|Drenth, Andre - UNIVERSITY OF QUEENSLAND|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 9, 1997
Publication Date: N/A
Interpretive Summary: Phytophthora infestans, the cause of late blight disease of potato and tomato, originated in central Mexico and has been spread around the world during the past 150 years. Previous analyses revealed that only one mating type (two are required for sexual reproduction) escaped Mexico during the initial migrations. The second mating type was probably introduced into Europe from northwestern Mexico during the 1970s and has since been introduced to other locations worldwide. Recently, mating type change, rather than migration, was proposed as a potential origin of the A2 mating type outside Mexico. The existence of two competing hypotheses to explain the same phenomenon has created confusion among plant pathologists and other scientists. The purpose of this paper was to explicitly test the mating type change hypothesis for the origin of the A2 mating type of P. infestans outside Mexico. The alternative hypothesis was that the A2 originated by migration. Cluster analyses were performed on existing genetic data from six locations worldwide where the A2 has appeared recently to test the predictions of the mating type change hypothesis. The data did not show the results predicted by mating type change, and thus this hypothesis was falsified in all six locations. Migration was the only viable hypothesis that could explain the results. Claims of A2 or self-fertile isolates of P. infestans in early literature reports were also evaluated and found to be groundless. These results clear up much of the confusion among plant pathologists and other scientists caused by the existence of two competing hypotheses, and establish a baseline against which future hypotheses can be judged.
Technical Abstract: Cluster analyses of genotypes of Phytophthora infestans from six locations where the A2 mating type was detected recently were used to explicitly test the mating type change hypothesis for the origin of the A2 outside Mexico. Origin by mating type change predicts that A1 and A2 genotypes should be very similar. However, in all six locations (Northwestern Mexico, South America/Costa Rica, Eastern Europe, East Asia, Western Europe, the United States/Canada) A2 genotypes did not cluster with the previously existing A1s and the mating type change hypothesis was falsified. Isolates in new populations worldwide were very different genetically from the predominant clonal lineage in old populations. The mean number of genetic differences from the old US-1 clonal lineage in new populations of P. infestans was not significantly different from that in Mexican populations. Migration is the only viable explanation for these results. Early reports of oospores of P. .infestans were also evaluated to determine whether the isolates described in them were homothallic. In all cases, oospores were only produced sporadically in old cultures under special conditions. The few oospores produced rarely had antheridia and were usually aborted. None of the isolates described in the old reports matched the characteristics of known self-fertile isolates and thus none could be classified as homothallic. A previous conclusion that A2 isolates were found in Japan during the 1930s was also evaluated and was not supported. There was no evidence for the occurrence of any A2 or homothallic isolates of P. infestans outside Mexico prior to their discovery in central Mexico during the 1950s.