The root biomass among these translocation lines ranked as follows: Pavon 1RS.1AL > Pavon 1RS.1DL > Pavon 1RS.1BL > Pavon 76. On the other hand, in Colorado, the ‘Amigo’ 1RS.1AL translocation from a different rye source in wheat cv. ‘Karl 92’, showed 23% yield increase under field conditions over its winter wheat check, Karl 92 . In 1RS.1BL translocation wheats grown in acid soils, roots were thinner and there was a higher root length density, and this likely enhanced the root surface area . The yield advantage of 1RS translocation lines may be partly attributed to the increase in root biomass that increases uptake of water and nutrients from the soil . To elucidate the mechanisms responsible for increase in root biomass in 1RS wheats, it is necessary to genetically map and identify loci responsible for enhanced root traits. The objective of this study was to develop a consensus genetic map of the 1RS- 1BS chromosome arms using a population of induced homoeologous recombinants, and subsequently use the genetic map to tag the 1RS chromosomal region responsible for increased root traits. Molecular linkage maps of cereals are being improved rapidly by adding new types of markers, merging different species-specific maps and comparative mapping of markers between related genomes. Efficient use of resulting dense maps requires detailed insights into the relationship between genetic and physical distances.PCR-based markers were developed for 1RS.1AL and 1RS.1BL wheat-rye translocations in wheat .
However,stackable planters these translocations have not been used extensively to generate consensus map of wheat and rye chromosomes which would be a useful tool to study different agronomic characters influenced by the presence of rye chromatin. Recombination mapping has an advantage over deletion-bin mapping in generating higher resolution maps. In deletion mapping, the number of available breakpoints, hence the number of bins, limits the resolution, and after the set developed by Endo there have been no further efforts to generate new breakpoints. In genetic mapping, resolution is limited primarily by the number of available markers, and these increase steadily. Another advantage of recombination mapping is the ability to study the genes on rye chromosomes, where, because of diploidy, it is difficult, if not impossible, to practice deletion bin mapping. In this study, using a set of recombinants, we generated a consensus map of 1RS-1BS that integrates physical and molecular markers and attempted to narrow the regions containing major QTL for root characteristics. Experimental material was provided by Dr. A.J. Lukaszewski, University of California, Riverside. It consisted of a set of rye-wheat recombinant lines in a nearisogenic background of bread wheat cv. Pavon 76. Pavon 76 is a spring hard wheat from the breeding program at Centro Internacional de Mejoramiento de Maíz y Trigo , Mexico. The set includes centric translocation 1RS.1BL in Pavon 76, where the 1RS arm is from cv. Kavkaz and 1BL arm is from Pavon 76 and a set of 68 recombinants of the 1RS arm with 1BS in Pavon 76 . 1RS-1BS recombination was induced by the absence of the Ph1 locus. All recombinants are single breakpoints; therefore, the short arm of each recombinant chromosome contains one segment of 1RS and a complementary segment of 1BS. Each recombinant has a normal 1BL long arm. Since they were produced by crossing over, they can be used to generate a genetic map.
However, since recombination in wheat is predominantly in the distal portion of each arm , physically, these recombination breakpoints cover only the terminal 40% of the arm’s length. A phenotyping experiment to study root characters was set up in the glasshouse in sand-tube cultures in PVC tubes, 80 cm long and 10 cm in diameter during 2006, 2007 and 2008. The study involved five lines: Pavon 76 and Pavon 1RS.1BL as the parents, and recombinants T-14, 1B+38, and 1B+2 . The three recombinant lines were chosen from the set of 68 lines to subdivide the recombining portion of the arms into three segments of roughly equal lengths. Seed of these five lines were surface sterilized with 5% commercial bleach for 5 min, washed for 10 min in distilled water, soaked in water for 24 hrs and then germinated on wet filter paper in Petri dishes. Five day old seedlings were transplanted to 80 cm PVC tubes containing 1 m polythene tubing, closed at one end, with 8.5 kg of silica sand #30. Two small holes were made at the bottom of polythene tube to allow drainage of excess water. The PVC tubes were supported in metal frames and arranged in a randomized complete block design with four replicates. Plants were harvested 45 days after germination when the differences for root characters could be efficiently measured among different recombinant lines. Data for different shoot characters were recorded and the tubes containing roots were stored at 4oC until processing. Roots were washed and recovered without damage using a floatation technique . The shoot characters measured were longest leaf length , maximum width of the longest leaf , leaf area , plant height , number of tillers , and dry shoot biomass .
