The Lr34/Yr18 adult plant resistance gene contributes significantly to durable leaf rust (caused by Puccinia triticina Eriks.) resistance. Simple and robust molecular markers that enable early detection of Lr34/Yr18 are a major advancement in wheat (Triticum aestivum L.) breeding. An insertion/deletion size variant located at the csLV34 locus on chromosome 7D within an intron sequence of a sulfate transporter‐like gene tightly linked to the Lr34/Yr18 dual rust resistance gene was used to examine a global collection of wheat cultivars, landraces, and D genome–containing diploid and polyploid species of wheat relatives. Two predominant allelic size variants, csLV34a and b, found among the wheat cultivars showed disparate variation in different wheat growing zones. A strong association was observed between the presence of Lr34/Yr18 and the csLV34b allele and wheat lines known to have Lr34/Yr18 that had the csLV34a allele were rare. All landraces with the exception of those from China were predominantly of the csLV34a type. Only one size variant, csLV34a, was detected among the diploid and polyploid D genome–containing species, indicating that csLV34b arose subsequent to hexaploid bread wheat synthesis. The lineage of the csLV34b allele associated with Lr34/Yr18 in modern wheat cultivars from North and South America, CIMMYT, Australia, and Russia was tracked back to the cultivars Mentana and Ardito developed in Italy by Nazareno Strampelli in the early 1900s. The robustness of the csLV34 marker in postulating the likely occurrence of Lr34/Yr18 across a wide range of wheat germplasm and its utility in wheat breeding was confirmed.

Significant progress has been made in the characterization of loci controlling traits of importance using molecular markers. A number of markers are currently available in wheat for genes of interest to the breeders. Markers can be used to better characterize parental material, thereby improving the efficiency and effectiveness of parental selection for crossing and to track genes in segregating progenies through the selection process. Although a number of breeding programs are using molecular markers at modest levels, the costs associated with marker assisted selection (MAS) are frequently cited as the main constraint to their wide-spread use by plant breeders. However, this is likely to change when user-friendly, high-throughput, automated marker technologies based on single nucleotide polymorphisms become available. These evolving technologies will increase the number of available markers, and will improve the efficiency, throughput, and cost effectiveness of MAS, thereby making it more attractive and affordable to many breeding programs. This article examines the extent to which molecular markers have been used at the International Maize and Wheat Improvement Center (CIMMYT) in applied wheat breeding and reviews the limited publicly available information on MAS from other wheat breeding programs. As markers are currently available for relatively few traits, we believe that MAS must be integrated with ongoing conventional breeding to maximize its impact. When used in tandem with phenotypic selection, MAS will improve response to selection for certain traits, thereby increasing rates of genetic progress.

Anthracnose, caused by Colletotrichum lindemuthianum, is an important fungal disease of common bean (Phaseolus vulgaris). Alleles at the Co–4 locus confer resistance to a number of races of C. lindemuthianum. A population of 94 F4:5 recombinant inbred lines of a cross between resistant black bean genotype B09197 and susceptible navy bean cultivar Nautica was used to identify markers associated with resistance in bean chromosome 8 (Pv08) where Co–4 is localized. Three SCAR markers with known linkage to Co–4 and a panel of single nucleotide markers were used for genotyping. A refined physical region on Pv08 with significant association with anthracnose resistance identified by markers was used in BLAST searches with the genomic sequence of common bean accession G19833. Thirty two unique annotated candidate genes were identified that spanned a physical region of 936.46 kb. A majority of the annotated genes identified had functional similarity to leucine rich repeats/receptor like kinase domains. Three annotated genes had similarity to 1, 3-β-glucanase domains. There were sequence similarities between some of the annotated genes found in the study and the genes associated with phosphoinositide-specific phosphilipases C associated with Co-x and the COK–4 loci found in previous studies. It is possible that the Co–4 locus is structured as a group of genes with functional domains dominated by protein tyrosine kinase along with leucine rich repeats/nucleotide binding site, phosphilipases C as well as β-glucanases.

Leaf rust, caused by Puccinia triticina, is an important disease of durum wheat (Triticum turgidum subsp. durum) and only a few designated resistance genes are known to occur in this crop. A dominant leaf rust resistance gene in the Chilean durum cv. Llareta INIA was mapped to chromosome arm 7BL through bulked segregant analysis using the amplified fragment length polymorphism (AFLP) technique, and by mapping three polymorphic markers in the common wheat (T. aestivum) International Triticeae Mapping Initiative population. Several simple sequence repeat (SSR) markers, including Xgwm344-7B and Xgwm146-7B, were associated with the leaf rust resistance gene. Resistance response and chromosomal position indicated that this gene is likely to be Lr14a. The SSR markers Xgwm344-7B and Xgwm146-7B and one AFLP marker also differentiated common wheat cv. Thatcher from the near-isogenic line with Lr14a, as well as durum ‘Altar C84’ from durum wheat with Lr14a. This is the first report of the presence of Lr14a in durum wheat, although the gene originally was transferred from emmer wheat ‘Yaroslav’ to common wheat. Lr14a is also present in CIMMYT-derived durum ‘Somateria’ and effective against Mexican and other P. triticina races of durum origin. Lr14a should be deployed in combination with other effective leaf rust resistance genes to prolong its effectiveness in durum wheat.

