In this article we will discuss about the life cycle of zoospores.
Mycelium in Zoospores:
The vegetative phase is variable. It ranges from a simple, unicellular thallus to an extensively developed, flamentous structure (mycelium) consisting of branched, coenocytic hyphae. The hyphae grow in and around the medium. The oomycete hyphal wall contains cellulose but no chitin. Glucans or mannans also occur in addition. In fact glucans may predominate.
A large number of amino acids appear to be typical of Oomycete hyphal walls. Crook and Johnson (1962) reported as many as seventeen in the hyphal walls of Saprolegnia and Phytophtora.
Hunsley and Burnett (1970) reported that all the Oomycetes so far studied have hyphal walls composed of B-1, 4 linked cellulose micro-fibrils and highly branched insoluble B-1, 3-glucans which has B-1, 6 linkages at the branching residues.
Most of the species are eucarpic and reproduce both asexually and sexually. During the growing season, the specialized plant pathogens of this class multiply by several successive, asexual generations. Sexual reproduction takes place only once and probably towards the end of the growing season.
Asexual Reproduction in Zoospores:
In the aquatic species such as Saprolegnia, it takes place by the formation of zoospores which are produced inside zoosporangia. The latter in the lower members are in no way different from the vegetative hyphae. In the advanced forms the sporangia are definite organs.
They have a characteristic shape and are borne on special, reproductive hyphae called the sporangiophores. The sporangiophores are not only well developed but are characteristically branched.
The zoospores may be formed in the sporangium itself by the cleavage of the protoplast-and released through the pore at the apex of sporangium. In some other species, however, the zoospores are differentiated in a vesicle formed at the apex of the sporangium into which the sporangium protoplast migrates. The vesicle then vanishes and the zoospores are set free.
Behaviour of Zoospores:
Depending upon the species the released zoospores behave in two different ways:
(a) In the majority of the species (Patheoposis), the liberated zoospores swim about actively. After a while they become quiescent, retract their flagella and round off. Each then secretes a wall around it and thus becomes encysted.
The clothed zoospore or cyst then germinates by putting out a germ tube which develops into a new plant. Such species which produce only one kind of zoospores each of which on germination directly develops into a parent plant are called monomorphic or monoplanetic. This kind of zoospore or behavior is called monoplanetism.
(b) There are, however, some simple and primitive genera in this class such as Sparolegnia and Leptolegnia in which the zoospores released from the zoosporangium behave in a different and quite interesting way. They are pear-shaped and biflagellate. The flagella are inserted at the anterior end. We call these as the primary zoospores.
Each primary zoospore after a brief period of farming comes to rest and encysts (secretes a wall around it). The encysted zoospore, instead of germinating by a germ tube, produce a second kind of zoospores. It is known as the secondary zoospore. The secondary zoospore differs from the primary zoospore in form and instered flagella. It is reniform (kidney-shaped) of and is literally biflagellate.
The two flagella are inserted on the concave side, one of these is directed forward and the other backward. The secondary zoospore after emergence from the cyst embarks upon a second swarming period there after it comes to rest and encysts. The encysted secondary zoospore germinates by a germ tube which produces the mycelium.
These genera and species which produce two kinds of zoospores (primary and secondary) in succession in the life cycle called dimorphic or diplanetic. This condition or phenomenon is called diplanetism. The advanced land forms which do not produce zoospores are called aplanetic.
Evolution of the Conidiospore:
Another interesting feature of this class is the light the different species throw on the transition from a sporangium to a spore-like conidium.
You have studied above the aquatic genera such as Saprolegnia reproduce entirely by the formation of Oospores which are produced within -sporangia. Many species of Pythium are a little more soil-minded. They grow in damp soil and reproduce by zoospores indicating thereby that they are basically aquatic.
Phytophthora and Pythium debaryanum are amphibious in their habit. They are adapted to life both in dry and wet habitats. The fungal hyphae in both bear
Sporangia which under wet conditions produce zoospores. However, under dry conditions or high temperature the zoosporangia get detached.
The detached sporangia are similar in size and shape to the zoosporangia, which produce zoospores, but function differently. Each detached sporangium, on germination, behaves like a conidium. It produces a germ tube which develops into a fresh fungus mycelium.
