In this article we will discuss about:- 1. Occurrence of Vaucheria 2. Plant Body of Vaucheria 3. Features 4. Reproduction 5. Taxonomic Status 6. Life Cycle.
Occurrence of Vaucheria:
The genus Vaucheria (named after J. P. Vaucher) is represented by about 54 species, out of which 9 species are found in India.
Most of the members are terrestrial (V. geminata, V. terrestris, K. hamata) or fresh water (V. uncinata). A member like V. sessilis is terrestrial as well as aquatic. Species like K amphibian is amphibious in habitat. The terrestrial forms grow in winter in the form of green and thick layer on damp soil. A few species grow in brackish water and some are marine (V. piloboloides). V. jonesii grows on snow.
Plant Body of Vaucheria:
Plant body of Vaucheria is filamentous, much branched, coenocytic and siphonaceous thallus (Fig, 3.83A). The coenocytic body contains many nuclei. Septa may form during injury or on development of sex organ. But the filament is normally septate in V. pseudohamata.
In terrestrial species the plant body remains attached to the soil surface with much branched thread like structure, the rhizoid or hapteron. In floating members the rhizoids are either absent or ill-developed. Species like V. mayyanadensis shows well-developed rhizoid and aerial branches.
The filamentous body has a thin outer wall, which is less elastic. It is made up of outer pectic and inner cellulosic layers. In the centre of the filament a continuous vacuole is present except at the apical region, which is filled with cell sap. The protoplast is present throughout the filament between the cell wall and vacuole which contains nuclei, chromatophores and other substances.
In the protoplast, nuclei remain towards vacuole and chromatophores towards periphery (Fig. 3.83B). Chromatophores are very small, circular, oval or elliptical in outline. Pyrenoids are absent.
Vaucheria contains the pigments like chlorophyll a, chlorophyll e, carotenoids and xanthophylls as in the class Xanthophyceae. The carotenoids are present in more amount than chlorophylls. The characteristic pigment of Chlorophyceae, chlorophyll b, is absent and the dominant pigment of the order Siphonales of Chlorophyceae, the siphonin and siphonoxan- thin are absent in’ Vaucheria.
The reserve food material is oil, it occurs as colourless droplets in the cytoplasm. If the filaments remain exposed under continuous illumination, the assimilatory product will be starch instead of oil.
Important Features of Vaucheria:
1. Plant body is filamentous, coenocytic and branched. It remains attached with the substratum by rhizoids.
2. Reserve food is fats and oils.
3. Reproduction takes place by all the three means : Vegetative, Asexual and Sexual.
4. Vegetative reproduction takes place by fragmentation.
5. Asexual reproduction takes place by compound multiflagellate zoospores i.e., synzoospores, aplanospores, hypnospores and akinetes.
6. Sexual reproduction is oogamous.
7. The oogonium contains single large, uninucleate egg, while the antheridium contains many biflagellate antherozoids.
8. The oospore or zygote undergoes meiosis during germination and forms new Vaucheria plant.
Reproduction of Vaucheria:
Vaucheria reproduces by all the three means: vegetative, asexual and sexual.
Vegetative reproduction’ It takes place due to accidental breakage of the vegetative filament. Septa are formed in the injured region of the filament.
Vaucheria reproduces asexually by the formation of various types of spores such as zoospores, aplanospores and akinetes.
Zoospores are formed singly inside the zoosporangium, developed at the apical region of the filament (Fig. 3.84A). During formation of the zoosporangia, more amount of nuclei and cytoplasm accumulate towards the apex, thereby the vacuole becomes reduced in size. The upper portion gets separated from rest of the filament by cross-wall and forms apical sporangium (Fig. 3.84B).
Consequently the sporangium becomes club-shaped and dark green in colour. After some time, the orientation of nuclei and chloroplast get reversed, thereby the nuclei present towards the centre of the vegetative filament come to the periphery and chloroplasts become shifted from periphery towards the centre.
Before liberation, entire protoplast of the zoosporangium contracts slightly. During liberation a narrow aperture develops at the apex through which the zoospore is liberated. Flagella are formed during liberation (Fig. 3.84C, D).
The zoospores are yellowish-green, ovoid, multinucleate and multiflagellate structures, having small central vacuole and many chromatophores situated between the vacuole and peripheral nuclei (Fig. 3.84F). The paired flagella are unequal in length.
