Genetic Analyses of Interspecific Hybrids between Phytophthora infestans and Phytophthora mirabilis

Goodwin, S. B., and Fry, W. E. 1994. Genetic analyses of interspecific hybrids between Phytophthora infestans and Phytophthora mirabilis. Experimental Mycology 18, 20-32. Four crosses were made between isolates of two host-specific Phytophthora species. Phytophthora infestans and Phytophthora mirabilis. In the two most successful crosses involving a common P. infestans A2 parent, allozyme analysis confirmed that 79 of 86 progeny were interspecific hybrids, 3 were presumed selfs, and 4 were either selfs or nonrecombinant parental types. Mating type, alleles at the allozyme locus glucose-6-phosphate isomerase, and the + alleles at a number of DNA fingerprinting loci segregated independently according to Mendelian expectation. Three DNA fingerprinting loci were tightly linked in P. mirabilis, but no other linkages were detected among these markers. Mitochondrial DNA was uniparentally inherited, mostly from the P. infestans parent. Growth rate segregated as a quantitative character. None of the 68 progeny tested infected Mirabilis jalapa (the host of P. mirabilis), 3 infected potato, and 4 were weakly pathogenic to tomato. Because most of the F₁ hybrids could not infect any of the hosts infected by the parents, host specialization could provide a postzygotic as well as a prezygotic reproductive isolating mechanism for P. infestans and P. mirabilis in central Mexico. These results indicate that P. mirabilis probably is capable of a regular outcrossing mating system.