Oomycetes Vs True Fungi: Similarities and Differences

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.

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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)

Difference between Oomycetes and True Fungi

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.FeaturesOomycetesTrue Fungi
1Neighboring taxonomic groupDiatoms and golden-brown algaeAnimals
2Hyphal architectureAseptate and coenocytic tubular hyphaeEither single cell or septate hyphae, with 1 or 2 nuclei per cell
3Ploidy of vegetative hyphaeDiploidHaploid or dikaryotic
4Dikaryotic phaseAbsentPresent in higher groups
5Size of genomeLarge: 50-250 MbSmall: 10 – 40 Mb
6Cell wall carbohydrateCelluloseChitin
7Pigmentationun-pigmentedPigmentation very common
8Toxic secondary metabolitesNot reportedCommonly present
9Mating hormoneNon-peptide typeUsually peptides
10Sexual sporesMultinucleate sporangiaUni-nucleate conidia
11Motile asexual sporesPresentVery uncommon, (present only in Chytrids)
13Sexual sporesOospores are sexual sporesVarious types of sexual spores
14Spore bearing structuresWell organized sore bearing structures absentSpore bearing structures such as perithecia, apothecia, basidiocarp etc. present
15Major energy reserveMycolaminarinGlycogen
16MitochondriaWith tubular cristaeWith flattened cristae
17FlagellaHeterokont, of two types, one whiplash, another ciliatedFlagella usually absent, If present, only whiplash type
18Lysine biosynthetic pathwayThrough diaminopimelic acid pathway (DAP)Through α-amino adipic acid (AAA) pathway.

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