COMPLEX GENETIC CONTROL OF POLYKETIDE TOXIN PRODUCTION BY COCHLIOBOLUS HETEROSTROPHUS

A fundamental issue in microbe/plant interactions is how pathogens alter their levels of virulence or change host range. What genetic events cause the appearance of new pathogenic races? The filamentous Ascomycete Cochliobolus heterostrophus, a pathogen of corn, offers a clear example of the appearance of a new race at a distinct point in time. Until 1969 only one pathogenic form (race O) of C. heterostrophus was known. During the period 1969-1970, a new form (race T) of the fungus appeared and was distinguished by having extreme virulence on corn carrying Texas (T) male sterile cytoplasm (1). The distribution of mating type frequencies in the field populations of race T and race O indicate that race T is a recent mutant of race O (2). Genetic analyses involving the two races revealed that the essential difference between them is a single genetic locus designated Toxl (3, 4). This gene controls production of an unusual secondary metabolite, a large (C41) polyketide called T-toxin, which specifically affects T cytoplasm corn (5). Thus, C. heterostrophus provides an opportunity to understand a distinct step in the evolution of a new race if the difference between the “alleles” at Toxl in race T vs. race O can be determined.