of phenotypic variation for GI7, 43% for VI and 20% for SI, respectively. On the other hand, QPhs.dpivic.4A.1 which accounted for 31% of the phenotypic variation in GI7 in 2004 Horsham field trial, was not stable across environments. Physical mapping of two SSR markers, Xgwm937 and Xgwm894 linked to the major QTL for PHS resistance, using Chinese Spring deletions lines for chromosome 4AS and 4AL revealed that the markers were located in the deletion bins 4AL-12 and 4AL-13. The newly identified SSR markers (Xgwm937/Xgwm894) showed strong association with seed dormancy and PHS resistance in a range of wheat lines reputed to possess PHS resistance. The results suggest that Xgwm937/Xgwm894 could be used in marker-assisted selection (MAS) for incorporating preharvest sprouting resistance into elite wheat cultivars susceptible to PHS.     

Genetic diversity in Chinese modern wheat varieties revealed by microsatellite markers

Genetic diversity of 1680 modern varieties in Chinese candidate core collections was analyzed at 78 SSR loci by fluorescence detection system. A total of 1336 alleles were detected, of which 1253 alleles could be annotated into 71 loci. For these 71 loci, the alleles ranged from 4 to 44 with an average of 17.6, and the PIC values changed from 0.19 to 0.89 with an average of 0.69. (1) In the three genomes of wheat, the average genetic richness was B>A>D, and the genetic diversity indexes were B>D>A. (2) Among the seven homoeologous groups, the average genetic richness was 2=7>3>4>6>5>1, and the genetic diversity indexes were 7>3>2>4>6>5>1. As a whole, group 7 possessed the highest genetic diversity, while groups 1 and 5 were the lowest. (3) In the 21 wheat chromosomes, 7A, 3B and 2D possessed much higher genetic diversity, while 2A, 1B, 4D, 5D and 1D were the lowest. (4) The highest average genetic diversity index existed in varieties bred in the 1950s, and then it declined continually. However, the change tendency of genetic diversity among decades was not greatly sharp. This was further illustrated by changes of the average genetic distance between varieties. In the 1950s it was the largest (0.731). Since the 1960s, it has decreased gradually (0.711, 0.706, 0.696, 0.695). The genetic base of modern varieties is becoming narrower and narrower. This should be given enough attention by breeders and policy makers.     

Retrotransposon-based molecular markers for linkage and genetic diversity analysis in wheat

Retrotransposon-based molecular markers have been developed to study bread wheat ( Triticum aestivum) and its wild relatives. SSAP (Sequence-Specific Amplification Polymorphism) markers based on the BARE-1/ Wis-2-1A retrotransposons were assigned to T. aestivum chromosomes by scoring nullisomic-tetrasomic chromosome substitution lines. The markers are distributed among all wheat chromosomes, with the lowest proportion being assigned the D wheat genome. SSAP markers for BARE-1/ Wis-2-1A and three other wheat retrotransposons, Thv19 , Tagermina and Tar1, are broadly distributed on a wheat linkage map. Polymorphism levels associated with these four retrotransposons vary, with BARE-1/ Wis-2-1A and Thv19 both showing approximately 13% of bands polymorphic in a mapping population, Tagermina showing approximately 17% SSAP band polymorphism and Tar1 roughly 18%. This suggests that Tagermina and Tar1 have been more transpositionally active in the recent evolutionary past, and are potentially the more useful source of molecular markers in wheat. Lastly, BARE-1 / Wis-2-1A markers have also been used to characterise the genetic diversity among a set of 35 diploid and tetraploid wheat species including 26 Aegilops and 9 Triticum accessions. The SSAP-based diversity tree for Aegilops species agrees well with current classifications, though the Triticum tree shows several significant differences, which may be associated with polyploidy in this genus.Communicated by M.-A. Grandbastien     

Assessment of genetic diversity within and among germplasm accessions in cultivated sorghum using microsatellite markers

Microsatellite markers are increasingly being used in crop plants to discriminate among genotypes and as tools in marker-assisted selection. Here we evaluated the use of microsatellite markers to quantify the genetic diversity within as well as among accessions sampled from the world germplasm collection of sorghum. Considerable variation was found at the five microsatellite loci analysed, with an average number of alleles per locus equal to 2.4 within accessions and 19.2 in the overall sample of 25 accessions. The collection of sorghum appeared highly structured genetically with about 70% of the total genetic diversity occurring among accessions. However, differentiation among morphologically defined races of sorghum, or among geographic origins, accounted for less than 15% of the total genetic diversity. Our results are in global agreement with those obtained previously with allozyme markers. We were also able to show that microsatellite data are useful in identifying individual accessions with a high relative contribution to the overall allelic diversity of the collection. Received: 10 August 1999 / Accepted: 27 August 1999     

Mapping of FHB resistance QTLs in tetraploid wheat.

