Submitted to: World Journal of Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/1999
Publication Date: 3/1/2000
Citation: VALVERDE, M.E., VANDEMARK, G.J., MARTINEZ, O., LOPEZ-PAREDES, O. GENETIC DIVERSITY OF USTILAGO MAYDIS STRAINS. WORLD JOURNAL OF MICROBIOLOGY AND BIOTECHNOLOGY, 16(1):49-55. 2000. Interpretive Summary: Considerable economic losses on a global scale occur annually due to the infection of crop plants by fungi. The fungus Ustilago maydis causes a severe disease of corn that results in the fungus growing structures called "galls" in the ear of corn in place of the kernels. However, in many Latin American cultures, the fungus itself is prized as an edible delicacy, called in Spanish "Huitlacoche". The fungus is cultivated commercially by artificially infecting corn plants with the fungus and harvesting the fungal galls. This situation has led to some complications in the breeding of corn varieties. Varieties that are resistant to the fungus are necessary for corn production, but varieties that are susceptible to the fungus are required for the cultivation of fungal galls. To reduce the losses in corn production attributed to the fungus, it would be very useful if strains of the fungus could be identified that only caused disease on a few corn varieties used for production of the fungus. To identify such strains of the fungus, it is first necessary to determine which strains are genetically different. We evaluated thirty wild isolates of the fungus from five different locations in Mexico and two laboratory strains with molecular techniques that can detect genetic differences among isolates. All the strains were genetically quite distinct, and considerable differences were found even between isolates collected from the same field. The results suggest that enough genetic diversity exists for identifying strains of the fungus that can be used to produce edible galls for consumption while not infecting corn varieties used for grain production.
Technical Abstract: The basidiomycete fungus Ustilago maydis causes smut disease of corn, which is characterized by the growth of fungal galls in the ear of corn. However, in Latin American cultures the fungal galls are prized as an edible food delicacy known in Spanish as "Huitlacoche". The fungus is produced commercially by artificial inoculation of corn plants with the fungus followed by harvesting of fungal galls. We chose to examine levels of genetic diversity among isolates of the fungus as a first step towards identifying strains of the fungus that could be improved for the production of edible galls. Two laboratory strains of U. maydis and thirty wild isolates obtained from five different locations in Mexico were characterized for restriction fragment length polymorphisms (RFLPs). Twenty three different clones derived from a Pst 1 library were used as probes for Southern blots of genomic DNA digested with two different restriction enzymes. All loci identified were highly polymorphic, including one locus with thirty one different alleles. Genetic distances were calculated from the molecular marker data for each pairwise comparison. No significant correlation was observed between genetic and geographic distances among isolates. Considerable genetic variation was observed between isolates collected from the same field. The results demonstrate that U. maydis is a genetically heterogeneous species and that RFLPs can be successfully used for detecting genetic variation among different fungal isolates.