RFLP MAPPING OF THE HA2 CEREAL CYST-NEMATODE RESISTANCE GENE IN BARLEY

The cereal cyst nematode (CCN), Heterodera avenae Well., is an economi cally damaging pest of barley in many of the world's cereal-growing ar eas. The development of CCN-resistant cultivars may be accelerated thr ough the use of molecular markers. A number of resistance genes agains t the pest are well known; one of them, the single dominant Ha2 resist ance gene, has been shown to be effective against the Australian patho type and maps to chromosome 2 of barley. Segregation analysis identifi ed two restriction fragment length polymorphism (RFLP) markers flankin g the resistance gene in two doubled-haploid populations of barley. AW BMA 21 and MWG 694 mapped 4.1 and 6.1 cM respectively from the Ha2 loc us in the Chebec x Harrington cross and 4.0 and 9.2 cM respectively in the Clipper x Sahara cross. Analysis of a further seven sources of CC N resistance in the form of near-isogenic lines (NILs) indicates that all available sources of resistance to the Australian pathotype of CCN in barley represent the Ha 2 locus.