The following points highlight the top five evolutionary trends on Oomycetes.
1. There is progressive evolution from the aquatic to land habitat. Members of orders Lagenidiales, Saprolegniales and Leptomitales are chiefly aquatic. The advanced order Peronosporales includes aquatic, amphibious and terrestrial forms.
2. The representatives of this class show a whole range from saprophytes (Saprolegnia) to facultative parasites (Pythium) and finally to obligate parasites (Peronospora and Albugo). The advanced land forms are all obligate parasites.
3. In the primative parasitic members, the mycelium is chiefly intracellular but the more advanced forms have intercellular mycelium which draws nourishment by means of haustoria. Of the haustoria the branched ones are considered more advanced than the vesicular or elongated type.
4. Evolution of conidia. An interesting evolutionary trend exhibited by the different species of this class is the transition of sporangium from a zoosporangium to a spore-like conidium. The primitive aquatic genera like Saprolegnia and Achlya reproduce asexually entirely by means of zoospores produced within zoosporangia. In Phytophthora, Pythium and Plasmopara the sporangia behave in two ways.
In the presence of moisture and at low temperature they behave like zoosporangia and thus produce zoospores but at high temperature the zoosporangia behave like spore-like conidia and germinate directly by a germ tube. Peronospora produces sporangia which invariably function as conidia.
5. The Oomycetes are, as a rule, oogamous and the gametes are non-motile. The Saprolegniales (Saprolegnia and Achlya) exhibit primitive type of oogamy. The number of oospores in the oogonium varies from 2 to several and there is no differentiation of oogonial protoplast into periplasm and ooplasm.
Pythium, Phytophthora and other Peronosporales produce a single oosphere in the oogonium and zonation of the oogonial protoplast gets well established.