The genetics of Phytophthora infestans: Segregation of allozyme markers in F2 and backcross progeny and the inheritance of virulence against potato resistance genesR2 andR4 in F1 progeny

Abstract

The purpose of this work was to investigate the inheritance of allozymes and virulence in Phytophthora infestans. For two previously known allozyme systems (glucose phosphate isomerase and peptidase) we discovered new alleles, Gpil (98) and Pepl ( 83 and 96). We also determined that a previously uncharacterized enzyme system, mannose phosphate isomerase, had at least two loci and four electromorphs (93, 100, 105, and 111) whose allelic relationships are not yet elucidated. We observed normal segregations in F1 progeny, but there were some unexpected segregations in backcrosses and sibcrosses. One backcross produced predominantly heterozygous progeny for both Gpil and Pepl, with the expected classes of homozygotes virtually absent. An excess of heterozygotes for Gpil was observed in another backcross, and in sibcrosses there were deficiencies of expected homozygous classes. In F1 progeny, virulence/ avirulence against potato resistance geneR2 segregated in a 1:1 ratio (P = 0.3), while the segregation of virulence/avirulence toR4 departed somewhat from a 1:1 ratio (P = 0.07). These results support the gene-for-gene hypothesis in P. infestans and suggest that virulence againstR2 is controlled at a single locus, while virulence againstR4 may be determined at one or two loci.