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Section C

General characteristics of Mastigomycotina :-

  • One large group of the Mastigomycotina is aquatic. While another group of the Mastigomycotina are primarily terrestrial, although the organisms still form motile zoospores when open water is available
  • The members of Mastigomycotina produce flagellated zoospores in their life cycle.
  • Most of them are filamentous and have coenocytic mycelium. However, unicellular form are present, and some genera show the pseudosepta (false cross wall) formation.
  • Rhizoids are present in some of unicellular forms.
  • They show centric nuclear division. Their centrioles remain functional during nuclear division.
  • Live either as saprophytes or parasites.Due to presence of haustoria in a majority of Mastigomycotina , the mode of nutrition is typically absorptive.
  • The sexual reproduction takes place by different methods , oospores formation are common in almost all Mastigomycotina

Three classes are included in this sub-division, on the basis of zoospore and oospore and comprise 204 genera and 1160 species:-

  • Chytridiomycetes produces posteriorly uniflagellate zoospores Chytridiomycetous fungi occur as saprobes on plants and animal remains in water while other members occur as parasites on algae and aquatic animals.
  • Hyphochytridiomycetes :- Zoospores are anteriorly uniflagellate.The Hyphochytridiomycetes are those aquatic fungi whose thallus is holocarpic or eucarpic, monocentric or polycentric and their vegetative system is rhizoidal or hypha-like with intercalary swellings.
  • Oomycetes :- The Oomycetes contain 74 genera and 580 species, which are mostly aquatic, though some are terrestrial and live as parasites or saprophytes.Includes classic “water molds” in the Order Saprolegniales and the “downy mildews” in the Order Peronosporales.

General characteristics of Class :- Oomycetes :-

  • Vegetative body is filamentous and coenocytic except the unicellular Lagenidiales.
  • Holocarpic or Eucarpic
    1. Holocarpic - Entire thallus converted into reproductive structures
    2. Eucarpic - Reproductive organs arise from only a portion of the thallus and remainder continues as somatic Majority of species are eucarpic.
  • Cell wall contains cellulose and glucans.Chitin is absent.
  • Asexual reproduction is by biflagellate heterokont (different) and anisokont (unequal) zoospores that are produced in zoosporangia.
  • Zoosporangia- Modified hyphae that are usually terminal and delimited by a septum
  • Zoospores are diploid formed by mitosis
  • Anteriorly directed flagellum is tinsel type and posteriorly directed is whiplash type .Depending on genera single type-monomorphic or two types of zoospores are formed-dimorphic . Two types of zoospores are formed in the life cycle are :-
    1. Primary zoospores :- First formed, pip-shaped, and the flagella are located anteriorly. Primary zoospore is released from the zoosporangium, encyst and germinates to form the secondary zoospore.
    2. Secondary zoospores :- The secondary zoospore which is reniform or bean-shaped and laterally flagellated.
  • Zooporangium and zoospores are the major dispersal agents for most species.
  • Sexual reproduction:- Sexual reproduction is heterogamous (oogamous) by oogonia (female) and antheridia (male). Female gamete (oosphere) produced by an oogonium. Depending on taxon, there may be one to many oospheres per oogonium Male gamete is produced by antheridium and transferred to the oogonium by gametangial contact and migration of male nuclei into oogonia and fertilize oospheres Homothallic– self-fertile or Heterothallic– opposite mating types required for sexual reproduction. A swimming sperm is absent in the Oomycetes. This type of sexual reproduction is referred to as gametangial copulation. In antheridia and oogonia meiosis take place. The eggs and sperms are products of meiosis and the only parts of the life cycle that are haploid. Diploid zygote develops into thick-walled resistant oospore that germinates and give rise to vegatative diploid hyphae that reproduce asexually by production of zoospores.

 

The vegetative body is diploid and the life cycle is diplontic.

Class Oomycetes is divided into four orders.

  • Lagenidiales (Salilagenidiales)
  • Leptomitales
  • Saproleginales
  • Peronosporales :- This order has some of the most well known pathogens (fungi cause diseases) cause diseases to many a crop plants. Peronosporales:- divided into three families :-
    1. Pythiaceae - Pithium,Phytophthora
    2. Peronosporaceae-Plasmopara
    3. Albuginaceae -Albugo.
  • Peronosporales differs from the Saprolegniales in producing only secondary zoospores in a zoosporangium
  • That is differentiated from hyphae (eucarpic) and one oosphere (egg) per oogonium.
  • Zoosporangia often deciduous

  • Zoospores often formed in vesicle
  • They are aquatic, amphibious, terrestrial and some of the most destructive plant pathogens .

The most economically important group of Oomycetes is the Peronosporales that contain the late blight of potato fungus Phytophthora infestans and relatives such as Peronospora, Bremia, Plasmopara and others that cause “downy mildews”, the “damping off” fungi, Pythium spp., and the white rust fungi, Albugo spp.

However, although members of Mastigomycotina are morphologically similar, they share no close phylogenetic relationships to fungi. The combination of cellulose cell wall, biflatellated zoospores, one flagellum of the tinsel type and the other of the whiplash type, and gametangial copulation are characteristics that are shared with some members of the algal divisions Phaeophyta and Chrysophyta. This has recently led to recognition of yet another kingdom, Stramenopila, which includes the divisions Bacillariophyta, Chrysophyta, Phaeophyta, Hyphochytridiomycota and Oomycota. These divisions are now thought to be derived from a common ancestor.