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Characters of the Lower Fungi (Phycomycetes)

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Characters of the Lower Fungi (Phycomycetes) Characters of The lower fungi (Phycomycetes) General feature of Phycomycetes The lower fungi comprise the simplest and primitive group of true fungi. The majority of these are aquatic (Saprolegnia) , some are amphibious (Phytophthora) and others are terrestrial (Rhizopus). Oogonium of Infected fish by Saprolegnia Saprolegnia Thallus growth of Saprolegnia on different media. Sporangia of Phytophthora Rhizopus Rot disease caused by Rhizopus Phycomycetes live either as strict parasites or saprophytes. Parasitic species live on algae, ferns and seed plants. Some of them cause diseases to economic plants, such as the downy mildew of grape, the brown rot of lemon and potato. Downy mildew of grapes Brown rot of lemon and potato Phycomycetes are called the algal fungi because they are resemblance to some algae in the following: 1- Morphology of the algal thallus . 2-Methods of asexual reproduction of algal members. 3- Sex organs of algae . Lower fungi characterized by: 1- The somatic phase consists of either a unicellular thallus or a non- septate mycelium (coenocytic structure). 2- Asexual reproduction takes place by zoospores , sporangiospores and sometimes by conidia. 3- Sexual reproduction, when present, is either isogamy, anisogamy or oogamy. 4- Karyogamy generally follows plasmogamy almost immediately so that there is no dikaryophase in life cycle. Followed Plasmogamy Karyogamy Immediately Evolution of thallus The thallus in these forms are microscopic and unicellular. In Olpidium endogenum (parasitic fungus), it consists of a tiny mass of naked uninucleate protoplast (rhizomycelium) which enters the host cell, enlarges and lays down the fungal cell wall which contains chitin. Germinating zoospore zoospore promycelium mycelium Evolution of fungal thallus At the reproductive stage, the entire cell protoplast is involved in zoospore formation. In the more advanced unicellular forms, the thallus is fixed to the substratum by a few thin hair-like structures which called rhizoids. The coenocytic thallus is (holocarpic form). The fungal mass of hyphae, known as the mycelium penetrates bb the bread and produces the fruiting bodies on the top of the stalks, called sporangiospores Rhizoids Mycelia = a mass of hyphae or filaments At maturity, it is converted either into a single reproductive organ (zoosporangium or gametangium) as in family Olpidiaceae or into a cluster of such organs as in family Synchytriaceae. Some more advanced aquatic members have a little more complex vegetative body. At maturity, zoosporic fungi form the reproductive organse such as zoosporangium or gametangium. Allomyces hypha Blastocladiella consist of two multinucleate compartments. One of these is vegetative in function and the other is reproductive. The gametothallus of Allomyces which is a soil fungus, consists of branched, multinucleate hyphae forming a small mycelium, it comprise : 1-A group of rhizoids which fixed the thallus to the substratum. 2-Precense of a short, slender trunk-like body. 3-Dichotomously branched hyphae bear sex organs (as seen before). The majority of the lower fungi are aquatic as well as terrestrial, they have a well developed, filamentous thallus, called the mycelium (coenocetic thallus) It is differentiated into distinct, sterile and fertile portions and thus is (eucarpic). The eucarpic mycelium is of two types: In the first type, the mycelium is differentiated into an aerial portion and holdfast as (Rhizopus). The holdfast is branched and tapering. It penetrates the substratum (rhizoids) . Eucarpic thallus of Spizellomyces The aerial portion consists of usually stout, cylindrical, branched and coenocytic (aseptate hyphae) called (sporangiophore) . In the second type, the mycelium (as in Peronosporales) consists of slender, cylindrical, coenocytic hyphae which grow hidden view within the tissues of the host or in dead organic matter in the soil or water. In the land forms, the sporangiospores are released by the rupture of the sporangial wall. The released spores may be motile in the aquatic species and nonmotile (immotile) in the land forms. The former group are called zoospores (motile) but the latter are aplanospores (immotile). In the holocarpic species, the entire thallus functions as the sporangium. The eucarpic species have the sporangia borne on special reproductive hyphae known as the sporangiophores which may be simple or branched. The zoospores may be uniflagellate, as in members of (Chytridiomycetes and Hyphochytridiomycetes) or biflagellate , as in fungal species of (Plasmodiophoromycetes and Oomycetes). Chytridiomycetes produced uniflagellate zoospores with a whiplash flagellum inserted at the posterior end. Plasma membrane Lipid globules Microbodies Nucleus Ribosomes Endoplasmic reticulum Microtubules (cytoskeleton) Mitochondria flagellum Types of Zoospore Biflagellate zoospores whiplash and tinsle posterior position. In Hyphochytridiomycetes, the single flagellum is of tinsel type. It is borne at the anterior end. A small group of endoparasitic slime molds which belong to the class Plasmodiophoromycetes produce biflagellate of unequal size at the anterior end. The smaller flagellum has a blunt tip and the longer has a pointed tip. Types of zoospores Uniflagellate zoospores with posteriore and anterior end. Biflagellate zoospores with unequal size at the anterior end. The more advanced members of the lower fungi which belong to the class Oomycetes produce biflagellate zoospores with the two flagella inserted either at the anterior or laterally position. One of these is of tinsel type and the other is whiplash. The anteriorly flagellate zoospore is usually pear shaped while the laterally flagellate zoospore is reniform. The phenomenon of the presence of two types of zoospores is called diplanetism , as in Saprolegnia . In the advanced lower fungi, the asexual reproduction takes place by aplanospores which are strictly terrestrial, is by means of non-motile, encapsulated, wind disseminated spores known as the aplanospores. They are produced in globose sporangia borne on simple or branch sporangiophores. The primitive genera of the mucorales (Mucor, Rhizopus) have large multispored, columellate sporangia borne singly and terminally on unbranched sporangiophores. The sporangiophores in other gener of the Mucorales are branched, each ultimate branch bearing one or more sporangia which in general are of three types 1- Large sporangia with many spores as in Mucor (A). 2- Small sporangia or sporangiole with few spores even two or three (B & C). 3- Single spored sporangia (D & E). Conidia: Certain of the more advanced families (Peronosporaceae and Albuginaceae) have deciduous sporangia which get detached. They are dispersed, mainly by air currents. The detachable sporangia are often called the conidia and the hyphae bearing them by conidiophores. Some mycologists call them conidiosporangia. On germination in a drop of water (at high humidity condition) they may function as zoosporangia , as Albugo. Therefore , their contents produce many of zoospores. But, when the environment is dry they function as conidia, each conidium germinate directly by putting out one short, slender hypha (called the germ tube), and produce a new fungus , also as Albugo. Chlamydospores: Some members of lower fungi produce thick walled resting cells called chlamydospores. They are , either terminal, intercalary or lateral in position. Under unfaivourable conditions for growth, the protoplasmic contents of coenocytic hyphae accumulate at intervals. The intervening portions remain empty. Each localized accumulation is delimited by septa. Later these thin walled segments of the hyphae secrete thick walls around them to become chlamydospores. The chlamydospores are rich in reserved food materials and are very resistant to desiccation. Chlamydospores Oidia: The Mucorales frequently produce rounded or oval thin walled cells when grown in nutrients. The hyphae undergo segmentation and produce yeast- like cells called oidia. Each oidium during germination, produces a new mycelium under suitable conditions. Chlamydospores Oidia .
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