Classification:
Kingdom-Straminopila
Phylum-Oomycota
Order-Saprolegnales
Family-Saprolegniaceae
Genus-Saprolegnia
Occurrence: The genus is commonly known as water mould. They occur in fresh water and soil. Most saprolegnia are saprophytes, living on plant and animal debris in water and damp soil. Some species are parasitic on microscopic animals, fish, amphibians and their eggs.
Vegetative structure: The vegetative body of saprolegnia is a well developed mycelium, it is aseptate, unbranched hyphae. The hyphal wall consist of cellulose. The thallus is diploid and each hypha consist of a wall and a multinucleate mass of colourless and highly vacuolated cytoplasm.
Reproduction
It takes place by three methods they are fallowing
Vegetative reproduction
a) By Fragmentation: In favourable condition the hyphae break up into many fragments. Each fragment germinates into a new mycelium.
b) By Gemmae or Chlamydospores: During the formation of Gammae, certain proteins of the hyphae acquire dense cytoplasm and become cut off from the hyphae by means of septa. These unicellular bodies are termed as the gemmae. These may be terminal or intercalary; single or in chains.
The gemmae are ovoid, globose, or irregular in shape. After rest period they separate from the parent hyphae and each gemmae germinate into a new mycelium.
Asexual reproduction
Saprolegnia reproduce asexually by means of diploid and biflagellate zoospores, in zoosporangium.
During the formation of a zoosporangium, the tip of the hypha enlarges and becomes filled with cytoplasm. A large number of nuclei migrate to this hyphal tip and accumulate there. This swollen tip is finally cut off from the remaining hypha by cross wall, thus forming a zoosporangium. The multinucleate cytoplasm within the zoosporangium becomes divided to give rise to a large number of biflagellate zoospores.
When they mature, the zoospores are liberated one by one through an apical pore of the zoosporangium. These zoospores are called primary zoospores. After liberation primary zoospore swims for a while, withdraws its flagella, secretes a thin wall to form spherical primary cyst. After a period of rest, the protoplast of each cyst comes out of the wall, and give rise to another from the zoospores which is of different shape and is called secondary zoospore. The occurrence of two morphological types of zoospores is known as the phenomenon of diplanetism. The species, showing such two motile periods separated by period of encystment, are called diplanetic. The secondary zoospores, after a brief period of motility, again encysts and germinates by producing a germ tube that develops into a mycelium.
Sexual reproduction
The sexual reproduction in Saprolegnia is oogamous and occurs by gametangial contact. The sex organs called antheridia and oogonia are usually cut off from the mycelium by cross walls. Many species are saprolegnia are homothallic; others are heterothallic.
Development of oogonium: Each oogonium originates as a terminal spherical structure at the end of main hypha. Later, it is delimited from the vegetative hypha by a septum. The oogonium is diploid. At first, the oogonium is multinucleate, but eventually some of the nuclei disintegrate and the protoplasm differentiate into a number of uninucleate eggs.
Development of antheridium: The antheridium develops singly as a tubular outgrowth from a hypha; below the oogonium, or from a different hypha other than the oogonium. The antheridium, developing from the sane hypha that bear the oogonium, is called monoclinous. If the antheridium develops from different hypha are called declinous.
Many nuclei and the cytoplasm pass into the slender outgrowth which is cut off from the rest of vegetative mycelium by a basal wall; eventually an antheridium is formed.
Each antheridium is a tubular structure and contains several nuclei. Sometimes, the antheridium may be branched.
The antheridium comes in contact with the oogonium and meiosis occurs in each organ. The antheridium penetrates the oogonial wall with elongate fertilisation tubes, sending one tube to each haploid oosphere. One haploid male nucleus passes into each oosphere to effect plasmogamy. Eventually, the nuclei fuse and the diploid oospore undergoes a resting period before germination to form a new thallus.