Fungal Asexual Reproduction

8 Key excerpts on "Fungal Asexual Reproduction"

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  Fungal Diversity, Ecology and Metabolites

  • Nwosu, Obasi(Authors)
  • 2018(Publication Date)
  • Agri Horti Press

    (Publisher)

  Reproduction Fungal reproduction is complex, reflecting the differences in lifestyles and genetic makeup within this diverse kingdom of organisms. It is estimated that a third of all fungi reproduce using more than one method of propagation; for example, reproduction may occur in two well- differentiated stages within the life cycle of a species, the teleomorph and the anamorph. Environmental conditions trigger genetically determined developmental states that lead to the creation of specialized structures for sexual or asexual reproduction. These structures aid reproduction by efficiently dispersing spores or spore-containing propagules. Asexual Reproduction Asexual reproduction occurs via vegetative spores (conidia) or through mycelial fragmentation. Mycelial fragmentation occurs when a fungal mycelium separates into pieces, and each component grows into a separate mycelium. Mycelial fragmentation and vegatative spores maintain clonal populations adapted to a specific niche, and allow more rapid dispersal than sexual reproduction. The “Fungi imperfecti” (fungi lacking the perfect or sexual stage) or Deuteromycota comprise all the species that lack an observable sexual cycle. Sexual Reproduction Sexual reproduction with meiosis exists in all fungal phyla (with the exception of the Glomeromycota). It differs in many aspects from sexual reproduction in animals or plants. Differences also exist between fungal groups and can be used to discriminate species by morphological differences in sexual structures and reproductive strategies. Mating experiments between fungal isolates may identify Polyporus squamosus This ebook is exclusively for this university only. Cannot be resold/distributed. 82 Fungal Diversity, Ecology and Metabolites species on the basis of biological species concepts.

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  Genetics and Evolution of Infectious Diseases

  • Michel Tibayrenc(Author)
  • 2010(Publication Date)
  • Elsevier

    (Publisher)

  4.7.1. Reproductive System

  Fungi present a striking diversity of life cycles, and studying their reproductive biology is a challenging task. However, this information is critical to assess the risk posed by pathogens and for the design of disease management strategies ( McDonald and Linde, 2002 ). For instance, outcrossing promotes genetic exchange and can accelerate the spread of new mutations in combination with other beneficial alleles, which is critical in the context of an arms race between hosts (or the humans that breed or grow them) and pathogens. By contrast, selfing or asexual reproduction provides insurance of reproduction for species having a low probability of finding a mate, and these species can therefore invade distant territories more easily and/or more rapidly ( Taylor et al., 2006 ). Asexual reproduction is also an expeditious way of multiplying rapidly favorable combinations of genes built by past selection ( Otto, 2009 ) and a more efficient strategy of transmitting genes to the next generation. Indeed, an asexual parent transmits 100% of its genes to the next generation, against only 50% for a sexual parent, which is called “the twofold cost of sex” in anisogamous species ( Bell, 1982 ). In the following, we briefly define the terminology used to qualify different aspects of fungal reproductive systems, and then we provide an overview of the methods available for their analysis, with some case studies among the fungal pathogens.

  Terminology

  Inconsistent use of key terms might be a cause of the slow integration of fungi in the field of evolution, and more generally unclear definitions of concepts are often an obstacle in the progress of science ( Neal and Anderson, 2005 ). A proper identification of the key features of the reproductive system of fungal pathogens is also fundamental for the correct selections of appropriate models to study population structure ( Giraud et al., 2008a

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  Fungi, Algae, and Protists

  • Britannica Educational Publishing, Kara Rogers(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  Typically in asexual reproduction, a single individual gives rise to a genetic duplicate of the progenitor without a genetic contribution from another individual. Perhaps the simplest method of reproduction of fungi is by fragmentation of the thallus, the body of a fungus. Some yeasts, which are single-celled fungi, reproduce by simple cell division, or fission, in which one cell undergoes nuclear division and splits into two daughter cells. After some growth, these cells divide, and eventually a population of cells forms. In filamentous fungi the mycelium may fragment into a number of segments, each of which is capable of growing into a new individual. In the laboratory, fungi are commonly propagated on a layer of solid nutrient agar inoculated either with spores or with fragments of mycelium.

  Budding, which is another method of asexual reproduction, occurs in most yeasts and in some filamentous fungi. In this process, a bud develops on the surface of either the yeast cell or the hypha, with the cytoplasm of the bud being continuous with that of the parent cell. The nucleus of the parent cell then divides. One of the daughter nuclei migrates into the bud, and the other remains in the parent cell. The parent cell is capable of producing many buds over its surface by continuous synthesis of cytoplasm and repeated nuclear divisions. After a bud develops to a certain point and even before it is severed from the parent cell, it is itself capable of budding by the same process. In this way, a chain of cells may be produced. Eventually, the individual buds pinch off the parent cell and become individual yeast cells. Buds that are pinched off a hypha of a filamentous fungus behave as spores; that is, they germinate, each giving rise to a structure called a germ tube, which develops into a new hypha.

