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In this article we will discuss about the features and significance of oomycetes.
Salient Features of Oomycetes:
Oomycetes contain 74 genera with 580 species. The members of the class Oomycetes are characterized by the oogamous type of sexual reproduction and the presence of biflagellate zoospores which lack cell wall. The gametes are non-flagellate.
The members of the class are typically aquatic, either free-living (i.e., saprophytic) or parasitic on algae, acquatic fungi, small animals, and other forms of aquatic life. Some Oomycetes are terrestrial and many of them cause downy mildew diseases on higher plants.
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The salient features of the class are as follows:
(i) Most of the members are aquatic but some grow on the soil and the others attack the shoots of terrestrial plants.
(ii) The primitive forms are unicellular, whereas the advanced forms have a well-developed branched, filamentous, and coenocytic mycelium.
(iii) The cell wall is mainly composed of cellulose-β-glucan and chitin is altogether absent.
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(iv) A sexual reproduction takes place by means of pyriform or reniform biflagellate zoospores, which are devoid of cell wall. In pyriform zoospores, the flagella are attached anteriorly and in reniform laterally. One flagellum is of whiplash type and the other of tinsel type.
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(v) In primitive aquatic form, zoosporangia are not well differentiated from somatic hyphae, whereas in advanced forms distinct zoosprangia are found.
(vi) Some forms also produce non-motile asexual spores, known as conidia.
(vii) Most of the Oomycetes are eucarpic, but the members of the order Lagenidiales are holocarpic.
(viii) The sexual reproduction is of oogamous type. The male and female sex organs are known as antheridia and oogonia, respectively. The sex organs are either terminal or intercalary.
(ix) It is now established that meiosis in Oomycetes takes place within the diploid nuclei of gametangia (antheridia and oogonia) hence is called gametangial-meiosis. That is, zygotic meiosis is absent. As a result, the vegetative thallus is diploid.
(x) The gametes are non-flagellate. The fusion of gametes results in the formation of thick-walled oospores.
(xi) Oomycets show interesting parallel changes in their biology and morphology, which are closely connected with their phylogeny.
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These changes are the following:
(a) Transition from aquatic to terrestrial existence.
(b) Replacement of sporangia producing zoospores by conidia, a change from water-dispersal to air-dispersal.
(c) Alteration is biological requirements from saprophytism to obligate parasitism.
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(d) Refinement of parasitism increasing host specificity and even organ specificity.
Significance of Oomycetes:
(i) Many terrestrial Oomycetes cause serious plant diseases such as late blight of potato (Phytophthora infestans), downy mildew of grape-wine (Plasmopara viticola), white rust of crucifers (Albugo candida), damping off of seedlings (Pythium spp.) and others.
(ii) Late blight of potato (Phytophthora infestans) and downy mildew of grape-vine (Plasmopara viticola) are historically very important. The late blight of potato resulted in Irish Famine’ in mid-19th century that caused death of millions of people and Europe. Downy mildew of grape-vine (Plasmopara viticola) helped Prof. Millardet to discover the first fungicide called ‘Bordeaux mixture’.
(iii) Many members of this group help understand the evolutionary migration of fungi from aquatic to terrestrial environments.
