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Ustilago maydis, the corn smut fungus, has an unusual diploid mitotic stage
Karen Snetselaar1, Michael McCann1
1a Department of Biology , Saint Joseph's University , 5600 City Avenue, Philadelphia , Pennsylvania 19131.
Abstract:
Ustilago maydis causes common smut disease in maize. Although pathogenic diploid strains of the fungus have been known for many years, the normal life cycle was thought to involve an extended dikaryotic stage, with nuclear fusion occurring in immature teliospores. However, microscopic examination of both living and fixed tumor material showed that nuclei fuse long before sporulation begins and that tumors are filled with uninucleate cells undergoing mitosis. Quantification of DNA in the nuclei confirmed these observations. Additionally, fungal cells from tumor material placed on nutrient agar produced colonies of diploid budding cells. Time-lapse observations showed that at least some of these colonies arose from thin-walled fungal cells rather than from immature spores. Ultrastructural examination of developing teliospores from tumors confirmed that they were uninucleate. Condensed chromatin and other structures characteristic of nuclei in prophase I of meiosis were observed. These observations support revising the U. maydis life cycle to include a diploid mitotic stage that corresponds with rapid tumor enlargement and conversion of plant to fungal biomass. Because mitotic division of diploid nuclei is so unusual as a life cycle feature in the fungi, it will be interesting to explore the consequences of its presence in U. maydis.
Insights
The fungus Ustilago maydis, which causes maize smut, has a revised life cycle. It includes a diploid mitotic stage during tumor development, challenging previous understandings of fungal reproduction.
Area of Science:
- Plant Pathology
- Mycology
- Fungal Genetics
Background:
- Ustilago maydis is a pathogenic fungus causing common smut disease in maize.
- The traditional U. maydis life cycle model proposed an extended dikaryotic stage with late nuclear fusion.
Purpose of the Study:
- To investigate the nuclear behavior and life cycle stages of Ustilago maydis during maize infection.
- To re-evaluate the established life cycle of U. maydis based on new observations.
Main Methods:
- Microscopic examination (light and electron microscopy) of infected maize tumor material.
- DNA quantification to assess nuclear ploidy.
- Culturing of fungal cells on nutrient agar.
- Time-lapse microscopy to observe cell division and colony formation.
Main Results:
- Maize tumors are primarily composed of uninucleate cells undergoing mitosis, indicating an early diploidization.
- Fungal cells from tumors can form diploid budding colonies on agar, originating from non-spore cells.
- Developing teliospores are uninucleate, with evidence of early meiotic stages.
Conclusions:
- The life cycle of Ustilago maydis should be revised to include a significant diploid mitotic stage.
- This diploid mitotic stage is crucial for rapid tumor growth and biomass conversion.
- The presence of diploid mitotic division as a life cycle feature in fungi warrants further investigation.