  Although fragmentation, fission, and budding are methods of asexual reproduction in a number of fungi, the majority reproduce asexually by the formation of spores. Spores that are produced asexually are often termed mitospores, and such spores are produced in a variety of ways.

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  Handbook Crop Diseases

  • Nwosu, Obasai(Authors)
  • 2018(Publication Date)
  • Agri Horti Press

    (Publisher)

  A feature which is of widespread occurrence is the presence of lomasomes. These are made up of membrane bounded tubules or vesicles. The function of lomasomes is not known. Reproduction Fungi reproduce in both ways, i.e. , asexually as well as sexually. Asexual reproduction takes place by a variety of methods. This may be by: • Fragmentation of the soma. • Fusion of the somatic cells. • Budding of the somatic cells. • Production of spores, such as conidia, sporangiospores - motile or non-motilc-etc. Sexual reproduction is of widespread occurrence in fungi. Fertilization can be brought about in a variety of ways. The most common methods are: • Planogametic copulation. • Gametangial contact. • Gametangial copulation. • Spermatisation. • Somatogamy. Life Cycle As in other organisms, in fungi too there is generally a cycle of haploid and diploid structures, corresponding to the gametophyte and sporophyte in the green plants. The diploid phase begins with karyogamy and ends with meiosis. In the majority of fungi there is no distinct alternation of generations. Raper recognised seven basic types of life cycles. However, Burnett recognises five basic life cycles In fungi. These are: • Asexual, in which sexual reproduction is lacking, namely, Deuteromycetes. • Haploid, in which meiosis immediately follows nuclear fusion. This is very commonly seen in fungi or some Ascomycetes. Here the diploid phase is of minimum duration. • Haploid-dikaryotic, which is similar to the second type, excepting that paired, potentially conjugant nuclei persist in close physical This ebook is exclusively for this university only. Cannot be resold/distributed. Handbook Crop Diseases 96 association in the same hyphal segment (hence dikaryon). The examples of this are many Ascomycetes. Binucleate ascogenous hyphae develop just prior to ascus development and such a dikaryon cannot exist independently of the haploid phase.

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  Essential Microbiology

  • Stuart Hogg(Author)
  • 2013(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Figure 8.3 ). The conidia may be naked or protected by a flask-like structure called the pycnidium. Asexual reproduction by conidia formation is a means of rapid propagation for the fungus in favourable conditions. The characteristic green, pink or brown colour of many moulds is due to the pigmentation of the conidia, which are produced in huge numbers and dispersed by air or water currents. The conidia germinate to form another haploid mycelium.

  Figure 8.3 Asexual reproduction in the Ascomycota. Chains of conidia develop at the end of specialised hyphae called conidiophores.

  Figure 8.4 Budding in yeasts. Yeast cells in various stages of budding. A protuberance or bud develops on the parent yeast; the nucleus undergoes division and one copy passes into the bud. Eventually the bud is walled off and separated to form a new cell.

  In the case of the unicellular yeasts, asexual reproduction occurs as the result of budding , a pinching off of a protuberance from the cell, which eventually grows to full size (Figure 8.4 ).

  Plasmogamy is the fusion of the cytoplasmic content of two cells. Karyogamy is the fusion of nuclei from two different cells.

  Although some ascomycetes are self-fertile, sexual reproduction often involves separate ‘plus’ and ‘minus’ mating strains. Reproductively distinct, these two types are, however, morphologically identical, so it is not appropriate to refer to them as ‘male’ and ‘female’. The hyphae involved in reproduction are termed the antheridium (+ strain) and ascogonium (− strain). Hyphae from the different strains grow together and there is a fusion of their cytoplasm (Figure 8.5

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  Fungi

  Biology and Applications

  • Kevin Kavanagh(Author)
  • 2005(Publication Date)
  • Wiley

    (Publisher)

