ABSTRACT
INTRODUCTION A large number of filamentous fungi are notoriously familiar to most people for their dashing colonisation of foods, often resulting in spoilage, even under cold storage (Fisher, 2002). They also share the ability of producing large numbers of asexual spores. This apparently harmless feature renders them ubiquitous in natural and human environments, including thoroughly sanitised food storage and processing facilities. Indeed, prolific spore production and dispersal is at the very heart of their unwelcome success. This chapter aims to provide an overview of spore production as well as the stimuli which are involved in triggering this important biological phenomenon. Given the fundamental differences at the phylogenetic, cellular and developmental level between the Zygomycetes, which produce sporangiospores, and other fungal groups, which normally form conidia (Deuteromycetes and Ascomycetes), sporulation in these two groups of organisms will be presented separately. CONIDIAL FUNGI The Process of Conidiation Conidia are cellular propagules which commonly emerge from aerial hyphae at zones which lie behind the growing colony edge, and therefore, no longer participate in vegetative growth. Their purpose is to provide the fungal
colony with a means of dispersal in a rapidly changing environment. Hence, conidial production (conidiation) typically relies on relatively simple cellular transformations which can be completed relatively swiftly in every aerial hypha, resulting in a concerted and massive production of spores. In line with the above requirements, conidia rely on a readily available dispersion method: aerial transport. However, they may also be carried by insects and other living organisms with remarkable success (Ngugi and Scherm, 2004). Conidiation has attracted considerable interest in the food industry (Gray, 1981), since conidia can be used as biotransformation catalysts (Larroche and Gros, 1997) and as inoculum for industrial fermentations (Smith and Calam, 1980). In addition, fungal spores are at the start of food spoilage and decay processes, and are well known for harbouring mycotoxins (Pestka, 1995). Further, the health risks associated with the presence of fungi and their spores in human living environments is an increasing concern (Nielsen, 2003). In this section, we shall focus on three representatives of the most frequently encountered conidial fungi: Penicillium, Aspergillus and Neurospora. Conidiation in Penicillium species involves the differentiation of the hyphal apex into a specialised reproductive cell called phialide, which undergoes mitotic divisions, each resulting in a new specialised daughter cell: the conidium (Cole and Kendrick, 1969).
