In this article we will discuss about reproduction of fungi in phylum oomycetes.
Reproduction in Saprolegnia:
It is accomplished by zoospores. Cylindrical zoosporangia develop at the tips of hyphae which produce numerous biflagellate zoospores.
When mature, they can be seen jostling one another within the zoosporangium. The apex of the zoosporangium suddenly breaks releasing the zoospores. At first, the zoospores come out rushing, as if discharged by pressure, but later, the remaining zoospores escape slowly, one by one.
A cine film of the emptying sporangium has shown that the zoospores escape with the blunt end forward, but as soon as the zoospores are out, their direction of motion reverses and they swim with the pointed end forward. The zoospores are propelled by the flagella, one pointing forward and the other trailing behind.
It is a characteristic feature of Saprolegnia. The apically biflagellate primary zoospores, which are liberated through a pore at the tip of the zoosporangium, swim about for a few minutes, come to rest and encyst i.e. withdraw the flagella and form a thin wall.
After a short period of rest of a few hours, a papilla develops on the cyst. Its tip dissolves and a reniform zoospore, bearing two lateral flagella (secondary zoospore) creeps out leaving the empty cyst behind. The secondary zoospore comes to rest and forms a cyst. The cyst germinates by forming a hypha that forms a new colony.
It is an interesting phenomenon characteristic of Saprolegnia, which enables the formation of several zoosporangia at the same site. When a zoosporangium has liberated the primary zoospores, a second zoosporangium is formed within the old one, which pushes out of the remnants of the previous zoosporangium.
A third zoosporangium may be formed similarly within the second sporangium. The septum at the base of the empty sporangium bulges out and forms a new hyphal tip, which grows through the old sporangium and forma a new sporangium within the old one. The emptied sporangia, as a rule, remain attached to the hypha.
Dimorphism and zoosporangial proliferation are the two curious phenomena characteristic of Saprolegnia.
Some portions of the hyphae become septate and thick-walled to form chlamydospores. After death of the hyphae, these get detached and germinate to form new thalli.
It is brought about by gametangial contact (gametangiogamy). Club-shaped antheridia and globular oogonia are formed on the same hypha. The oogonium is bigger in size than the antheridium and thick walled. At first, it is multinucleate, but some of the nuclei degenerate and the protoplasm differentiate into one to several uninucleate eggs or oospheres.
The mature antheridia are multinucleate, which get attached to the oogonium and form fertilization tubes that enter the oogonium and form branches which approach the oospheres. One male nucleus enters each oosphere and fuses with the female nucleus forming a diploid zygote-nucleus.
Each fertilized oosphere secretes a smooth and thick wall and gets transformed into an oospore. The oospores contained within the oogonium undergo a resting period. The oospores are usually not dispersed and remain inside the oogonium. After a period of rest, the oospores germinate in situ. The germ tube pushes out of the oogonium wall and forms club-shaped germ sporangium at the tip. Zoospores are formed in the sporangium which, on liberation, germinates to give rise to a new thallus.
In most species of Saprolegnia, development of oogonia into oospores, without fertilization, has been noticed. Such oospores are haploid.
Reproduction in Achlya:
Zoospores are formed in cylindrical zoosporangia borne on undifferentiated zoosporangiophores. The primary zoospores, which are all ejected together, and not one by one as in Saprolegnia, transform into cysts as soon as they come out.
The cysts are arranged like a hollow ball at the tip of the sporangium. Each cyst, after a short period of rest, gives rise to a laterally biflagellate secondary zoospore, which after a swimming period encysts and later germinates to give rise to a new thallus. So, there is only one swarming spore in the life cycle of Achlya.
Achlya lacks dimorphism and sporangial proliferation. However, after ejection of the zoospores, a new zoosporangium is formed by its side.
The differences in Zoosporangial behaviour between Achlya and Saprolegnia are given below:
It is similar to that of Saprolegnia. But, the antheridia and oogonia are borne on different thalli and their formation is controlled by two hormones, antheridial and oogoniol. The hyphae of opposite strains induce each other to form the sex organs.