The root characters measured were number of roots greater than 30 cm , longest root length , total length of roots greater than 30 cm , shallow root weight , deep root weight , dry root biomass , and root biomass to shoot biomass ratio . The shoot and root data were subjected to the analysis of variance for each year . The combined ANOVA across years was performed for each measured and calculated trait. The overall rooting ability of each genotype was calculated by ranking each genotype for individual root traits in each replication in each of the three years. Genotypes with the highest values were ranked 5 and those with the lowest values were ranked 1. Subsequently, all the ranks of root characters for each genotype were summed providing a measure of the rooting ability index for each genotype at different replications . The genotypic rank sums averaged across the years were subjected to the non-parametric Quade analysis developed for randomized complete block designs to differentiate genotypes for overall rooting ability. A calculative approach was used to generate a genetic map of the 1RS-1BS arms. The 1RS-1BS map from the previous study , based on twelve markers and 103 recombinants, was enriched by eight additional molecular markers and recalculated using 68 lines. The population of recombinants was divided into two configuration groups, 1B+ lines with distal 1RS, and T- lines with distal 1BS, comprising 34 lines each. Both groups were scored seperately for the presence/absence of each of the 20 markers, with the presence of a marker denoted as a score of 1 and the absence as 0. Each line was then ranked according to its total score for all markers and each group was further divided into 11 subgroups on the basis of its ranking. Thus,stacking pots the entire mapping population was divided into 22 subgroups . In this fashion, the 20 markers subdivided the genetic maps of the arms into 15 intervals . Phenotyping of recombinants was necessary to test the general applicability of the consensus 1RS-1BS map in locating a 1RS region showing better rooting ability. Various shoot and root traits were studied using two parents and three recombinants covering the whole 1RS-1BS map. There were significant differences among years for all shoot characters measured, except for the maximum width of the longest leaf . Significant differences were found among the genotypes for shoot characters, except for maximum width of the longest leaf and leaf area. Genotype × year interaction was significant only for the number of tillers per plant . This interaction was due to changes in the magnitude of the genotypic means across different years rather than changes in ranking of the means. Therefore, shoot characters in Table 2 are represented by means averaged across years. Pavon 1RS.1BL was taller, had longer leaf length and a greater root to shoot ratio than Pavon 76. Since Pavon 1RS.1BL and Pavon 76 had similar shoot biomass , greater root to shoot biomass ratio in the former genotype indicated greater root biomass in 1RS.1BL than Pavon 76 . Leaf area in 1B+2 was the highest followed by Pavon 1RS.1BL .
Despite significant differences observed among the genotypes for shoot characters, the differences were relatively small, except for the shoot biomass in 1B+2 in the third year which was due to greater number of tillers per plant and plant height . Otherwise, the rest of the genotypes did not show large differences for combined as well as for individual years for most of the shoot traits. There were significant differences among years for all root characters measured . Significant differences were found among the genotypes for all the root characters measured, except for longest root length. The genotype × year interaction was significant only for the number of roots greater than 30 cm. Therefore, means for root characters were averaged across three years . The number of roots greater than 30 cm and root biomass in Pavon 1RS.1BL were greater than those in Pavon 76 which confirmed the results reported earlier . Number of roots greater than 30 cm in Pavon 1RS.1BL, 1B+2 and 1B+38 were similar, but greater than those in Pavon 76 and T-14 . A similar trend was observed for the total length of roots greater than 30 cm. Shallow root weight was highest in Pavon 1RS.1BL followed by 1B+2 , and 1B+38 . The lowest shallow root weight belonged to T-14 . Deep root weight in Pavon 1RS.1BL, 1B+2, and 1B+38 were similar, but greater than those in Pavon 76 and T-14. Greater dry root biomass observed in Pavon 1RS.1BL compared to Pavon 76 was due to a combination of greater shallow and deep root weight in the former than the later genotype . Quade analysis was used to compare the rooting ability index of the examined genotypes based on the mean rank sums of root characters. The Quade statistic ranged from -11.0 for T-14 to 11.5 for Pavon 1RS.1BL . Differences among Sj were statistically significant at P< 0.10 . Pavon 1RS.1BL had the highest RAI followed by 1B+2 and 1B+38 . Pavon 76 and T-14 had the lowest RAI . Genetic mapping in wide hybrids has been performed for many plant species, particularly in diploids including barley , chickpea , lentils , onion , Nicotiana species , and tomato . In the early days of genetic mapping with molecular markers, wide hybrids were the approach of choice, for it guaranteed much higher levels of polymorphism than in intra-specific hybrids. Despite notable instances of non-Mendelian segregation and skewed distribution of recombination, wide hybrids produced useful genetic maps with higher marker saturation at considerably less cost and effort . Use of wide hybrids in allopolyploids is more complicated than in diploids as allopolyploids tend to have some kind of chromosome pairing control system in place that limits crossing over to homologues. Hence, homoeologous pairing may be low or even non-existent. In this study, 68 recombinants produced by crossing over were used. These recombinants were selected at random from a population of 103 such recombinants developed by Lukaszewski . The entire recombinant population was selected from a population of ca. 17,000 progeny with the Ph1 system disabled. If the assumption is made that crossing over in the Ph1+ and Ph1- wheats is the same, and they appear to be, except for the absence of multiple crossovers per arm then the sample analyzed here would be equivalent to a population of 136 back cross progeny, a sensible number giving the maximum resolution level of 0.7 cM. Physical distribution of 68 recombinant breakpoints used in the present study is shown in Figure 3. With a total of 20 physical and molecular markers, we constructed the combined genetic map of 1RS-1BS recombinant breakpoints in Pavon 76 background. The genetic map produced here has an average density of 2.5 cM. The maps shown in Figures 2 and 4 represent only the physical 40% of the distal ends of the 1S arms as no recombination took place in the proximal 60% of the arm. Any loci in this region would show complete linkage with the centromere.