Gene flow analysis of molecular markers confirms that Phytophthora mirabilis and P. infestans are separate species

Abstract

The taxonomic status of Phytophthora mirabilis, one of six host-specific, foliar pathogens in Phytophthora group IV, has been uncertain. At various times this taxon has been given three different names: P. infestans var. mirabilis; P. mirabilis; and P. infestans forma specialis mirabilis. Which of these names is correct depends on the degree of reproductive isolation between this taxon and the closely related species, P. infestans. The purpose of this paper was to evaluate the hypothesis that P. infestans and P. mirabilis are conspecific using a large battery of molecular markers. Analyses of one isozyme, 44 DNA fingerprint, and 85 presumed RAPD loci revealed little, if any, gene flow between P. infestans and P. mirabilis. Thus, host specificity apparently functions as an effective pre-and postulating reproductive isolating mechanism in nature. Gene flow analysis indicated that these two taxa are as reproductively isolated from each other as they are from the other four species in Phytophthora group IV. There were 26 fixed differences between P. infestans and P. mirabilis that only could have developed in the absence of gene flow. Attempts to obtain F2 progeny from F1 interspecific hybrids failed, indicating the existence of genetic mechanisms of reproductive isolation in addition to host specificity. Despite the differences between P. infestans and P. mirabilis, growth rate on seven commonly used laboratory media could not be used to separate them in the laboratory. These data clearly reject the hypothesis that P. infestans and P. mirabilis are conspecific. Therefore, two of the three names given to this taxon, P. infestans var. mirabilis and P. infestans forma specialis mirabilis, are invalid. We propose that the correct name for this species is P. mirabilis.