The club-shaped antheridium, borne on antheridiophore on male hyphal branches, lies appressed to the globular oogonium, borne on female hyphal branches of opposite strain. The antheridium forms a fertilization tube, which penetrates the oogonium and forms branches. Each branch approaches one oosphere.
The oogonium contains usually 8 oospheres. The oospheres, after fertilization, transform into oospores which remain inside the oogonium and undergo a resting period. On germination, the oospores form a germ sporangium, which, in turn, forms zoospores with anterior flagella. The zoospores encyst and germinate to give rise to a new thallus.
Reproduction in Pythium:
Globose or oval sporangia are borne terminally on the hyphae. Distinct sporangiophores are not formed. At the time of zoospore formation, a bubble-like vesicle with a long stalk emerges on the sporangium. The sporangial protoplast moves into the vesicle. The zoospores are formed in this vesicle. This process is completed within 15-20 minutes.
When the crowded zoospores start rocking motion, and bounce on the wall, the delicate vesicle bursts, like a soap bubble, releasing zoospores. The zoospores, which are reniform and biflagellate, after swimming for some time in the film of soil water, come to rest; encyst (that is withdraw the flagella and form a wall) and germinate by forming a germ tube.
The zoosporangia in dry weather germinate directly by a germ tube rather than by forming zoospores and, thus, behave like conidia.
Sexual reproduction is oogamous and occurs by gametangial contact. The oogonia and antheridia may be terminal or intercalary and borne on the same or different branches. The oogonium is globose and has a single multinucleate oosphere in the centre, surrounded by a layer of periplasm. Antheridia, which are elongated or club-shaped and smaller than the oogonia, appear around the oogonium.
Mature oogonia and antheridia are uninucleate. On gametangial contact, each antheridium forms a fertilization tube which grows through the oogonial wall and the periplasm and finally enters the oosphere. The male nucleus is released into the oosphere where plasmogamy takes place. Karyogamy may follow immediately or may be delayed until spore germination.
The oosphere forms a thick, smooth or spiny wall, which in P. debaryanum remains loose in the oogonium (aplerotic). It is fused with the oogonial wall (plerotic) in some other species, for example, P. torulosum. It undergoes a period of rest before germination. The mode of germination of oospore is determined by temperature.
At higher temperatures (28°C) the oospore germinates directly by forming a germ tube, while at lower temperatures (10-17°C) it germinates by forming a ‘germ sporangium’ at the tip of a stout hypha. The first division of the zygote is assumed to be meiotic. Zoospores (or the germ tube), formed by the oospores, infect susceptible seedlings and cause damping off disease.
Reproduction in Phytophthora:
Chlamydospores- terminal or intercalary chlamydospores are abundantly formed on the hyphae.
Low temperature (18 – 20°C), accompanied by high relative humidity (91-100 per cent), are the requisites for a sporangial production. Sporangiophores emerge through the stomata from the lower surface of infected leaves and bear terminally, lemon-shaped sporangia with a distinct beak.
The renewed growth of the sporangiophore gives it a sympodial zig-zag shape. On potato tubers, the sporangiophores emerge in large numbers from the cut surfaces.
The sporangia are deciduous (fall off) and get widely disseminated through soil water (especially irrigation water) or through wind. Wind- disseminated sporangia germinate directly by germ tube and, thus, functionally become conidia. They are highly susceptible to desiccation. In presence of water, the sporangium germinates directly (by forming a germ tube) or form zoospores.
This is dependent on the temperature. Low temperature (12°C) favours zoospore production, while high temperature (24°C) induces direct germination. Reniform, biflagellate zoospores, after swimming for some time, come to rest, encyst and germinate by a germ tube. On leaf, the tip of the germ tube develops into a flattened bulbous structure, called appresorium (plural appresoria).
An infection hypha originates from the appresorium which enters the leaf tissues and establishes an intercellular mycelium. Dead, blighted (brown coloured) areas appear on the leaves. After a few days, numerous sporangiophores emerge from the lower surface of the infected leaf and from zoosporangia. Zoospores in the soil infect the tubers.
It is of oogamous type and occurs by gametangial contact. P. infestans is heterothallic i.e., two mating types are required for sexual reproduction. In nature, sexual reproduction is very infrequent as the two mating types are rarely found in the same locality.
