L. 6-Biotechnology Mycology D. Ebtihal Muiz Oomycota Division: Oomycota




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L.6-Biotechnology Mycology D.Ebtihal Muiz

Oomycota

Division: Oomycota

The division Oomycota, unlike the Chytridiomycota, is a group of aquatic fungi that have cell walls composed of cellulose and a diploid dominant lifecycle (=gametic lifecycle). The mycelium is coenocytic and produce septa only to separate the reproductive structures from the assimilative portion of the thallus. Asexual reproduction is by zoospores that are produced in zoosporangia. The zoospores produced are biflagellated with one flagellum of the whiplash type and the other of the tinsel type. Sexual reproduction is heterogamous and occurs by direct injection of the male nuclei (=sperms) from the antheridium into the eggs contained in the oogonium . A swimming sperm is absent in the Oomycota. This type of sexual reproduction is referred to as gametangial copulation. The eggs and sperms are products of meiosis and the only parts of the life cycle that are haploid. Only one class, the Oomycetes, is recognized in this division.

The combination of cellulose cell wall, biflatellated zoospores, one flagellum of the tinsel type and the other of the whiplash type, and gametangial copulation are characteristics that are shared with some members of the algal divisions Phaeophyta and Chrysophyta. This has recently led to recognition of yet another kingdom, Stramenopila, which includes the divisions Bacillariophyta, Chrysophyta, Phaeophyta, Hyphochytridiomycota and Oomycota. These divisions are now thought to be derived from a common ancestor .

• This phylum of organisms differs in fundamental ways from the rest of the organisms in the Kingdom Fungi – cell wall chemistry, lysine biosynthesis, rDNA sequences
•  Basic form of thallus in most is the mycelium with hyphae, some form holocarpic thalli
• Significant economic group – contains species that are important plant pathogens

Evolutionary trends in Oomycota

•      Vegetative thallus – in some species, it is holocarpic, endobiotic – most for well developed aseptate hyphae (eucarpic, polycentric)
•      Nutrition – some species are aquatic (both saprotrophic and parasitic). Others are highly specialized biotrophic parasites of higher plants
•      Asexual reproduction – In many, zoospores are produced in a zoosporangium, in some the number of zoospores/sporangium is reduced. In a few species, the sporangium functions as a conidium (germinates with a germ tube)

Oomycota

•      Contains one class – Oomycetes
•      Two subclasses:
–  Saprolegniomycetidae – one order
•   Saprolegniales
–  Peronosporomycetidae – five orders, we will discuss representatives of two
•   Pythiales
•   Peronosporales


Class: Oomycetes

The characteristics of the class Oomycetes is the same as those of the division.



We will examine two representatives in the Oomycota: Saprolegnia sp. (Saprolegniales) and Phytophthora palmivora (Peronosporales).

Saprolegniales

•      Vegetative thallus varies from relatively simple holocarpic thallus (parasitic on algae and fungi) to well developed mycelium
•      Hyphae are aseptate
•      Commonly called “water molds”
•      Most are saprotrophs, some parasitize fish and fish eggs – can cause large economic losses to fish hatcheries

Asexual reproduction

•      Zoospores produced by long cylindrical zoosporangia typically formed at the hyphal tips (formed when immersed in water)

Zoospores

•      Two types of zoospores produced sequentially = dimorphic
•      If they produce only one type of zoospore = monomorphic
•      Primary zoospores are pear shaped, poor swimmers
•      Secondary zoospores are oval to kidney shaped and better swimmers

Primary & secondary zoospores

•      Species vary in sequence of events in formation of primary and secondary zoospores (see handout)

Zoospores

•      Saprolegnia – primary zoospore swims away before encysting, then forms secondary zoospore
      Achlya – 1º zoospore encysts right outside zoosporangium, then forms 2º zoospore
      Dichtyuchus – No 1º flagellated zoospores, they encyst in the zoosporangium, cysts germinate to form 2º zoospores or they may germinate with a germ tube

 Asexual reproduction

•      May also produce gemmae – irregularly shaped hyphal segments that separate from the thallus and can germinate (germ tube or zoospores) to form a new thallus

Sexual life cycle

•      For a long time the Oomycetes were thought to have a haploid vegetative phase like most other fungi
•      Since nuclei are small, it is difficult to determine where meiosis takes place
•      Microspectrophotometry – allowed concentration of DNA in nuclei to be measured in situ – indicated that vegetative mycelium is diploid, meiosis takes place in gametangia
•      Exhibit a diploid life cycle (unusual for fungi

Life cycle

•      Vegetative hyphae are diploid
•      Undergo asexual reproduction by formation of zoospores in zoosporangia

