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In this article we will discuss about the life cycle of saprolegnia with the help of suitable diagrams.
Mycelium of Saprolegnia (Fig. 6.2 A):
The mycelium is coenocytic and branched.
During the vegetative phase it is composed of two kinds of hyphae:
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(i) Rhizoidal or intramatrical hyphae:
These are short hyphae which penetrate the substratum. They anchor the mycelium and absorb nutrition.
(ii) Extramatrical hyphae (Fig. 6.1):
These are long hyphae which grow out from the surface of the substratum into the water and extend in all directions. They form the visible portion of the mycelium and produce the reproductive organs (Fig. 6.2A). The extramatrical hyphae are long, slender and extensively branched. They are aseptate and coenocytic.
Septa, however, appear in connection with the formation of reproductive organs and rarely in older hyphae. The hyphal wall contains cellulose and not chitin. In addition glucans or mannans are also involved. Next to the hyphal wall is the vacuolate cytoplasm in the form of a thin lining layer containing several nuclei (A). Food is stored in the form of oil globules and glycogen. The mycelium is eucarpic.
Reproduction of Saprolegnia:
After a certain period of growth and under suitable conditions of food, temperature and other environmental factors Saprolegnia enters the reproductive phase. It bears reproductive organs at the tips of hyphae. Saprolegnia reproduces asexually as well as sexually.
1. Asexual reproduction:
It takes place by vegetative methods and sporulation.
(a) Vegetative reproduction:
It takes place by the following two methods:
(i) Fragmentation:
Under favourable circumstances the hyphae break up into pieces of variable lengths. Each such piece or fragment by further elongation and nuclear division develops into a mycelium.
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(ii) Formation of Gemmae (Fig. 6.2B):
They are formed at the tips of hyphae and are unicellular. The formation of gemmae is marked by a conspicuous swelling of the hyphal tips. The swollen portion contains abundant food material and possesses several nuclei. It is separated from the hypha by means of a cross wall. The gemmae vary in shape and size.
They may be rounded, ovoid or irregular and developed singly or in chains of variable lengths. At maturity the gemmae get detached. The detached gemma germinates by giving out a tubular hypha on a new substrate and develops into a mycelium or short-stalked sporangium typical of the species. Some mycologists call these gemmae as chlamydospores.
(b) Sporulation (Fig. 6.3):
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It takes place by means of pip or pear-shaped, biflagellate zoospores which are produced in long, cylindrical tapering zoosporangia. The zoosporangia are formed at the tips of somatic hyphae which are not differentiated into sporangiophores.
The zoosporangia are only slightly greater in diameter than the hyphae bearing them. They are densely filled with protoplasm whereas the somatic hyphae are only lined with a thin layer of protoplasm.
Development of Zoosporangia (Fig. 6.3, A-J):
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The apical portion of external or extramatrical hyphae shows a slight amount of swelling (A). The swollen portion is conspicuous by its denser granular cytoplasmic contents. A large number of nuclei migrate into this club-shaped apical portion.
Finally it becomes cut off from the remaining hypha by a cross wall and is known as the zoosporangium (B) which is usually club-shaped or it may be tubular with ends slightly tapering (cigar-shaped). The mature zoosporangium has a well-defined thin-walled apex.
Differentiation of Zoospores (Fig. 6.4):
The colourless multinucleate protoplast of the zoosporangium divides into several uninucleated portions by progressive cleavage of the entire protoplast (A). Each unicleate daughter protoplast rounds off and later assumes a pear-shaped form (A). It is furnished with two apically inserted flagella. One of these is of tinsel or pantonematic type and the other of whiplash or acronematic type (B).
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These pyriform, biflagellate, uninucleate structures are known as the primary zoospores. They are released into the surrounding water, through an apical pore that suddenly develops at maturity in the zoosporangium (A).
Most of them emerge with a rush as if under pressure within the zoosporangium. The remaining ones come out one by one slowly. The zoospores emerge backwards with the blunt end foremost (Greenwood). The liberated zoospores swim about in water with the pointed end forward for some time (B).
They are propelled by their flagella with the shorter tinsel type directed forward and the longer whiplash type trailing behind. The tinsel flagellum bears two rows of fine, lateral fibrillar hairs 2-3 um long.
The fibrillar hairs end in shapely defined fine tips. After a swarming period of brief duration which according to Ingold is of a few minutes and according to Manton extends over 1-2 hours, each one of them comes to rest and withdraws its flagella.