[Single flagellum develops from a basal granule or basal body. For each pair of flagella, a pair of basal granules is present. The basal granules remain firmly attached together at the tip of the nuclei.] Some algologists considered this multi-flagellate and multinucleate unit as synzoospore or compound zoospore (Fig. 3.84E).
After about 15 minutes of swimming the zoospore becomes sluggish, deflagellated, rounded and then secretes a thin wall around. One or more elongated siphonaceous tubes develop at different direction. One of the tubes develops into rhizoid and other one into new thallus (Fig. 3.84G, H).
Aplanospores are developed singly within aplanosporangium, during unfavourable conditions (Fig. 3.85). Like zoospores, the aplanospores are developed singly inside the aplanosporangium at the apical side of the aerial hyphae. During development it also cuts off from rest of the filament by transverse septum.
The aplanosporangium is club-shaped, which develops single non-flagellate club-shaped or globose aplanospore (Fig. 3.85B, C). The aplanospore is liberated by rupturing the apical wall. After liberation the aplanospore germinates into new filament like the mother (Fig. 3.85D).
Rarely the protoplast of the sporangium gets separated into small units, the micro-aplanospores. On being liberated they germinate into new filaments.
Akinetes are formed during unfavourable condition. During akinete formation the content of the filament divides into small segments by thick gelatinous wall. These thick walled small multinucleate portions or segments are called akinetes, also called as hypnospores or cysts. The protoplasts of akinetes are often laden with oil. During favourable condition akinete germinates into new filament.
The above stage of akinete chain looks like another alga Gongrosira and therefore this stage is called Gongrosira stage (Fig. 3.85E).
Sometimes the protoplast of the akinete divides into many small uninucleate amoeboid segments. After liberation by breaking the akinete wall, they develop into individual filaments.
Sexual Reproduction of Vaucheria:
The sexual reproduction in Vaucheria is of oogamous type. It takes place by antheridium, the male sex organ and oogonium, the female sex organ. Most of the species are homothallic or monoecious but a few (V. litorea, V. dichotoma and V: mayyanadensis) are heterothallic or dioecious (Fig. 3.84).
In homothallic or monoecious species (V. sessilis, V. aversa) antheridia and oogonia are borne very close or adjacent on a single filament or on adjacent filament. But in heterothallic or dioecious species (V. dichotoma), they are borne on different plants.
[The sex organ shows great variation in their position not only in different species but also in the same species. The sex organs are stalked or sessile (V. sessilis). In V. geminata, a short stalk bears single terminal antheridium flanked by 3-4 oogonia. In V. gardneri single, terminal elongated antheridium is surrounded by a group of oogonium. In V. terrestris the stalk of antheridium and oogonium is stout, very long and well-developed. In V. bilateral is however, 2-6 oogonia are arranged bilaterally in a series and the antheridia are short-stalked and curved and open towards oogonium.]
Initially a short lateral branch like vegetative branch projects out (Fig. 3.87A). Large number of nuciei and chromatophores accumulate towards the apical region. The apical region then bends like a horn (Fig. 3.87B, C) and a transverse septum is laid down which differentiates the apical antheridium (Fig. 3.87D).
The nuclei of antheridium aggregate in the centre and divide mitotically. Each nucleus along with some cytoplasm metamorphoses into single spindle shaped biflagellate antherozoid (Fig. 3.87E). The flagella are unequal in length, dissimilar (one whiplash and other tinsel) and laterally inserted.
The antherozoids are generally liberated through an opening developed at the apical region of antheridium (Fig. 3.87F). Several pores are developed on anthridial wall in V. debaryana for the liberation of anthrozoides.
Initially a small protuberance develops at or near the base of antheridial branch, due to accumulation of cytoplasm. The cytoplasm of this region is colourless which has many nuclei and without any chromatophore. This colourless multinucleate mass of cytoplasm is called wanderplasm (Oltmanns, 1895) (Fig. 3.87A).
The wall of the thalli near the wander- plasm bulges out and forms oogonial initial (Fig. 3.87B, C), which gradually enlarges and consequently many chromatophores migrate into it. With further development the oogonium initial becomes round or ovoid in shape with a beak at the apex and gets separated from rest of the filament by transverse septa (Fig. 3.87D).
