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Phycomycetes
Phycomycetes comes from the Greek words "phykos" (algae) and "mykes" (fungus). They were historically considered a link between algae and fungi due to specific characteristics, such as their coenocytic thallus. This group includes some of the most primitive fungi. They are typically found in aquatic habitats, on decaying wood in moist and damp places, or as obligate parasites on plants.
1.0Characteristics of Phycomycetes
Habitat :
- All the fungi included in this class are called as lower fungi.
- Members of phycomycetes are found in aquatic habitats (members of this class known as algal fungi) and on decaying wood in moist and damp places or as obligate parasites on plants.
Mycelium :
- The fungal filament (mycelium) of the fungi included in this class are coenocytic, aseptate and branched.
- Asexual reproduction = by zoospores, aplanospores and conidia.
- Sexual reproduction = may be isogamous, anisogamous and oogamous.
- Phycomycetes includes further groups like oomycetes, zygomycetes
2.0Classification of Phycomycetes
The classification of Phycomycetes has changed over time. They were once a formal class, but modern fungal taxonomy, based on molecular and genetic data, has reclassified them into separate phyla.
Oomycetes:
- Also known as water molds.
- Their cell wall is made of cellulose.
- Asexual reproduction occurs via zoospores.
- Sexual reproduction is oogamous, producing oospores.
- Phycomycetes examples from this group include Saprolegnia (a water mold) and Albugo (a parasitic fungus causing white rust on plants). Phytophthora infestans, the causal agent of late blight of potato, is another significant example.
Asexual reproduction :-
- By the formation of zoospores and conidia.
Sexual reproduction :-
- By Gametangial contact
- Type of sexual reproduction
- Oogamous - All the members
(i) Phytophthora infestans: - Causes late blight of potato. This disease is known as " Famine of Ireland" - 1845
(ii) Pythium species: - Causes "Damping off" disease in tobacco & "vegetable crops".
(ii) Albugo candida or Cystopus candidus: - It causes "White rust or white spots disease" in the members of cruciferae family.
Zygomycetes:
- Also known as bread molds or pin molds.
- Their cell wall is composed of chitin.
- Asexual reproduction occurs via non-motile spores (aplanospores).
- Sexual reproduction results in a thick-walled, resistant zygospore formed by the fusion of two gametangia.
- Types of sexual reproduction :
- Isogamous: - All the members
- e.g.: - (i) Pilobolus : -
- (ii) Rhizopus & Mucor :- These are known as bread mold . They prefer to grow on bread.
3.0Life Cycle of a Representative Phycomycete: Rhizopus
To truly grasp the concepts, let's look at the life cycle of a classic Phycomycetes example, Rhizopus (bread mold), which belongs to the Zygomycetes.
Asexual Reproduction
- The mycelium of Rhizopus is a network of coenocytic hyphae.
- Specialized hyphae called sporangiophores grow upwards and develop a sac-like structure called a sporangium at their tips.
- Inside the sporangium, spores are formed through mitosis.
- When the sporangium matures, it ruptures, releasing a large number of non-motile spores (aplanospores) into the air.
- If a spore lands on a suitable substrate (like a piece of bread), it germinates and develops into a new mycelium.
Sexual Reproduction
- Sexual reproduction in Rhizopus occurs when two compatible mating types (+ and - strains) come into contact.
- Hyphae from both strains grow towards each other.
- They form short, multinucleate, club-shaped structures called progametangia.
- The tips of these progametangia are cut off by septa to form gametangia.
- The walls separating the two gametangia dissolve, and their protoplasts fuse (plasmogamy).
- The nuclei of the two gametes fuse (karyogamy), forming diploid nuclei.
- This fusion leads to the formation of a thick-walled, dormant structure called a zygospore.
- Under favorable conditions, the zygospore germinates. The diploid nucleus undergoes meiosis, and a new sporangium (germ sporangium) is formed, which produces haploid spores.