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United States Department of Agriculture

Agricultural Research Service


item Goodwin, Stephen - Steve
item Legard, Daniel
item Smart, Christine
item Levy, Morris
item Fry, William

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Identification and naming of pathogenic organisms is an important first step in controlling plant diseases. However, identification of plant pathogenic fungi often is complicated by a lack of variation. This makes it difficult to define species boundaries. Furthermore, biological species concepts developed for other organisms often do not transfer easily to asexually reproducing fungi. Thus, there is a great need for new methods to define species of plant pathogenic fungi. This is particularly evident in the genus Phytophthora, a group causing some of the most devastating plant diseases known. For example, one species, P. mirabilis, has been given three different names during the past 40 years, and a debate still rages about which is correct. The closest relative of P. mirabilis appears to be P. infestans, the incitant of late blight disease of potato and tomato and cause of the Irish potato famine during the 1840s. The purpose of this study was to use molecular markers to define species boundaries an test whether P. infestans and P. mirabilis are separate species. Analyses of data from 130 molecular marker loci revealed that these two species have little, if any, exchange of genes in nature, even though both are native to the same area of central Mexico. In fact, for the molecular markers tested they are as separated from each other as they are from four other species in the same genus. The mechanism of separation is host specificity, and a change in host specificity probably was the first step in the speciation process. These results can be adapted by mycologists to define species boundaries in other fungi, and will be used by evolutionary biologists to understand how new species form. An understanding of how host shifts occur could help plant pathologists combat emerging diseases in the future.

Technical 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 post- mating 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 o 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.

Last Modified: 10/18/2017
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