Oomycota, Kingdom - Chromista

•      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

Oomycota

•      Major characterisitics
–  Biflagellate zoospore – 1 whiplash and 1 tinsel type flagellum
–  Sexual reproduction results in production of oospore
–  Large aseptate hyphae

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

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

Peronosporomycetidae

•      Most highly evolved members of the Oomycota
•      Include aquatic, amphibious and terrestrial forms
•      Saprotrophs and parasites
•      Oogonia produce only one oosphere

Oogonium

•      In Saprolegniomycetidae – multiple oospheres in oogonium – formed from a large central vacuole that produces furrows that cleave out oospheres before fertilization
•      In Peronosporomycetidae – single oosphere in oogonium – oosphere is not delimited by membrane until plasmogamy occurs, no central vacuole, periplasm is present (cytoplasm in oogonium that is not incorporated into oosphere)

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

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