Oomycetes are characterised by oogamous type of sexual reproduction and the presence of biflagellate zoospores .
Some of the orders of oomycetes are -
1. Saprolegniales - Achlya , Saprolegnia
2. Peronosporales - Phytophthora , Pythium , Albugo
Order Saprolegniales
Saprolegniales are commonly known as water moulds . Moulds mean fungus made up of hyphae ( multicellular filaments ) . They are found in water . Saprolegnia and Achlya grow as parasites on fishes and their eggs . Plasmogamy takes place by gametangial contact ( by fertilization tube ) .
Saprolegnia
Class - Oomycetes
Order - Saprolegniales
Family - Saprolegniaceae
Genus - Saprolegnia
S. parasitica causes salmon disease in fishes . Myceliun is profusely branched and coenocytic . Septa are formed only at the base of sex organs . Hyphae may be rhizoidal and external . Asexual reproduction takes place by biflagellate zoospores .One of the flagellum is of whiplash type and second one is of tinsel type . Zoosores are released through apical pore of zoosporangium . Zoospores are of two types - primary zoospores ( pear shaped ) and secondary zoospores ( kidney shaped ) .Secondary zoospores have lateral flagella . Production of two types of spores is called diplanetism . Primary zoospore encysts and then germinates in to secondary zoospore . Secondary zoospore deflagellates and germinates in to hyphae .
One of the characteristic feature of Saprolegnia is proliferation of zoosporangium . New zoosporangium is formed inside the old zoosporangium (from the septum of first one ) . In some species of Saprolegnia this process is repeated several times .
Antheridia and oogonia develop terminally on the lateral branches of the mycelium . Fertilization takes place by gametangial contact ( by fertilization tube ) . Fertilized egg called oospore germinates to form zoosporangium .
Achlya
Achlya shows similarities with Saprolegnia in classification and in most of the characters . Some of the species of Achlya show polyplanetism . Raper studied the effect of hormones on development of sex organs in Achlya .