•      Gametangia formed & meiosis occurs–
•      Homothallic & heterothallic species
•      Male gametangium – antheridium
•      Female gametangium – oogonium that contains multiple gametes – oospheres in the Saprolegniales

•      Antheridium grows to oogonium
•      Forms fertilization tubes to oospheres
•      Nuclei from antheridia migrate through fertilization tubes
•      Plasmogamy and karyogamy take place
•      Oosphere is now diploid
•      Develops thick wall and becomes an oospore

Oogonium

•      Oospore can remain dormant
•      Germinates to produce zoosporangium or  2n mycelium


Saprolegnia:

Asexual Reproduction: Zoosporangia are not differentiated, in this genus, from the mycelium and resemble hyphal cells. However, they can be easily located since they are delimited by septa and have much denser protoplasm than the rest of the mycelium. Saprolegnia zoosporangia can be identified by the internal proliferation of a new zoosporangium inside of the previous one (Fig. 1). Compare this genus with Achyla, which as lateral proliferation of zoosporangia, next to and below the previous zoosporangium (Fig. 2).Two types of zoospores are produced: The primary zoospore, which is posteriorly biflagellated, is released from the zoosporangium, encyst and germinates to form the secondary zoospore which is reniform (=bean-shaped) and laterally flagellated.





Figure 1: Zoosporangium of Saprolengia: Note proliferation of zoosporangium through previous zoosporangium. Youngest zoosporangium has dense protoplasm with basal septum.









Figure 2: Zoosporangium of Achyla: Note lateral proliferation of zoosporangium below previous zoosporangium. Primary zoospores do not have a swarming period. Instead, they encyst immediately

upon being released from the zoosporangium.




Sexual Reproduction: The oogonium (Fig. 3) contains from 2 to 20 eggs and typically have several antheridia attached to the oogonial wall at maturity. Sperm nuclei from the antheridium are injected directly to the

eggs and form the zygote. The egg and sperm are the products of meiosis and are the only haploid stage in the life cycle.





Figure 3: Oogonia and Eggs of Achyla, with attached Antheridia. Note that there are more than a single egg in each oogonium, a characteristic of the Saprolegniales.

Peronosporomycetidae

•      Sporangia may produce zoospores but only secondary zoospores or,
•      Trend in this subclass is for the zoosporangia to produce smaller number of zoospores
•      In some species, sporangia may germinate with a germ tube (called conidia)

Pythiales

•      Pythium – a large genus that includes saprotrophs (soil and water) and facultative parasites of algae, fungi and plants
•      One species causes “damping off” of seedlings – grows intercellularly in stems and roots of seedlings and rots tissue
•      Asexual reproduction by zoosporangia and zoospores (secondary zoospores only)

•      Contents of zoosporangium empty into a vesicle, cleave and form zoospores outside zoosporangium

Pythiales

•      Phytophthora – facultative plant parasites
      In plant host, form intercellular – between cells - and intracellular – penetrate cells - hyphae
      Intracellular hyphae produce haustoria – exchange of nutrients from host to parasite

 Phytophthora

•      Forms lemon shaped sporangia that detach from sporangiophore in asexual reproduction – sporangia can germinate by forming zoospores or forming a germ tube directly


Phytophthora palmivora:

Asexual Reproduction: The Zoosporangium (Fig. 4-6) is typically broad-ellipsoid and well differentiated from the vegetative myucelium in this species. Because the zoosporangia are readily deciduous, they will usually not be attached to the mycelium when mounted on a microscope slide for observation. Only the secondary zoospore is produced in this species. The micrographs below were taken under phase interference optics, which is responsible for the dark appearance of the images.





Figure 4: Intact zoosporangium of Phytophthora palmivora before release of zoospore. Note that it is morphologically differentiated from the hyphae, unlike those produced by Achyla and Saprolegnia.









Figure 5: Zoosporangium of Phytophthora palmivora with zoospores clustered together and oozing out of operculum.









Figure 6: Zoospores of Phytopthora palmivora coming apart. Two zoospores swiming away can be seen at the top of the screen.

 

Sexual Reproduction: The oogonium in this species, unlike Saprolegnia, contains only one egg.


Peronosporales

•      Albugo – obligate plant parasite, causes white rust
      Sporangia detach, dispersed by wind
      If moisture available, sporangia produce 4-12 zoospores
      Can germinate directly with a germ tube

Peronosporales

•      Peronospora and related genera – obligate parasites of plants – cause downy mildews
      In some species, sporangia always germinate with a germ tube, never form zoospores – called conidia









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