The quiescent zoospore becomes spherical and secretes a thin wall around it (C). The zoospore enclosed, in a sac or cyst is said to encyst. The cyst wall bears numerous delicate projections on the outside. The projections are double headed hooks on long, slender stalks.
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The encysted primary zoospore rests for a while (a few hours). After the resting period the cyst breaks and the contents emerge as a single bean or kidney-shaped secondary zoospore through the dissolved tip of a slender papilla developed on the cyst wall (D).
The ruptured cyst is left behind. The escaping secondary zoospore is somewhat smaller than the primary zoospore and is reniform (kidney-shaped). It is furnished with two laterally inserted and oppositely directed flagella (E). They arise from the shallow groove of the concave side. One flagellum is of tinsel type and the other of whiplash type. The hind whiplash flagellum is about twice the length of the fore-tinsel flagellum.
The secondary zoospore embarks upon a second swarming period. It is of a longer, duration. According to Alexopoulos and Mims (1979), the swarming period of both kinds (primary and secondary) of zoospores is of considerable duration.
Finally the secondary zoospore comes to rest. It withdraws its flagella and encysts (F). The cyst immediately germinates like a fungal spore. There is no emergence of a zoospore in this case. On a new substrate it produces a short germ tube (G). The germ tube grows into a hypha and finally into the mycelium.
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Ultrastructure of zoospore:
The zoospore, whether primary or secondary has no cell wall. It has a pear-shaped nucleus with a distinct nucleolus. The nuclear membrane has pores. A pair of basal bodies, from which arise the two flagella, lie near the tapered end of the nucleus. A number of microtubules radiate from the basal bodies into the cytoplasm.
The zoospore cytoplasm contains numerous ribosomes, mitochondria, lipid bodies and electron dense inclusion bodies scattered in the cytoplasm. Besides there are one or two golgi bodies located near the surface of the nucleus. Contractile vacuoles have also been reported.
Zoospore Behaviour:
Saprolegnia is peculiar in producing two types of zoospores (primary and secondary) in succession. Such species are called dimorphic in contradistinction to the monomorphic which produce only one kind of zoospores. The successive production of two different types of zoospores by a single fungal organism is termed diplanetism. The species with two swarming stages separated by a resting stage is called diplanetic.
The process of repeated encystment followed each time by the production of a further motile stage is called repeated zoospore emergence. The species with one swarming stage involving the formation of only one kind of zoospores are called monoplanetic.
This phenomenon involving the production of one kind of zoospores with one motile stage is called monopianetism. The species with no swarm stage and no production of zoospores are called aplanetic. Geolegnia is an example of this kind. Saprolegnia is an example of a diplanetic species.
The functional significance of diplanetism (two-phase behaviour of zoospores) and polyplanetism is not definitely known. However, in the absence of suitable substratum the fungus may resort to successive production of zoospores of secondary type separated by the resting stage till it makes contact with a suitable substratum.
Hickman (1967) found that repeated emergence of zoospores was suppressed in the presence of nutrients and favoured by cooler temperatures.
Proliferation of Zoosporangia:
It is an interesting and characteristic feature in Saprolegnia. The basal septum of the empty zoosporangium initiates the development of a secondary zoosporangium (Fig. 6.3 J). It grows and bulges into the empty sporangium to form a new sporangium. The new sporangium matures within the primary one.
Sometimes it matures some way beyond the primary zoosporangium. This process may be repeated and several zoosporangia may thus be formed within the walls of the next older one. Each of these matures and releases its contents before the next one is formed. Intercalary and catenulate zoosporangia have also been reported in some species such as S. torulosa.
2. Sexual Reproduction (Fig. 6.5):
It is oogamous and by gametangial contact. The sex organs are called antheridia and oogonia. They are formed at the tips of somatic hyphae (A) when conditions favourable for sexual reproduction appear or conditions unsuitable for somatic growth set in. Many species are monecious or homothallic. A few are dioecious or heterothallic. Following is an account of sexual reproduction in a homothallic species.
1. Oogonium (A):
(a) Structure:
The oogonium is generally a more or less a spherical structure with a thick wall. It is delimited from the supporting hypha (stalk) by a septum which is a solid plate. The oogonial wall is usually smooth. In some species it is spiny or papillate. Rarely it has pits (S. ferax). Usually the oogonia are produced singly and terminally on short lateral branches of the hyphae projecting from the substratum (A).
