Mapping of avirulence genes in Phytophthora infestans with amplified fragment length polymorphism markers selected by bulked segregant analysis

In this study we investigated the genetic control of avirulence in the dipl oid oomycete pathogen Phytophthora infestans, the causal agent of late blig ht on potato. The dominant avirulence (Avr) genes matched six race-specific resistance genes introgressed in potato from a wild Solanum species. AFLP markers linked to Avr genes were selected by bulked segregant analysis and used to construct two high-density linkage maps, one containing Avr4 (locat ed on linkage group A2-a) and the other containing a cluster of three tight ly linked genes, Avr3, Avr10, and Avr11 (located on linkage group VIII). Bu lked segregant analysis also resulted in a marker linked to Avr1 and this a llowed positioning of Avr1 on linkage group IV. No bulked segregant analysi s was performed for Avr2, but linkage to a set of random markers placed Avr 2 on linkage group VI. Of the six Avr genes, five were located on the most distal part of the linkage group, possibly close to the telomere. The high- density mapping was initiated to facilitate future positional cloning of P. infestans Avr genes.