MICROBIOLOGY Study Guide - 2012

CHAPTER 5. FUNGI

5.3. REPRODUCTION OF FILAMENTOUS FUNGI

Filamentous Fungi can reproduce vegetatively, asexually, and sexually.

Vegetative reproduction is carried out by fragments of the mycelium, which, after separating from the main thallus, can continue to develop independently.

Asexual reproduction is associated with The formation of various types of spores:

Sporangiospores are produced by terrestrial Zygomycetes, which represent lower fungi. These are small, thick-walled resting bodies formed As a result of multiple asexual nuclear divisions inside a spherical Structure called a sporangium. Upon rupture of the sporangium, the sporangiospores are released. In aquatic representatives of lower fungi, sporangiospores possess flagella and are therefore called zoospores. After the sporangium ruptures, the zoospores swim in the Water like flagellated Protozoa. Having swum some distance, each zoospore begins to develop and gives rise to a new mycelium.

Conidiospores or conidia are produced by higher fungi, Ascomycetes, and many imperfect fungi, the Deuteromycetes. Conidiospores are located at the free ends of spore-bearing hyphae. The shape, size, arrangement, and color of conidia are Characteristic Features of various genera and species of Molds.

Arthrospores. In molds with a septate mycelium, The breakdown of hyphae into fragments produces numerous oval-shaped Cells. Such cells are sometimes called oidia (from Latin oidium — egg-shaped body), although it is more accurate to call them arthrospores. Arthrospores have a thickened Cell wall and withstand desiccation, thereby contributing to the survival of the species. Each arthrospore continues to divide vegetatively, giving rise to a new mycelium. Fungi of the genus Endomyces, notably the dairy mold Endomyces lactis (synonym Geotrichum candidum), reproduce by means of arthrospores. Chlamydospores are resting cells formed in the following manner. One or several cells become surrounded by a thick wall, their contents fill with granules of reserve nutrients, and the cells undergo partial dehydration. Metabolic activity in chlamydospores is sharply reduced. The biological function of chlamydospores is to ensure the long-term survival of The Cell under conditions of starvation or low humidity. Fungi of the genus Fusarium can reproduce by chlamydospores.

Sexual reproduction in lower fungi (using Rhizopus nigricans as an example) is possible only when two physiologically compatible mycelia meet — a (+)-strain and a (—)-strain. The ends of the mycelium swell, transforming into progametangia and subsequently into gametangia. The protoplasts of the gametangia fuse after the lysis of their cell walls in the contact zone, followed by the fusion of (+)- and (—)-nuclei to form a coenozygote. The latter increases in size and turns into a thick-walled zygospore. The zygospore remains dormant for some time, and then, following meiotic nuclear division, germinates to form a germ sporangium.

In higher fungi — Ascomycetes — the principal spore-bearing organ is the ascus. Nuclear division (Meiosis) takes place within the ascus, and ascospores develop inside it. The formation of asci typically occurs within fruiting bodies. Three forms of fruiting bodies are distinguished: cleistothecia — enclosed fruiting bodies; perithecia — flask-shaped fruiting bodies; and apothecia — open cup-shaped fruiting bodies.

In Basidiomycetes, the characteristic spore-bearing organ is the basidium. Typically, four mononuclear haploid basidiospores are pinched off from the basidium. Like ascospores, they are formed as a result of plasmogamy, karyogamy, and meiosis.