Oomycete vs Fungi : Oomycetes, commonly called as water molds, are a group of fungi with distinct phylogenetic importance. Apart from its phylogenetic importance, Oomycetes are disreputable for their plant pathogenic properties. This group includes the notorious plant pathogen, Phytopthora, which cause late blight of potato, the major culprit in Irish Famine. Even though Oomycetes are traditionally included in the Kingdom Fungi, the current developments in the phylogenetic studies showed that Oomycetes are rather a controversial group which shows more phylogenetic lineage with photosynthetic organisms such as diatoms and brown algae. Morphologically Oomycetes are more allied to the Kingdom Fungi, however, in their phylogeny and physiological features they shows considerable difference from true fungi.
Recent molecular studies using DNA sequencing confirmed that Oomycetes are closely related to algae than true fungi and with this modern evidence, the Oomycetes should be regarded as “colorless” algae rather than true Fungi. In this post we will discuss the similarities and differences of Oomycetes from true fungi which will help you to understand the phylogenetic importance of this group.
[table “” not found /]Similarities between Oomycetes and true Fungi
Ø Oomycetes and true fungi are eukaryotes
Ø Both Oomycetes and true fungi, the vegetative plant body composed of mycelium formed by organized hyphal networks.
Ø Both Oomycetes and true fungi have heterotrophic mode of nutrition.
Ø Both groups obtain nutrients by absorption
Ø Both groups are devoid of chlorophyll pigments and hence they cannot do photosynthesis.
Ø Both groups include parasitic members
Ø In both groups, cell possess cell wall (the chemical nature of cell wall varies)
Ø Both groups includes flagellated structures (flagellated structures are absent in the higher groups of true fungi)
Ø Both groups undergo sexual reproduction
Ø Both groups produce spores as a mode of reproduction
Difference between Oomycetes and True Fungi
(1). Oomycetes
1. Neighboring taxonomic group: Based on the phylogenetic analysis with 18S rRNA, oomycetes are more related to Heterokontae which contain diatoms and golden-brown algae
2. Hyphal architecture: Aseptate and coenocytic tubular hyphae
3. Ploidy of vegetative hyphae: Vegetative phase diploid, except for transient haploid nuclei in the gametangia
4. Dikaryotic phase: No dikaryotic phase in the life cycle, gametic nuclei immediately undergo fusion to form diploid zygote
5. Size of genome: Large genome with 50-250 Mb of DNA
6. Cell wall chemistry: Cell wall composed of cellulose and β-1,3 and β-1,6 linked glucose polymers
7. Pigmentation: Usually un-pigmented vegetative and reproductive structures
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8. Toxic secondary metabolites: No toxic metabolite has been described so far from Oomycetes
9. Mating hormone: Non-peptide, probably lipid like
10. Predominant sexual spores: Sexual spores are un-desiccated, unicellular sporangia (multinucleate cells)
11. Motile asexual spores: Motile sexual spores are nearly universal in Oomycetes. They are biflagellated zoospores.
12. Oospores: Oomycetes produce oospores during sexual reproduction
13. Sexual spores: Sexual spores are oospores, formed on the terminal or specialized hyphae, each containing one viable zygotic nucleus
14. Major energy reserve: Mycolaminarin is the major food reserve. Lipids and polyphosphate are also food reserves
15. Mitochondria: With tubular cristae
16. Flagella: Heterokont, of two types, one whiplash, directed posteriorly, the other fibrous, ciliated, directed anteriorly
17. Lysine biosynthetic pathway: Different from true fungi, Oomycetes has diaminopimelic acid pathway (DAP) similar to higher plants
(2). True Fungi
1. Neighboring taxonomic group: Phylogenetic analysis with 18S rRNA showed that true fungi are related to Animals
2. Hyphal architecture: Either single cell or septate hyphae, with one or more nuclei per cell
3. Ploidy of vegetative hyphae: Vegetative phase is typically haploid or dikaryotic; often with a stable or semi-stable diploid stage following mating
4. Dikaryotic phase: A prominent dikaryotic phase is present in higher groups (Ascomycetes, Basidiomycetes), gamete nuclei stay together without fusion and they undergo simultaneous division when the cell divides
5. Size of genome: Comparatively small genome with only 10 – 40 Mb
6. Cell wall chemistry: Cell wall primarily composed of chitin (β-1,4 linked N-acetylglucosamine) and/or chitosan (β-1,4 linked glucosamine)
7. Pigmentation: Pigmentation is very common in hyphae or spores. Common pigments are melanin and carotenoids.
8. Toxic secondary metabolites: Toxic metabolites are common in true fungi. They are typically aromatic, heterocyclic compounds or proteins.
9. Mating hormone: Usually small peptides or lipo-peptides
10. Predominant sexual spores: Sexual spores are desiccated single or multicellular conidia (one nucleus per cell)
11. Motile asexual spores: Motile spores are very uncommon in true fungi. They are present only in Chytrids. The zoozpores of Chytrids are monoflagellate
12. Oospores: Oospores are absent in true fungi
13. Sexual spores: Various types of sexual spores are formed in true fungi. They are often formed in large numbers within complex enclosures such as perithecia, apothecia or mushroom caps.
14. Major energy reserve: Glycogen is the primary food reserve. Other food reserves are trehalose, sugar alcohols and lipids
15. Mitochondria: With flattened cristae
16. Flagella: If flagellum produced, usually of only one type: posterior, whiplash
17. Lysine biosynthetic pathway: Different from Oomycetes, True fungi has α-amino adipic acid (AAA) pathway.
Oomycete vs Fungi
Sl. No. Features Oomycetes True Fungi 1 Neighboring taxonomic group Diatoms and golden-brown algae Animals 2 Hyphal architecture Aseptate and coenocytic tubular hyphae Either single cell or septate hyphae, with 1 or 2 nuclei per cell 3 Ploidy of vegetative hyphae Diploid Haploid or dikaryotic 4 Dikaryotic phase Absent Present in higher groups 5 Size of genome Large: 50-250 Mb Small: 10 – 40 Mb 6 Cell wall carbohydrate Cellulose Chitin 7 Pigmentation un-pigmented Pigmentation very common 8 Toxic secondary metabolites Not reported Commonly present 9 Mating hormone Non-peptide type Usually peptides 10 Sexual spores Multinucleate sporangia Uni-nucleate conidia 11 Motile asexual spores Present Very uncommon, (present only in Chytrids) 12 Oospores Present Absent 13 Sexual spores Oospores are sexual spores Various types of sexual spores 14 Spore bearing structures Well organized sore bearing structures absent Spore bearing structures such as perithecia, apothecia, basidiocarp etc. present 15 Major energy reserve Mycolaminarin Glycogen 16 Mitochondria With tubular cristae With flattened cristae 17 Flagella Heterokont, of two types, one whiplash, another ciliated Flagella usually absent, If present, only whiplash type 18 Lysine biosynthetic pathway Through diaminopimelic acid pathway (DAP) Through α-amino adipic acid (AAA) pathway.
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