On the basis of the position of the sex organs, the species are designated as paragynous or amphigynous. In the paragynous type (e.g., P. cactora), the sex organs lie laterally, while in the amphigynous type (e.g., P. infestans and P. colocasiae), the young oogonium grows through the antheridium and after emerging above, develops into the mature globose structure.
The antheridium forms a funnel or collar-shaped structure at the base of the oogonium. The oogonium, which is initially multinucleate, at maturity, contains a single nucleus. The protoplasm at the centre of the oogonium is called ooplasm, which contains the female nucleus and serves as the female gamete-called egg or oosphere.
The cytoplasm, surrounding the ooplasm, is called periplasm. The club- shaped antheridium, which too is initially multinucleate, finally contains a single male nucleus. The male nucleus reaches the oosphere through a fertilization tube, formed by the antheridium. Plasmogamy, followed by karyogamy, results in a diploid, zygote nucleus.
Parthenogenetic development of unfertilized oosphere into oospore is also known. The oospore fills the cavity of the oogonium (Plerotic type of oospore). After a rest period, the oospore germinates and forms a zoosporangium at the tip of the germ tube. Zoospores of the two mating types are formed in equal numbers, which give rise to thalli of opposite mating types.
Reproduction in Albugo:
After attaining a certain stage of maturity, hyphae aggregate at several places under the epidermis. Club-shaped sporangiophores of determinate growth, with thick lateral walls and a thin apex, arise to form a palisade layer. These cut multinucleate sporangia which remain attached to form a chain at the apex.
The oldest sporangia lie at the top and youngest at the base of the chain. The walls between the sporangia fuse to form a gelatinous disc-like structure called disjunctor or separation disc. The disjunctors are dissolved by water, and the sporangia are set free.
The numerous sporangia that are produced at the apical end of sporangiophores, push against the epidermis which bulges out and ultimately breaks. When the epidermis bursts, the white mass of sporangia become visible as a white crust on the leaf surface. These areas with broken epidermis and creamy mass of sporangia appear as pustules or blisters on the leaves.
The young sporangia are globose, but due to mutural pressure, become hexagonal. They are disseminated by wind. Germination occurs when the sporangia reach a suitable host. The mode of germination depends on the availability of water. If water is available, zoospores are formed; otherwise, the sporangium germinates directly by forming a germ tube and thus behaves like a conidium.
Mature 4-12 biflagellate and reniform zoospores are formed in the sporangium which move into a vesicle formed on the sporangium, before they are released. After a swimming period, the zoospores come to rest, encyst and germinate by a germ tube. The germ tube enters the host and establishes an intercellular mycelium. The repeated production of sporangia spreads infection to more and more plants.
When the crop season approaches its end, sexual reproduction ensues. The hyphae penetrate deeper into the tissues and form sex organs. The onset of sexual reproduction is usually indicated by hypertrophy of the infected part.
The sexual reproduction is by gametangial contact of dissimilar sex organs (oogamous). There are cytological differences in the fertilization of various species. The following description is for A. Candida.
The antheridia and oogonia, borne terminally on somatic hyphae, are multinucleate in the beginning, but later become uninucleate by disorganization of the nuclei. The antheridia, as usual, are club shaped while the oogonia are globose. There is a single egg (oosphere) surrounded by a periplasm.
The fertilization tube, formed by the antheridium, carries the male nucleus into the oosphere, where the two nuclei fuse to form a diploid zygote nucleus. The oosphere develops a warty wall and becomes the oospore. In the meantime, the diploid nucleus undergoes meiosis, followed by several mitotic divisions to form a multinucleate oospore.
The ridges on the oospore wall provide a useful criterion for the identification of the various species of Albugo. The multinucleate oospore, after a prolonged resting period (which extends up to the next crop season), germinates and forms reniform biflagellate zoospores. The zoospores are released from a vesicle, borne on the germinated oospore. The zoospores, after swarming for some time, encyst and germinate to form a germ tube which infects the host plant.
Parthenogenetic development of the oospore has been reported for A. evolvuli.