The mature oogonium contains a large nucleus at the centre with many chromatophores and oil droplets dispersed throughout the cytoplasm. All the nuclei except one degenerate during development.
According to Oltmanns (1895) all nuclei except one go back to the main thallus before separation]. The protoplast along with nucleus rounds off and forms single ovum or egg. It has an hyaline area towards the anterior, known as the receptive spot (Fig. 3.87E).
Fertilisation. Both the sex organs open almost at the same time (Fig. 3.87F). The tip of the beak of oogonium gelatinises and forms an aperture. Simultaneously an aperture is also formed at the apex of antheridium (the time of opening may vary from a few minutes to 2 hours).
A small drop of colourless cytoplasm oozes out through the beak of oogonium and several antherozoids coming out from the antheridium get entangled in this drop. Out of many, only one enters into the oogonium. Coming in contact with the egg it looses its flagella and enters into it.
The nucleus of antherozoid is smaller than the egg nucleus (Fig. 3.87G). They remain side by side, till the nucleus of the antherozoid attains almost same size of the egg nucleus. Two nuclei then fuse and form diploid (2n) zygote (Fig. 3.87H).
The zygote secretes 3-7 layered wall around it (Fig. 3.88A). The protoplast accumulates oil droplets. Initially the zygote remains green, but later it becomes red due to breakdown of chlorophyll. It remains within the oogonium and later comes out by decay of oogonial wall. It remains dormant for long period which varies with the duration of unfavourable condition like high temperature, desiccation etc.
During favourable condition the zygote germinates to form new thallus. The nucleus (2n) of zygote undergoes first meiotic division followed by several mitotic divisions thus forming a coenocytic condition. The zygote wall cracks at a point and inner protoplasm elongates, which gradually forms lower rhizoids and an aerial hypha like the mother (Fig. 3.88B, C).
Taxonomic Status of Vaucheria:
There has been a great controversy regarding the taxonomic status of Vaucheria. Fritsch (1935) included Vaucheria along with other three genera (Vaucheriopsis, Dichotomosiphon and Pseudodichotomosiphon) in the family Vaucheriaceae under the class Chlorophyceae. This was also followed by lyenger (1951) and Venkataraman (1961).
It was placed under the class Chlorophyceae, based on the following characteristics:
1. Plant body is siphonaceous filament.
2. Asexual reproduction by synzoospore or compound zoospore.
3. Sexual reproduction is oogamous type.
4. Presence of chlorophyll a.
5. Change of assimilatory product into starch, instead of oil under continuous illumination.
However, it differs from other members of Chlorophyceae, based on the following characteristics:
1. Absence of macerised cellulosic cell wall.
2. Absence of pyrenoid.
3. Paired flagella of synzoospores are unequal.
4. Absence of chlorophyll b.
5. Reserve food is oil.
Later Smith (1950) followed by Greenwood et al. (1957), Prescott (1969), Round (1973), Bold and Wynne (1978) separated Vaucheria from Chlorophyceae and placed it under the class Xanthophyceae.
They shifted Vaucheria to Xanthophyceae, based on the following characteristics:
1. Unequal length of flagella of antherozoids.
2. Presence of chlorophyll e, a pigment common in Xanthophyceae.
3. Absence of two xanthophylls, siphonin and siphonoxanthin.
Davis (1904), Williams (1926) and Printz (1927) observed the phylogenetic relationships between Vaucheria with fungi:
1. Presence of thalloid coenocytic plant body.
2. Similarity in the development of sex organs with oomycetous fungi.
3. Presence of both whiplash and tinsel type of flagella like some phycomycetous members.
4. Presence of similarity in external structure with some members of Monoblepharidaceae.
5. Synzoospore of Vaucheria is like the gonidium of various members of Peronos- poraceae.
For the above evidences someone believed that the Phycomycetes have been derived from Vaucheria-like ancestors. But it is very difficult to make comment on whether this similarity in characteristics is due to their real affinity or it is due to convergent evolution.
Vaucheria sessilis, V. amphibian, V. geminata, V. uncinata, V. polysperma etc.
Life Cycle of Vaucheria:
Fig. 3.89 and 3.90 depict life cycle of Vaucheria.