Study guide: Oomycete Biology and Life Cycles

📋 Course Outline

1. Oomycete Identity and Fungal Similarities
2. Cellular and Biochemical Traits
3. Diploid Biology and Sexual Reproduction
4. Zoospores and Asexual Dispersal
5. Zoosporangia and Spore Release
6. Oomycete Orders and Diversity
7. Plant and Animal Pathogens

📖 1. Oomycete Identity and Fungal Similarities

🔑 Key Concepts & Definitions

• Oomycetes: Stramenopilous organisms called water molds that resemble fungi in lifestyle but do not belong to kingdom Fungi.

★ Must-know

• Oomycetes have complex, branching, tip-growing hyphae that form mycelia and obtain heterotrophic nutrition by absorption after enzyme exudation. - Beakes and Sekimoto, 2009

Further detail

• Oomycetes have fungal-like ecological roles and modes of heterotrophic nutrition. - Richards et al., 2006

• Some oomycetes infect plants or animals.

• More than 700 oomycete species are known, with aquatic and terrestrial representatives and saprophytic or pathogenic lifestyles.

💡 Memory Hook

Fungal-like lifestyle, but oomycetes are not fungi

📖 2. Cellular and Biochemical Traits

★ Must-know

📌 Oomycete cell walls primarily contain cellulose and β-1,3 and β-1,6 glucans, whereas true fungal walls contain chitin and β-glucans.

📌 Oomycetes have motile cells with two heterokont flagella, whereas true fungi lack motile cells or have cells with one posterior flagellum.

Further detail

• Oomycete mitochondria have tubular cristae, whereas true fungal mitochondria have lamellar cristae.

• Oomycetes have Golgi stacks called dictyosomes, whereas the fungal Golgi apparatus is reduced to single cisternae.

• Oomycetes synthesize lysine through the diaminopimelic acid pathway, whereas true fungi use the alpha-aminoadipic acid pathway.

• Oomycetes store mycolaminarins, whereas true fungi store glycogen.

• Oomycete lysine biosynthesis uses the diaminopimelic acid pathway, which is also found in plants.

• Fucosterol acts as a hormone and is required for oomycete sexual reproduction.

📖 3. Diploid Biology and Sexual Reproduction

🔑 Key Concepts & Definitions

• Oospore: The resistant sexual spore formed as a diploid zygote after fertilization of an oosphere.

★ Must-know

• Oomycetes are predominantly diploid, and their somatic nuclei are diploid.

• 🔄 Sexual reproduction proceeds through these stages:
  1. Meiosis occurs in the antheridia and oogonia.
  2. Haploid oocytes and antheridial nuclei form.
  3. A fertilization tube transfers an antheridial nucleus to an oosphere.
  4. Fertilization produces a diploid oospore.

Further detail

• Oomycetes may be monoecious or dioecious and homothallic or heterothallic.

• An oogonium produces one or more haploid oospheres, while an antheridium produces haploid polar or sperm nuclei.

• An oospore has a double wall, which may be ornate, and can remain dormant for up to 6 years.

• An oospore germinates to produce vegetative diploid hyphae, some of which develop into zoosporangia.

💡 Memory Hook

Meiosis in gametangia → fertilization → diploid oospore

📖 4. Zoospores and Asexual Dispersal

🔑 Key Concepts & Definitions

• Zoospore: A diploid asexual dispersal spore produced by mitosis in a zoosporangium.

★ Must-know

• Oomycete zoospores are biflagellate, with a tinsel flagellum bearing mastigonemes and a whiplash flagellum.

Further detail

📌 Primary zoospores have anteriorly attached flagella, whereas secondary zoospores have laterally attached flagella.

📌 Monoplanetic spores have one swimming period followed by encystment, diplanetic spores have two swimming periods separated by encystment and emergence, and polyplanetic spores have multiple swimming and encystment periods.

• Zoospores encyst after adhering to a host or substrate and are not gametes.

💡 Memory Hook

Primary zoospores carry anterior flagella; secondary ones carry lateral flagella

📖 5. Zoosporangia and Spore Release

🔑 Key Concepts & Definitions

• Zoosporangium: A modified hypha, usually terminal, delimited by a septum, that produces zoospores.

📝 Essential Points

📌 Zoosporangia may be persistent or deciduous, and deciduous zoosporangia can act as dispersal agents.

📖 6. Oomycete Orders and Diversity

★ Must-know

📌 Saprolegniales produce primary and secondary zoospores, with spores always forming inside the sporangium, whereas Peronosporales produce only secondary zoospores and spores may form in the sporangium or an evanescent vesicle.

Further detail

• Typical Saprolegniales traits include: large-diameter coenocytic hyphae, mostly hermaphroditic and homothallic reproduction, long cylindrical terminal zoosporangia

• The Saprolegniales include Saprolegnia and Achyla.

• Named Peronosporales include:

  • Phytophthora
  • Pythium
  • Peronospora
  • Plasmopara
  • Hyaloperonospora
  • Pseudoperonospora

💡 Memory Hook

Saprolegniales form primary and secondary zoospores; Peronosporales form only secondary ones

📖 7. Plant and Animal Pathogens

★ Must-know

• Phytophthora infestans causes late blight of potato.

Further detail

• Phytophthora ramorum causes sudden oak death, bleeding canker, and tan oak blight.

• Phytophthora cinnamomi causes dieback of Jarrah.

• Phytophthora rubi and Phytophthora fragariae are associated with root rot.

• Pythium species cause damping off and are described as annoying but not devastating plant pathogens.

• Pythium insidiosum is a facultative animal pathogen.

• Peronosporales are aquatic, amphibious, or terrestrial organisms and include devastating plant pathogens.

📊 Synthesis Tables

Saprolegniales and Peronosporales

Trait	Saprolegniales	Peronosporales
Zoospore types	Primary and secondary	Secondary only
Spore formation	Always in sporangium	In sporangium or evanescent vesicle
Hyphae	Large diameter	Not specified
Representative genera	Saprolegnia, Achyla	Phytophthora, Pythium