Development of a deletion map of wheat chromosomes 5A and 5B

The International Wheat Genome Sequence Consortium, aimed to physical mapping of bread wheat genome, assigned to Italian research groups the genetic and physical mapping of 5A chromosome. This chromosome is one of the most lacking in genetic markers, so the aim of this work was to find new molecular markers on chromosome 5A to saturate the existing genetic map, and to develop a cytogenetic deletion map of the homeologous chromosomes 5A and 5B. A total of 134 microsatellite markers were screened for polymorphism between Chinese Spring and Chinese Spring-5A dicoccoides (a Chinese Spring whose 5A chromosome was replaced with the 5A from the Triticum turgidum var. dicoccoides accession #TA106), which are the parental lines of the RIL population that will be used for the SSR genetic mapping (still in development). 115 of the markers were genomic microsatellites (gSSR) and 19 were EST-derived SSRs developed by La Rota et al. (2005) and available at http://wheat.pw.usda.gov. Polymorphic bands were visualized on agarose and acrylammide gels, and by capillary electrophoresis performed using an ABI PRISM 3100 Avant Genetic Analyzer. Both polymorphic and monomorphic SSR markers among the two parental lines were cytologically mapped to bins of chromosomes 5A and 5B by using a set of aneuploid lines derived from Chinese Spring, represented by nulli-tetrasomic, di-telosomic and deletion bin lines. In particular, the physical mapping on 5A chromosome was conducted with the lines N5A-T5B, N5AT5D, DT-5AL, and 14 deletion lines of which 4 belonging to the 5A chromosome short arm, and 10 to the long arm. All the lines were tested for carrying the correct homozygous terminal deletion by PCR amplification with specific SSR markers belonging to the missing chromosome region. A total of 92 markers were physically mapped, of which 43 on 5A, 30 on 5B and 4 on 5D; 17 SSRs produced multiple loci mapped to bins of 5A and 5B (13) and 5B and 5D (4), and two had bands located on 5A, 5B and 5D. For both the homeologous the majority of the markers were physically mapped in the bins of the long arm. The most represented bins were 5AL3-0.56-0.64 for 5A and 5BL6-0.29-1.00 for 5B, while the regions which were the most lacking of markers were bins C-5AL3-0.56 and C-5BS4-0.43 respectively for 5A and 5B.