Dimorphic Structures
Introduction
Dimorphic means two forms; pleomorphic means more than one form. The terms are applied to fungi because they can have different forms, of hyphae or spores. At the most obvious, fungi commonly have sexual and asexual stages in their life cycles. What is less commonly understood, is that some fungi may have many morphologically different types of asexual spore. LINK Further, the thallus may switch from yeast-like to filamentous under specific environmental conditions. These forms are independent of one another. In practice, the different forms may also lead to considerable confusion during identification.
Many fungi produce two different spore types during asexual proliferation. Unfortunately, these states can cause confusion when the spore resembles those of other fungi. Similarity of form does not necessarily indicate relatedness, and care must be taken when naming fungi using asexual structures. In all systems, the teleomorph (sexual) takes precedence over the anamorph (asexual), when the link between the two has been confirmed.
Many fungi modify their growth according to the environmental conditions under which they grow. For instance, soil fungi have a sparse filamentous growth in culture when organic carbon is poorly available. When organic carbon is plentiful, the hyphae branch frequently, and the mycelium is dense. The net effect for the fungus is that in conditions of plentiful energy, the fungus is able to absorb large quantities, but when food is lacking, little energy is spent on exploring the location. The consequence of the variation in growth patterns in soil, is that soil is thoroughly explored and efficiently exploited.
Rust Fungi
Many fungi have the potential to form different spore types. The asexual spores of rust fungi, in particular, take many forms. The rust fungi are extremely important plant pathogens with an enormous economic importance, greater than any other single biotic stress. LINK Rust fungi are all biotrophic fungi, incapable of growing apart from their specific hosts, except under extreme laboratory conditions. Their life cycle is complex, and in some cases involves two hosts for completion. However, most can continue in the absence of a sexual stage by cycling asexually on their primary host(s). The rust fungi are also interesting because many of them have more than one stage in the asexual process. The literature is complex and several different approaches to describing the cycle have been adopted. We will use a simplified approach which would never be used in published literature.
Rust life cycle (labelled to show hosts and stages).
The rust fungus Puccinia graminis passes through two hosts to complete its life cycle. The fungus is a heterothallic, macrocyclic and heteroecious species that cycles between grass hosts and either Berberis or Mahonia spp. Dikaryotic teliospores germinate in spring. Following karyogamy and meiosis, basidiospores are formed, released to the wind, where they may subsequently colonise leaves of Berberis or Mahonia if conditions are suitable. Homokaryotic mycelium develops in the plant tissue, where after several days, spermatia erupt on the leaf surface releasing spermatiospores within receptive hyphae. The spores are carried from one spermatia to another by insects, and if opposite mating types are transferred, fusion takes place. Following fusion, dikaryotic proliferation follows. On the lower leaf surface, the dikaryotic hyphae erupt to form aecia. Aeciospores are released to the wind and may travel to susceptible leaves of grasses (including wheat, oats, barley etc). In the second host, colonisation is followed by formation of uredinia. Urediniospores are released to the wind and may initiate colonies on adjacent hosts or leaves of the same host. Spread via urediniospores is extremely rapid, especially in crops of single plant genotypes. It is this stage of the life cycle that gives rise to the term rust fungus, as the spore masses are usually a dark brown. The spores are dikaryotic. Towards the end of summer, a different thick-walled spore is produced in further pustules, called telia. The teliospores are released to the soil. They are resting structures, capable of overwintering.
True Dimorphic Growth
True dimorphic growth is found in a number of fungi that can cause disease in mammals, insects and plants. Outside the body, the fungus is found as a filamentous mycelium. In general, the hyphae have the potential to invade bodies. They may also penetrate and ramify through solid substrates. However, the filamentous form lacks a capacity to be dispersed through the organism unless it can fragment. Fragmentation requires a major switch in form.
A range of pathogenic fungi of humans enter the lungs where they cause inflammation. The pathogens grow as mycelium in the tissue and eventually enter the blood stream of immunocompromised individuals where they immediately assume a yeast-like form. The fungi Histoplasma, Blastomyces and Paracoccidioides convert to yeast forms at 37C. Note that Candida is commonly found as a yeast at body temperature and a mycelium at high temperatures, indicating a different type of interaction with the mammalian body.
Pathogens of insects have been studied because of their potential to be used in biocontrol of insect pests. Two of these, Beauveria and Metarhizium, can be readily cultured on agar as mycelium. However, within insects, they exist as yeasts. The cells disperse through the circulatory system of the host, where they consume nutrients, proliferate and eventually kill the host through release of toxins and redirection of energy to the fungus.
The plant pathogen Ustilago colonises plants from mycelium. The fungus is culturable, though on nutrient-rich agar is found as a yeast.
Mechanisms
The mechanisms underlying the transition between forms is unclear. It appears to have evolved several times, as the capacity to switch is found in fungi from all major filamentous groups. Several components would need to be involved. The cytoskeleton would have to function differently in each form. Hyphal cell wall extension, in particular, and respiratory pathways, carbon acquisition and aerobiosis probably differ in each form. As dimorphism is widespread, though not common, different signalling systems seem likely to be operating.
Conclusion
Dimorphic growth is the result of environmental cues acting on genetic controls causing the fungus to change the form in which it is found. While the mechanisms appear to be multifaceted and complex, the different forms may have important consequences for survival in the different environments which are used. The diversity of forms has reached considerable complexity: different spore types and different hosts may be associated with different forms.
References
Gow NAR & Gadd GM 1995 The Growing Fungus. Chapman Hall.
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