Leaf rust, caused by Puccinia triticina, is an important disease of durum wheat (Triticum turgidum subsp. durum) worldwide, and the most effective way to control it is through the use of resistant cultivars. A partially dominant leaf rust resistance gene present in the International Maize and Wheat Improvement Center-derived Chilean cv. Guayacan INIA and its sister line Guayacan 2 was mapped to chromosome arm 6BS by identifying linked amplified fragment length polymorphisms (AFLPs) and mapping two of the molecular markers in common wheat (T. aestivum) linkage maps of the International Triticeae Mapping Initiative and Oligoculm × Fukuho-komugi populations. Comparison of infection type responses of the two resistant durums with common wheat testers carrying the previously mapped resistance genes Lr36 and Lr53 on 6BS, and their chromosomal positions, indicated that the resistance gene in durum wheat Guayacan INIA is a new leaf rust resistance gene, which was designated as Lr61. Gene Lr61 is effective against the P. triticina race BBG/BN predominant in northwestern Mexico and other races infecting durum wheat in various countries.

Moisture stress greatly limits the productivity of wheat in many wheat-growing regions of the world. Knowledge of the degree of genetic diversity among parental materials for key selection traits will facilitate the development of high yielding, stress tolerant wheat cultivars. The objectives of this study were to: (i) use amplified fragment length polymorphisms (AFLPs) to assess genetic diversity among bread wheat lines and cultivars with different responses to drought stress in two distinct environments and, (ii) compare genetic diversity estimated by AFLPs with diversity evaluated on agronomic performance under drought stress. Twenty-eight genotypes, 14 from Iran and 14 developed or obtained by CIMMYT, were evaluated in the study. Phenotypic data on the 14 Iranian lines were obtained in Iran, and data on the 14 CIMMYT lines were collected in Mexico. Ten AFLP primer pairs detected 335 polymorphic bands among the 28 cultivars. At the 5th fusion level of the resulting dendrogram, 6 genotype clusters were identified. Thirteen of the 14 CIMMYT genotypes grouped into one cluster while 4 of the remaining groups were comprised only of Iranian genotypes. When the agronomic performance of the Iranian materials was compared with the AFLP diversity analysis, 5 of the 6 drought susceptible genotypes clustered together in the agronomic dendrogram, and were located in the same cluster in the AFLP dendrogram. However, the drought tolerant Iranian materials did not show the same degree of relationship. The CIMMYT materials did not demonstrate a significant association between agronomic performance and genetic diversity determined using AFLPs. Clearly these data show that there are genotypes with similar agronomic performance and different genetic constitutions in this study that can be combined in a breeding program to potentially improve tolerance to drought stress.

The 1BL/1RS homozygous chromosome translocation involves the short arm of Secale cereale L. chromosome 1R (1RS) and the long arm of chromosome 1B (1BL) of Triticum aestivum L. The 1RS chromosome arm possesses leaf, stem, stripe rust and mildew resistance genes (McIntosh 1983, Zeller and Fuchs 1983, Heun and Fischbeck 1987. Presence of the rye arm has been further associated with high yield, stability and wide adaptability of wheat germplasm, specifically for the cultivar Veery cross (Rajaram et al. 1983, Villareal et al. 1994). The Veery lines were derived from the 1BL/1RS winter bread wheat variety Kavkaz. The translocation was simultaneously and independently identified in Veery "S" by Merker (1982) and Mujeeb-Kazi (1982). Global wheat varietal releases possessing the 1BL/1RS translocation currently occupy over five million hectares of cultivated area. Approximately 40.0% of our germplasm (Rajaram et al. 1990) and up to 85.0% of Pakistan wheat varieties in yield trials (Jahan et al. 1990, Ter-Kuile et al. 1991 possess the homozygous 1BL/1RS translocation.Recently it was suggested (Villareal et al. 1994) that 1BL/1RS wheat varieties exhibit a yield advantage over 1B wheat varieties. Further (Villareal et al. 1995) in a two year yield trial study based upon random F2 derived F6 advanced lines from a Nacozari (1B)/Seri 82 (1BL/ 1RS) cross concluded that there was a 4.3% significant yield advantage for the 1BL/1RS derivatives. We felt that a more stringent test was necessary and through this report describe the production of germplasm that shall more precisely elucidate the effects of the 1BL/1RS translocation in a bread and durum wheat variety. This was made possible by substituting in T. aestivum L. cv. Seri 82 (1BL/1RS, 1BL/1RS) a 1B chromosome, and a 1BL/1RS chromosome in T. turgidum L. cv. Altar 84 (1B, 1B) by using the backcross procedure. For critical testing of the 1BL effect on yield components the "extracted" Seri 82 and Altar 84 derivatives after eight backcrosses and a selfing were also selected. These "extracted" derivatives may differ from the original breeders Seri 82 and Altar 84 varieties by recombinational changes on the 1BL chromosome arm, a minor but significant feature not met by random F2 derived F6 germplasm also. The diagnostic biochemical, molecular and cytological procedures used in development of the substituted germplasm are also elucidated. Materials and methodsProduction and cytological diagnostics Triticum aestivum L. cv. Seri 82 (1BL/1RS, 1BL/1RS) as the female parent was pollinated by T. aestivum L. cv. Pavon 76 (1B, 1B). The resulting Seri 82 X Pavon 76 F1 (1BL/1RS, 1B) was backcrossed to Seri 82. BC1 plants were screened by C-or N-banding techniques (Jahan et al. 1990, Jewell andMujeeb-Kazi 1982) for identifying up to three 1BL/1RS, 1B heterozygote derivative plants which were then each backcrossed to Seri 82 again and similarly 1 Corresponding author: A . Mujeeb-Kazi.

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