engleski

Mapping QTLs for broomrape resistance in grain legumes

Orobanche crenata Forsk. is a root parasite that produces devastating effects on many crop legumes and has become a limiting factor for faba bean and pea production in the Mediterranean region. The efficacy of available control methods is minimal and breeding for broomrape resistance remains the most promising method of control. Resistance seems to be scarce and complex in nature, being a quantitative characteristic difficult to manage in breeding programmes. The best long term strategy for limiting damage by Orobanche is the development of Orobanche-resistant crops, but traditional plant breeding has generally failed to produce resistance that is stable across time or location as well as parasite pressure. The objectives of our studies were the development of genetic linkage maps of faba bean (Vicia faba L.) and peas (Pisum sativum L.) in F2 populations obtained by crossing lines resistant and susceptible to broomrape. The genetic linkage maps were subsequently used to identify and map quantitative trait loci (QTLs) associated with broomrape resistance. In faba bean (Vicia faba L.), 196 F2 plants derived from the cross between susceptible and resistant parent were analysed using isozymes, RAPD, seed protein genes, and microsatellites. The resistant line, Vf136, comes from the progeny selection of Vf1071 × Alameda. The linkage map was constructed by MAPMAKER V2.0 using a LOD score of 3.0 as the threshold for considering significant linkage. Of the 130 marker loci segregating in the F2 population, 121 could be mapped into 16 linkage groups covering 1445.5 cM of the faba bean genome. Nine out of 16 linkage groups were assigned to their specific chromosomes because a number of markers was common with previous studies which used primary trisomics in order to establish the chromosomal location of markers. F2-derived F3 lines were checked for broomrape resistance in field conditions. The 196 F2-derived F3 lines were grown together with the standard cultivar Brocal (susceptible to broomrape) in two replications in a field plot heavily infested with O. crenata seeds. Broomrape resistance was scored as the number of emerged broomrapes per faba bean individual at plant maturity. Simple regression has been carried out using the broomrape score in susceptible checks as an independent variable and the broomrape score in the F3 lines as a dependent variable to remove any statistically significant effects of field infestation variability. Regression-corrected values (residuals) were then calculated to correct for differences in broomrape seed density in the soil between plots. Regression residuals were considered to be the broomrape resistance index of the parental F2 individuals. Simple interval mapping (SIM) and composite interval mapping (CIM) were performed using QTL Cartographer. Composite interval mapping using the maximum number of markers as cofactors was clearly the most efficient way to locate putative QTLs. Three QTLs for broomrape resistance were detected. One of the three QTLs explained more than 35% of the phenotypic variance, whereas the others accounted for 11.2 and 25.5%, respectively. This result suggests that broomrape resistance in faba bean can be considered a polygenic trait with major effects of a few single genes. In order to identify and map the QTLs controlling broomrape resistance in peas, 115 F2 plants derived from the cross between Pisum sativum ssp. syriacum Berg (= P. humile Boiss and Nöe) × P. sativum cv. Messire were analysed using isozymes, RAPD and STS markers. The resistant maternal parent was the line P. sativum ssp. syriacum Berg and the male parent was P. sativum cv. Messire, highly susceptible to the parasite. One hundred and fifteen F3 lines were obtained by selfing of F2 individuals. F2-derived F3 lines were checked for broomrape resistance under field conditions and the broomrape resistance index of each F2 individual was constructed as mentioned. The linkage map was constructed with MAPMAKER V2.0. Of 217 markers, 120 could be mapped into 21 linkage groups covering 1770 cM of the pea genome. Simple Interval Mapping (SIM) and Composite Interval Mapping (CIM) were performed using QTL Cartographer. Two QTLs for broomrape resistance (Ocp1 and Ocp2) that explained a moderate portion of the observed variation. Ocp1 accounted for 9.59% of the total phenotypic variance and displayed a partial dominant gene action. Ocp2 explained 11.36% of the phenotypic variance and showed additive genetic effects. In the case of Ocp1 the resistance-enhancing alleles were originated from the susceptible parent. Thus, the genetic basis of resistance in pea seems to be more complex than in faba bean. The effectiveness of Marker Assisted Selection (MAS) procedure strongly depends on the accuracy of QTL mapping results and the analysis needs to be refined to determine precise positions and effects of the individual QTLs. Furthermore, QTLs found in a single environment must be validated across multiple test locations and independent mapping populations. Future research will be focused on saturating the regions of possible QTL locations with additional markers. Problems with reproducibility will be overcome by developing sequence or allele specific primers (SCARs) for the traits of interest. In order to check the accuracy of the QTL analyses and to analyze QTL stability in different environments, we have develop of recombinant inbred lines (RILs) derived from faba bean and pea F2 populations.

Orobanche crenata; Vicia faba; Pisum sativum; QTLs; broomrape resistance

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