Oomycota

Division: Oomycota

The division Oomycota, unlike the Chytridiomycota, is a group of aquatic fungi that have cell walls composed of cellulose and a diploid dominant lifecycle (=gametic lifecycle). The mycelium is coenocytic and produce septa only to separate the reproductive structures from the assimilative portion of the thallus. Asexual reproduction is by zoospores that are produced in zoosporangia. The zoospores produced are biflagellated with one flagellum of the whiplash type and the other of the tinsel type. Sexual reproduction is heterogamous and occurs by direct injection of the male nuclei (=sperms) from the antheridium into the eggs contained in the oogonium . A swimming sperm is absent in the Oomycota. This type of sexual reproduction is referred to as gametangial copulation. The eggs and sperms are products of meiosis and the only parts of the life cycle that are haploid. Only one class, the Oomycetes, is recognized in this division.

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

Class: Oomycetes

The characteristics of the class Oomycetes is the same as those of the division.

We will examine two representatives in the Oomycota: Saprolegnia sp. (Saprolegniales) and Phytophthora palmivora (Peronosporales).

Saprolegnia:

Asexual Reproduction: Zoosporangia are not differentiated, in this genus, from the mycelium and resemble hyphal cells. However, they can be easily located since they are delimited by septa and have much denser protoplasm than the rest of the mycelium. Saprolegnia zoosporangia can be identified by the internal proliferation of a new zoosporangium inside of the previous one (Fig. 1). Compare this genus with Achyla, which as lateral proliferation of zoosporangia, next to and below the previous zoosporangium (Fig. 2).Two types of zoospores are produced: The primary zoospore, which is posteriorly biflagellated, is released from the zoosporangium, encyst and germinates to form the secondary zoospore which is reniform (=bean-shaped) and laterally flagellated.

Figure 1: Zoosporangium of Saprolengia: Note proliferation of zoosporangium through previous zoosporangium. Youngest zoosporangium has dense protoplasm with basal septum.
Figure 2: Zoosporangium of Achyla: Note lateral proliferation of zoosporangium below previous zoosporangium. Primary zoospores do not have a swarming period. Instead, they encyst immediately
upon being released from the zoosporangium.

Sexual Reproduction: The oogonium (Fig. 3) contains from 2 to 20 eggs and typically have several antheridia attached to the oogonial wall at maturity. Sperm nuclei from the antheridium are injected directly to the eggs and form the zygote. The egg and sperm are the products of meiosis and are the only haploid stage in the life cycle.


Figure 3: Oogonia and Eggs of Achyla, with attached Antheridia. Note that there are more than a single egg in each oogonium, a characteristic of the Saprolegniales.


Phytophthora palmivora:

Asexual Reproduction: The Zoosporangium (Fig. 4-6) is typically broad-ellipsoid and well differentiated from the vegetative myucelium in this species. Because the zoosporangia are readily deciduous, they will usually not be attached to the mycelium when mounted on a microscope slide for observation. Only the secondary zoospore is produced in this species. The micrographs below were taken under phase interference optics, which is responsible for the dark appearance of the images.

Figure 4: Intact zoosporangium of Phytophthora palmivora before release of zoospore. Note that it is morphologically differentiated from the hyphae, unlike those produced by Achyla and Saprolegnia.

Figure 5: Zoosporangium of Phytophthora palmivora with zoospores clustered together and oozing out of operculum.

Figure 6: Zoospores of Phytopthora palmivora coming apart. Two zoospores swiming away can be seen at the top of the screen.

 

Sexual Reproduction: The oogonium in this species, unlike Saprolegnia, contains only one egg.

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