They may also arise from the main hyphae themselves. Sometimes oogonia are intercalary in position (S. litorales). The, multinucleate protoplasm of the oogonium which is uniformly granular, at first, becomes cleaved into several eggs or oospheres. The mature oogonium contains usually four to ten naked, uninucleate, spherical oospheres or eggs (E).
Rarely the number may go up to 32. Each oosphere contains a number of oil globules. These oil globules may unite to form a single, larger oil globule. In case this oil globule lies in the centre, the egg is known as centric. If it lies toward one side the oospohere is known as eccentric. The mature oosphere is dark in colour and is uninucleate.
(b) Development:
The tip of a lateral branch or the end of the main hypha swells. The swelling inflates to form a spherical structure. Thy cytoplasm and a large number of nuclei stream into the swollen portion. Finally a septum appears separating the terminal swelling from the supporting hypha (B). The former is the young oogonium and the latter is termed the stalk.
Following oogonium delimitation some of its nuclei and cytoplasm start degenerating. The process starts from the centre towards the periphery of the oogonium and continues till a conspicuous central vacuole forms. The nuclei left over along with the cytoplasm which is packed with lipid globules are restricted, at this stage, to a thin layer along the periphery of the oogonium (C).
The nuclei in the peripheral layer undergo another mitotic division which is again followed by the degeneration of some of them. The surviving nuclei gather cytoplasm around them and develop into uninucleate oospheres (D & E). The oospheres enlarge and become spherical. The entire protplasm of oogonium is used up in the formation of oospheres. The oogonium in Saprolegnia thus has no periplasm.
Antheridium:
(a) Structure:
The antheridia are borne on long or short, slender hyphae called the antheridiophores (A). The antheridiophores (one or more) may arise from the same hyphal branch which bears the oogonium and immediately below it or from a different hypha in the neighbourhood or on an entirely different thallus.
In the first case the antheridia are said to be monoclinous or androgynous and in the latter two cases diclinous. The mature antheridium is an elongated, tubular, multinucleate structure delimited by a basal cross wall (Fig. 6.5 B). It contains abundant protoplasm and is smaller than the oogonium. No sperms are organised.
(b) Development:
An antheridiophore arises as a lateral outgrowth. As it elongates its terminal portion becomes inflated to form a tubular or even an irregular, swollen structure. Several nuclei move up into the swelling along with sufficient amount of cytoplasm.
Finally a septum separates the- terminal portion from the rest of the antheridiophore. This structure is known as antheridium. It is stated that the nuclei in the antheridium undergo one or more mitotic divisions.
Fertilisation in the Saprolegnia:
At maturity, the antheridiophore grows towards the oogonium till the antheridium becomes closely attached to it (Fig. 6.5 E). One or more antheridia may become closely appressed against the oogonial wall in this way. A fine tubular outgrowth arises from the antheridium at the point of contact (F).
It is the fertilisation tube. It pierces the oogonial wall and may give out slender branches. Each branch makes its way to one of the oospheres within the oogonium. Sometimes each oosphere in the oogonium receives a separate fertilisation tube from another antheridium (F & G).
On coming in contact with the oosphere the fertilisation tube or its branch pierces it and discharges one male nucleus into it. The male nucleus fuses with the female nucleus to accomplish fertilisation.
The fertilised egg or oosphere secretes a thick smooth wall around it to become an oospore which is gorged with fatty reserve stored in the form of oil globules or droplets outside the large membrane bound cell inclusion termed the ooplast.
The reserve food may be stored outside the ooplast in a central (S. hypogyna), eccentric (S. aunisosphorio), subcentric (S. tenespora) or subeccentric (S. seccentrica) arrangement. The resultant oospore is a resting structure.
Germination of Oospore (Fig. 6.6 A-E):
Mature oospores are liberated by the degeneration of the oogonial wall. The oogonia degenerate after the mycelium bearing them has also undergone degeneration. The liberated oospores may remain inactive for a period of two to five months. In cultures oospores may not be liberated.
They remain within the oogonium and germinate in situ (Fig. 6.7). The germ tube grows out through the wall of the old oogonium. At the approach of favourable conditions oospore germinates on a new substrate. It absorbs water and swells.
The oospore wall becomes thinner. The protoplasmic contents grow out in the form of a tubular outgrowth, the germ tube (B). According to the older view (Fig. 6.8) diploid nucleus undergoes meiosis at the time of zygote germination. The gem tube contains haploid nuclei.
