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Dimorphic Structures Questions

Question 1

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Either. Pleomorphic refers to many forms and is used where more than one aspect of the biology of the fungus is being considered. Dimorphism is commonly used to refer specifically to transition between yeast-like and mycelial states of single fungi.

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Question 2

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There is no one switching mechanism. Some fungi switch from filamentous to yeast forms at high temperatures and/or high concentrations of available nutrients, others only require available nutrients. Still others switch from yeast-like to filamentous at high temperatures and low nutrient concentrations. As the transition is found in a diverse array of fungi, several, quite different processes are thought to be important in each case.

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Question 3

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In Puccinia gramininis var tritici, the stem rust of wheat, common in Australia, only the asexual stages are seen. The entire cycle is seen where both hosts are found. Monokaryotic basidiospores are released from a basidium on the ground after winter. These colonise the leaves of Berberis in spring and form spermagonia. Monokarytotic spermatia are released from the structure, and if they are placed on receptive hyphae in another spermatia, they initiate the dikaryotic state of the fungus. Dikaryotic hyphae grow through the leaf and form an aecium on the lower surface. Dikaryotic aeciospores are released and become air-borne. If they land on leaves of susceptible wheat and other grasses, they may form uredinia which produce dikaryotic urediniospores. The spores are transferred to further susceptible leaves, and in Australia, is the cycle which we see. As the conditions become cooler, dikaryotic teliospores are formed, which are released to the ground. In spring, the spores germinate, pass through karyogamy and meiosis and become the basidium. The cycle is complete. This is the most complex life cycle of the cereal rust fungi, having two hosts and six different types of spore.

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Question 4

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A range of soil fungi have the potential to cause disease in humans. In the first instance, if the individual in contact with the fungi has never experienced contact before, they are more likely to develop flu-like symptoms, as the fungus spreads over the inner surface of the lung. This is usually resolved quickly, and after subsequent contact the fungus has no effect. However, penetration into the blood stream which means the disease is especially serious, and may occur if the individual has a compromised immune system. Compromised immune systems are associated with age (very young or old), pregnancy, recreational use of a range of drugs, immune suppression following transplant operations, and a previous infection by an organism that kills immune cells such as HIV. The immune system has an amazing capacity to control fungal infection, so that when fungi enter the body, medical practitioners are often unprepared for it.

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Question 7

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