Triticum turgidum L var. durum is known to be particularly susceptible to infection by Fusarium graminearum, the causal agent for Fusarium head blight (FHB), which results in severe yield losses and grain contaminated with mycotoxins. This research was aimed at identifying FHB resistance in tetraploid wheat and mapping the location of FHB resistance genes. A tetraploid cross of durum wheat ('Strongfield') x Triticum carthlicum ('Blackbird') was used to generate a doubled-haploid (DH) population. This population was evaluated for type II resistance to F. graminearum in replicated greenhouse trials, in which heads were innoculated and the percent of infected spikelets was determined 21 days later. The population was also genotyped with microsatellite markers to construct a map of 424 loci, covering 2 052 cM. The FHB reaction and genotypic data were used to identify FHB resistance quantitative trait loci (QTLs). It was determined that 2 intervals on chromosomes 2BL and 6BS controlled FHB resistance in this tetraploid cross. The FHB resistance allele on chromosome 2BL (r2=0.26, logarithm of odds (LOD)=8.5) was derived from 'Strongfield', and the FHB resistance allele on chromosome 6BS (r2=0.23, LOD=6.6) was derived from 'Blackbird'. Two other loci, on chromosomes 5AS and 2AL, were shown to regulate FHB infection and to have an epistatic effect on the FHB resistance QTL on chromosome 6BS. Further, the FHB resistance QTL peak on chromosome 6BS was clearly coincident with the known FHB resistance gene Fhb2, derived from Sumai 3. The results show that FHB resistance can be expressed in durum wheat, and that T. carthlicum and Triticum aestivum likely share a common FHB resistance gene on chromosome 6BS.     

Genetic analysis of scab resistance QTL in wheat with microsatellite and AFLP markers.

Three chromosomal regions associated with scab resistance were detected in a common cultivar, Ning7840, by microsatellite and AFLP analysis. Six microsatellites on chromosome 3BS, Xgwm389, Xgwm533, Xbarc147, Xgwm493, Xbarc102, and Xbarc131, were integrated into an amplified fragment length polymorphism (AFLP) linkage group containing a major quantitative trait locus (QTL) for scab resistance in a mapping population of 133 recombinant inbred lines (RILs) derived from 'Ning7840' x 'Clark'. Based on single-factor analysis of variance of scab infection data from four experiments, Xgwm533 and Xbarc147 were the two microsatellite markers most tightly associated with the major scab resistance QTL. Interval analysis based on the integrated map of AFLP and microsatellite markers showed that the major QTL was located in a chromosome region about 8 cM in length around Xgwm533 and Xbarc147. Based on mapping of six microsatellite markers on eight 3BS deletion lines, the major QTL was located distal to breakage point 3BS-8. In total, 18 microsatellites were physically located on different subarm regions on 3BS. Two microsatellites, Xgwm120 and Xgwm614, were significantly associated with QTL for scab resistance on chromosome 2BL and 2AS, respectively. The resistance alleles on 3BS, 2BL, and 2AS were all derived from 'Ning7840'. Significant interaction between the major QTL on 3BS and the QTL on 2BL was detected based on microsatellite markers linked to them. Using these microsatellite markers would facilitate marker-assisted selection to improve scab resistance in wheat.     

Characterization of Spanish grapevine cultivar diversity using sequence-tagged microsatellite site markers.

A broad germplasm bank collection containing most of the autochthonous Spanish grapevine cultivars was analyzed using six sequence-tagged microsatellite site (STMS) loci: VVS2, VVMD5, VVMD7, ssrVrZAG47, ssrVrZAG62, and ssrVrZAG79. The number of alleles obtained ranged from 9 in ssrVrZAG47 to 13 in VVS2, and the observed genotypes per locus varied between 24 (ssrVrZAG47) and 41 (VVSS2). A total of 57 unique genotypes were obtained considering all 6 loci, and 40 varieties presented at least 1 of these specific genotypes. The genotypic combinations for the 6 loci have generated 163 different profiles in the 176 cultivars. Ten pairs of accessions and one group of four Garnacha's cultivars can not be differentiated. The observed heterozygosity varied between 75.6 (VVMD7) and 90.9% (VVMD5), without significant differences from the expected values for any loci. The VVMD5 locus was the most informative, and also showed the highest discrimination power. The cumulative discrimination power for all six loci was practically 1; however, in fact, these STMS loci have differentiated only about 93% of the accessions, probably owing to high relatedness of the plant material. Usefulness of this STMS set for characterization of a Spanish grapevine collection is emphasized, as well as the elaboration of databases with these molecular markers.