THE GENETICS OF WHEAT RESISTANCE TO MYCOSPHAERELLA GRAMINICOLA
PA BRADING, LS ARRAIANO and JKM BROWN
Cereals Research Dept, John lnnes Centre, Norwich, NR4 7UH, UK
Background and objectives
Septoria leaf blotch, caused by the fungal pathogen Mycosphaerella graminicola (anamorph Septoria tritici), is currently one of the most serious diseases of bread wheat (Triticum aestivum) in the world. In field trials, a number of wheat varieties have shown either specific or general resistance to isolates of M. graminicola . The genetics of these resistances are being studied as part of an EU funded consortium project, one part of which involves AFLPTM (amplified fragment length polymorphism) mapping  of resistance genes segregating in the progeny of crosses between wheat varieties. AFLP markers that cosegregate with resistance to M. graminicola will provide breeders with molecular tags for more rapid selection of Septoria-resistant wheat lines. Seedling tests and detached leaf assays (see accompanying poster by L.S. Arraiano) for studying wheat resistance to M. graminicola are being developed to permit faster screening of large numbers of varieties and isolates.
Materials and methods
For the AFLP mapping project, parents of crosses were chosen either because they lie at opposite ends of the general M. graminicola resistance/susceptibility spectrum or because they show resistance to specific isolates. The following crosses have been made: Shafir x Kavkaz-K4500, Veranopolis x Baidus and Arina x Riband. The AFLP technique used is a slightly modified version of the standard published method  using Ssel and Msel restriction enzymes with two selective bases for each primer.
The genetics of isolate specific resistance is being studied in five English wheat varieties. In field trials, the varieties Andante and Pastiche show specific resistance to isolate IPO 001 but are susceptible to isolate IPO 323. In contrast, Flame and Hereward are susceptible to IPO 001 and resistant to IPO 323. Longbow is susceptible to both isolates. Reciprocal crosses have been generated between Andante, Flame, Longbow, Pastiche and Hereward and the F2 generations of each cross are being assessed for segregation of resistance and susceptibility using detached leaf tests. This analysis aims to determine whether the specific resistances are due to single genes or to several interacting loci and also whether Andante and Pastiche or Flame and Hereward share common genes conferring resistance to IPO 001 or to IPO 323 respectively. Resistance is assessed in detached leaves according to the method described in the poster by L.S. Arraiano et al by scoring the percentage of leaf area covered by pycnidia.
Results and conclusions
The parental varieties for AFLP mapping are being screened to select differentiating AFLP markers. The average variation between Arina and Riband is approximately 9% and between Shafir and Kavkaz-K4500 approximately 15%. These markers will be used to screen doubled-haploid (DH) sets of Fl progeny or single-seed descent (SSD) lines at the F6 generation for each of the crosses, enabling the markers to be genetically mapped. Cosegregation of particular AFLP markers with Septoria resistance will allow resistance genes to be identified.
Analysis of specific resistance using detached leaf tests is in progress. This technique distinguishes between resistance and susceptibility to the isolates IPO 001 and IPO 323 in the parental varieties. Segregation data for specific resistance to these two isolates will be shown for the F2 progeny of the crosses between Andante, Flame and Longbow.
This work was supported by the EU Biotech programme, Praxis XXI and
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