Similarly Plasmopara viticola can adjust itself to both dry and wet conditions. Usually it reproduces by the formation of zoospores. Under dry conditions, however, the detached sporangia behave as conidia, each producing a germ tube.
In Pythium vexans sporangia normally function as conidia. The germ tube is formed in the place of a papilla of the sporangium. Rarely, however, the sporangia form zoospores.
Pythium ultimum and a terrestrial species of Plasmopara do not produce sporangia. The asexual reproductive organs always behave as conidia. Each germinates by producing a germ tube.
Finally in genera like Bremia and Perononspora the sporangia bearing hyphae (sporangiophores) are branched. Each branch bears either a single sterigma (Peronospora) or a number of them (Bremia). Each sterigma bears at its top a single spore-like conidium.
It is evident from the foregoing account that the conidium in this class has evolved from a sporangium, the protoplast of which forms a germ tube instead of producing zoospores. The change is an adaptation in response to a terrestrial mode of life which does not permit dispersal by zoospores but favours distribution by air.
Sexual Reproduction in Zoospores:
Excepting a few most primitive forms which are holocarpic, sexual reproduction is oogamous by gametangial contact. The sex organs, which are called antheridia and oogonia, are terminal or intercalary in position. The fusing gametes are non-flagellated.
The protoplast of the oogonium forms one or more oospheres which are uninucleate when mature. Either the entire protoplast (Saprolegniales) is utilised in the formation of oospheres or a part is left behind. The residual protoplast forms a peripheral layer termed the periplasm, around the oosphere. It is used up during maturation of oospore.
According to their position and occurrence, the antheridia are grouped into two categories, namely, androgynous and diclinous. The former develop on the stalk of the oogonium or a branch of it. When developed on a separate hypha the antheridium is described as diclinous.
Plasmogamy takes place by gametangial contact in the following ways:
1. Generally the antheridium gets applied to the side of the oogonium towards maturity.
2. Rarely, as in Phytophthom, the oogonium punctures the developing antheridium and grows through it to the top where it forms a rounded structure.
The antheridium, in the first case, is termed paragynous and in the second case amphigynous. The migration of the male nucleus into the oogonium usually takes place through a fertilisation tube or through a pore in the oogonial wall.
Place of Meiosis in the Oomycetes:
It is still under dispute. Stevens as early as 1899 suggested gametangial meiosis in Albugo. His interpretation was disputed by later workers. Since then it has been widely held that the Oomycetes, as a whole, are haploid and meiosis occurs in the oospore. Sansome (1963) working with Pythium debaryanum struck a discordant note.
She reported that the two successive divisions which occur in the gametangia constitute meiosis. From this one would conclude that the Oomycetes, as whole, would be diploid.
During the last two decades various investigators have reported meiosis in the gametangia of Phytophthora, Albugo and Peronospora, Sclerospora, Bremia, Achlya, Saprolegnia, and Apodachlya. From the investigations of these workers it would appear that both the Saprolegniales and Peronosporales are diploid.
On the other hand, Timmer, Castro, Irvin, Belsar and Zentmyar (1970) advanced a genetic evidence for zygotic meiosis in Phytophthora capsici. Stephenson working with Phytophthora capsici vehemently opposed gametangia meiosis in Phytophthora. They reported that meiosis takes place in the first divisions of the oospore nucleus.
More recently Sansome (1976) studied meiosis in Phytophthora capsici. She observed pachytene, diplotene and diakinesis stages in the oogonium. She further reported that metaphase and anaphase were followed by a second division without any increase in nuclear and chromosome size between the two divisions.
The haploid chromosome number was reported to be nine. She observed an association of 4 chromosomes in one or both parents.
The association of 4 chromosomes at diakinesis and metaphase provides an evidence that pairing of homologous parts occure during the first division and is a proof that the gametangial divisions are meiotic.
There is thus overwhelming and conclusive evidence in favour of gametangial meiosis in the Oomycetes according to which the somatic of gametangial meiosis in the sex organs (antheridia and oogonia) would be diploid the gametes alone represent the haploid structures in the Oomycete life cycle. In spite of this the mycologists, in general, stick to the view that there is zygotic meiosis and Oomycetes are haploid.