During its further growth the germ tube may behave in either of the following ways:
1. It may directly grow into a new mycelium.
2. It may swell up to form a club-shaped germ sporangium typical of the species (C). The terminal germ sporangium is delimited by a basal cross wall. The protoplasmic contents of the germ sporangium produce biflagellate zoospores (D). Each zoospore (E) germinates to form a new mycelium.
Oospheres which fail to be fertilized develop parthenogenetically. In some species such as S. ferax no antheridia are developed next to many oogonia. The eggs such oogonia function as parthenospores. Each parthenospore apparently is similar to an oospore. It develops parthenogenetically into a new mycelium.
Recent investigations have shown that in Saprolegins occurs at that time of gameteformation and not at the time of germination of oospore as is generally believed.
In case it is confirmed there will be a radical change in our concept of the life cycle of Saprolegnia. The somatic mycelium (vegetative phase) According to this view, will then be diploid and the only haploid structures in the life cycle will be the sexual nuclei or the gametes (Fig. 6.8 B).
Salient Features of Saprolegnia:
1. The majority of the species of Saprolegnia are aquatic saprobes.
2. The mycelium is well developed, profusely branched, and coenocytic. It is often seen as tiny tufts of cotton wool around some bit of decaying plant or animal tissue in water particularly the dead flies, a piece of meat, cut hemp seeds, etc.
3. The hyphae constituting the mycelium are of two kinds, rhizoidal and extrametrical hyphae. The former penetrate and ramify within the substratum. They serve to anchor the mycelium and absorb nutrition. The extramatrical hyphae grow on the surface of the substratum and project into the surrounding water. These are concerned with reproduction.
4. The hyphal walls contain cellulose and glucans but no chitin.
5. The mycelium reproduces vegetatively by fragmentation and formation of gemmae or chlamydospores.
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6. Asexual reproduction takes place by means of zoospores which are of two types, primary and secondary. For this reason is said to be dimorphic.
7. The primary zoospores are produced in elongated; cylindrical zoosporangia formed singly at the tips of somatic hyphae. They are pear-shaped, biflagellate, uninucleate structures. The flagella are inserted apically. One of these is of whiplash type and the other tinsel. The zoospores are set free in the surrounding water through an apical pore.
8. After some period of activity the primary zoospore comes to rest, retracts its flagella and secretes a thin wall around it. Instead of the cyst germinating by a germ tube i s contents develop into a single kidney-shaped secondary zoospore with the two oppositely directed flagella inserted laterally in the depression.
9. The liberated secondary zoospores embark upon a second swarming period after which each encysts and germinates by a germ tube to form a new mycelium. Saprolegnia is thus diplanetic.
10. Proliferation of zoosporangia is a characteristic feature of Saprolegnia.
11. Sexual reproduction is oogamous. The antheridia and oogonia are either formed on the neighbouring hyphae or on the same hypha, the antheridium arising below the oogonium from its stalk. The gametes are aplanogamic.
12. The mature oogonium is usually a rounded structure containing several uninucleate oospheres.
13. The mature antheridium is a multinucleate elongated structure much smaller than the oogonium. One or more antheridia become attached to the oogonium by the curvature of its antheridiophore.
14. Fertilisation takes place by means of the fertilisation tube. Entering the oogonium it may branch sending one branch to each oosphere or each oosphere receives a fertilisation tube from another antheridium. Only one male nucleus is discharged into the oosphere. No sperms are organised in the antheridium.
15. After fusion of the male and female nuclei each zygote secretes a thick smooth wall around it to become an oospore.
16. After a prolonged period of rest each oospore germinates by producing a germ tube which in some species directly develops into a new mycelium and in others ends in a terminal germ sporangium in which are produced the biflagellate zoospores. Each liberated zoospore germinates in the usual way by a germ tube to form a new mycelium.
17. Recent investigations have shown that meiosis in Saprolegnia is gametangial. It occurs at the time of differentiation of sexual nuclei or gametes.
18. The somatic phase in the life cycle is thus diploid.
19. The gamete nuclei are the only haploid structures. There is no haploid phase or gametophyte generation in Saprolegnia.
20. Such a life cycle in which there is no alternation of generation and meiosis is gametangial is called diploid or diplohaplontic (haplobiontic-diploid).
