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Detailed analysis of the inheritance of molecular markers was performed in the oomycete plant pathogen Phytophthora infestans. Linkage analysis in the sexual progeny of two Dutch field isolates (cross 71) resulted in a high-density map containing 508 markers on 13 major and 10 minor linkage groups. The map showed strong clustering of markers, particularly of markers originating from one parent, and dissimilarity between the parental isolates on linkage group III in the vicinity of the mating-type locus, indicating a chromosomal translocation. A second genetic map, constructed by linkage analysis in sexual progeny of two Mexican isolates (cross 68), contained 363 markers and is thus less dense than the cross 71 map. For some linkage groups the two independent linkage maps could be aligned, but sometimes markers appeared to be in a different order, or not linked at all, indicating chromosomal rearrangements between genotypes. Graphical genotyping showed that some progeny contained three copies of a homologous linkage group. This trisomy was found for several linkage groups in both crosses. Together, these analyses suggest a genome with a high degree of flexibility, which may have implications for evolution of new races and resistance development to crop protection agents.

Investigates the genetic control of avirulence (Avr) in the diploid oomycete pathogen Phytophthora infestans. Number of resistance genes that match the Avr genes; Number of Avr genes found located on the most distal part of the linkage group; Reason for the use of high-density mapping.