  1.6.2 Cellular reproduction Fungal growth involves transport and assimilation of nutrients, followed by their integration into cellular components, followed by biomass increase and eventual cell division. The physiology of vegetative reproduction and its control in fungi has been most widely studied in two model eukaryotes, the budding yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe. Budding is the most common mode of vegetative reproduction in yeasts and multilateral budding is typical in ascomycetous yeasts (see Table 1.11). In S. cerevisiae, buds are initiated when mother cells attain a critical cell size and this coincides with the onset of DNA synthesis. The budding processes results from localized weakening of the cell wall and this, together with tension exerted by turgor pressure, allows extrusion of cytoplasm in an area bounded by a new cell wall. Cell wall polysaccharides are mainly synthesized by glucan and chitin synthetases. Chitin is a polymer of N-acetylglucosamine and this material forms a ring between the mother cell and the bud that will eventually form the characteristic bud scar after cell division. Fission yeasts, typified by Schizosaccharomyces spp., divide exclusively by forming a cell septum, which constricts the cell into two equal-sized daughters. In Schiz. pombe, newly divided daughter cells grow in length until mitosis is initiated when cells reach a constant cell length (about 14 mm). The cell septum in Schiz. pombe forms by lateral growth of the inner cell wall (the primary septum) and proceeds inwardly followed by deposition of secondary septa. Cellular fission, or transverse cleavage, is completed in a manner resembling the closure of an iris diaphragm. In certain yeast species, the presence or absence of pseudohyphae and true hyphae can be used as taxonomic criteria (e.g. the ultrastructure of hyphal septa may discriminate between certain ascomycetous yeasts).

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  Physiology and Behaviour of Plants

  • Peter Scott(Author)
  • 2013(Publication Date)
  • Wiley-Interscience

    (Publisher)

  Longevity . Plants may, in many instances, be long-suffering and long-lasting, but if the great extinctions of the past teach us anything, it is that nothing lasts forever. No matter how old an individual plant is, one day conditions will change to make survival impossible. Plants need some means of surviving such adverse conditions, which may be local or temporary, and reproduction can offer a solution to this. Clonal plants may move away from the point of origin of the mother plant and therefore widen the spread of the plant, thereby escaping a particular stress. In addition, seeds offer even greater opportunities to disperse genetic material well away from the parent plant and also provide a structure that can remain dormant over prolonged periods. This permits a plant to survive temporary changes in conditions. Clones or seeds also produce new individuals which are free from fungal infections or damage present in the parent plant.

  3. Genetic variation . Finally, reproduction offers opportunities for genetic variation, which is essential in a changing habitat. Asexual reproduction offers little opportunity for genetic variation but the potential from sexual reproduction is immense.

  In this chapter we will look in greater detail at how plants reproduce and the merits and limitations of the different methods.

  Asexual reproduction

  Many plant species use asexual reproduction as a means of propagation. Asexual reproduction has several advantages and disadvantages (Figure 9.1 ). This form of reproduction produces individuals that are genetically identical and does not support much scope for phenotypic variation. This can lead to plants being vulnerable to changes in the habitat. In addition, offspring are generated close to the parent plant, so parent and offspring often have to compete for nutrients and light. However, asexually produced plants usually attain adulthood in advance of seedlings of the same parent and hence rapidly out-compete neighbouring plants reliant on seeds. Under many conditions, asexual reproduction is more successful than sexual reproduction for plants and a large number of species use this as their sole means of reproduction.

  Figure 9.1 Asexual reproduction in plants. Plants possess a range of different means of asexual reproduction. (A) Apomixis: using this method, the flowers spontaneously produce seeds without the need for any fertilization. The actual source of the embryo can come from the egg cell or the cells surrounding it. (B) A hyacinth bulb (Hyacinthus hybrid); bulbs frequently produce bulbils (small offset bulbs) at the base of the bulb. (C) A gladiolus corm (Gladiolus hybrid); corms frequently produce small offset corms at the base of the stem. (D) Strawberry plant (Fraxinus hybrid), forming runners, modified stems that form new plants. (E) Black poplar (Populus nigra

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  Food Mycology

  A Multifaceted Approach to Fungi and Food

  • Jan Dijksterhuis, Robert A. Samson, Jan Dijksterhuis, Robert A. Samson(Authors)
  • 2007(Publication Date)
  • CRC Press

    (Publisher)

  53 Chapter 3 Spore formation in food-relevant fungi Unai Ugalde 1 and Luis M. Corrochano 2 1 Unidad de Bioquímica II, Facultad de Química, Universidad del País Vasco, Apartado 1072, 20080 San Sebastian, Spain; 2 Departamento de GenØtica, Facultad de Biología, Universidad de Sevilla, Apartado 1095, 41080 Sevilla, Spain. INTRODUCTION A large number of filamentous fungi are noto-riously familiar to most people for their dash-ing 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 appar-ently harmless feature renders them ubiquitous in natural and human environments, including thoroughly sanitised food storage and process-ing facilities. Indeed, prolific spore production and dispersal is at the very heart of their un-welcome 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 fundamen-tal 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), sporula-tion in these two groups of organisms will be presented separately. CONIDIAL FUNGI The Process of Conidiation Conidia are cellular propagules which com-monly 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 pro-duction (conidiation) typically relies on rela-tively simple cellular transformations which can be completed relatively swiftly in every aerial hypha, resulting in a concerted and mas-sive production of spores.

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