The following points highlight the three families under which volvocales has been classified. The families are:- 1. Chlamtdomonadaceae 2. Sphaerellaceae 3. Volvocaceae.
Family # 1. Chlamtdomonadaceae:
This family includes algae possessing unicellular biflagellate vegetative cells. Asexual reproduction is by zoospores and hypnospores and sexual reproduction ranges from—isogamy, anisogamy to simple and advanced oogamy.
The genus Chlamydomonas represents one of the most primitive members of the living green plants. It is widely distributed in fresh-water and damp soil, often giving a bright green colour. The vegetative cell is spherical to ellipsoid, or pear-shapedin outline (Fig. 25 A).
It passes its ordinary vegetative life in a state of active movement. Motility is secured by means of two anteriorly placed flagella of equal length, which are longer than the length of the cell. The cell is surrounded by a cell wall which is differentiated into firmer and mucilaginous portions.
The cell contains a single large cup- shaped chloroplast, occupying most of the cell, opening at the top and much thicker at the base than elsewhere. In the lower part of the chloroplast a large pyrenoid is embedded. At the pointed end and in the hollow of the chloroplast there is clear cytoplasm, and embedded in this is the nucleus.
Two contractile vacuoles, which expand and contract, are generally situated in the anterior colourless cytoplasm, whilst a red eyespot, the most variable feature of the cell, is located at the side of the chloroplast. These cells swim about, through water with flagellate end foremost, rotating as they go and are positively phototrophic. During their movements the cells grow and ultimately divide.
There may be produced within the cell two to eight or occasionally more daughter swarmer’s which resemble the parent cell, except being smaller (Fig. 25B & G). These swarmer’s are liberated out by the dissolution of the parent cell wall and form new plants.
In forms growing on soil may result in the establishment of a palmella-stage by the failure of the daughter cells to escape or to become active, the result being a colony of many cells, commonly held together by a common mucilaginous investment (Fig. 25D). Ultimately, the cells acquire flagella and escape, again starting the active condition.
Most interesting of all is the sexual process, which takes place in three different ways, in different species. The simplest method is by the division of an ordinary cell into a larger number of parts than usual, so that the gametes are small and more slender in form compared with the vegetative cells.
The gametes swim about as usual, and when two meet they entangle each other with their flagella, and the pair goes waltzing through the water together. Then they come in contact, and begin to fuse, either end to end or side to side, depending on species (Fig. 26A). After union the pair still goes on swimming with its four flagella (Fig. 26B).
The corresponding parts of the cells unite, and the two nuclei meet and fuse (Fig. 26C). This process of gametic union is called isogamy. A cell wall is formed around the whole producing a resting zygote (Fig. 26D).
After some time, the diploid nucleus of the zygote undergoes reduction division with the production of four haploid nuclei which are incorporated into four zoospores which on being released form four Chlamydomonas plants (Fig. 26E & F and Fig. 28).
This type of isogamy is exhibited by C. moewusii. Again isogamy between two amoeboid gametes is encountered in C. eugametos, in which the fusing gametes do not possess any flagella. It is interesting to note that the gametes are nothing but under- sized vegetative cells which lack food energy to produced new individuals directly and behave in a different way. This is Known as ’hunger theory of sex’.
In the second type there is a marked and constant distinction between the two fusing gametes with a marked differentiation of sex. The microgametes are smaller and more active than the vegetative cells, and the macrogametes being much larger and less motile.
Both kinds of gametes are flagellate (Fig. 27A & B). In fusion both the gametes come in contact with each other by their anterior extremities where the membranes coalesce forming an open canal and the contents of the smaller gamete drift over into those of the larger one (Fig. 27C & D).
During the process of fusion the flagella disappear. A new and thick cell wall is developed around the fused contents consisting the zygote. (Fig. 27E). This is a case of anisogamy which is more advanced than that of the previous isogamous condition, is illustrated by Chlamydomonas braunii.
The third type represented by Chlamydomonas coccifera is more remarkable. Here the macrogamete is formed from the ordinary vegetative individual by the loss of flagella without undergoing any division, rather with a considerable enlargement accompanied by rounding off of the cell and may be designated as a female gamete.
The male gametes are much smaller and elongated than the vegetative individual. The male gamete unites with the female and both of them fuse to form a zygote. Although the male gametes are not the typical spermatozoids, the sexual reproduction is certainly of a remarkably high level for an unicellular plant and almost amounts to oogamy (Fig. 26G).
In all the three cases the product of sexual fusion becomes enveloped in a thick membrane. The diploid zygote nucleus divides meiotically resulting in the formation of four haploid nuclei which is followed by the cleavage of the protoplast and formation of zoospores. The zoospores are liberated by the dissolution of the zygotic wall. Chlamydomonas possesses both homo- and heterothallic species (Fig. 28).
The various sexual processes exhibited by different species of Chlamydomonas illustrate all the stages in the evolution of sex in algae and can very well be arranged as isogamy →anisogamy → oogamy from the evolutionary point of view. The origin and differentiation of sex can also be traced within the different species of this genus.
According to ‘hunger theory of sex’, both the zoospores and the gametes are remarkably similar and they start their development initially from the same material with their course of development being controlled by the food supply.
The small individuals lose the power of independent development and behave like gametes. Further advancement takes place during this gamete development when, as a result of slowing down of the process of gamete development, there results gametes of dissimilar size.
In the final stage of advancement, a vegetative cell instead of being divided starts enlarging losing its flagella and motility behaving like an ovum which is fertilized by an active male gamete and thus reaching the highest level of advancement in the sexuality that can be visualized.
Again, according to Smith the occasional occurrence of amoeboid gametes in Chlamydomonas, and the presence of a type of conjugation in C. eugametos suggest the possibility of a direct derivation of the Conjugales from motile, one-celled ancestors belonging to the order Volvocales.
Some Indian species:
Chlamydomonas ehrenbergii; C. eugametos Moewus; C. grandistigma.
1. Unicellular biflagellate and pear-shaped plant body.
2. Cup-shaped chloroplast.
3. Presence of eyespot and two contractile vacuoles.
4. Asexual reproduction by biflagellate zoospores.
5. Presence of palmella-stage.
6. Sexual reproduction iso-, aniso- to simple oogamy.
Family # 2. Sphaerellaceae:
The members of this family possess unicellular thick gelatinous-walled biflagellate vegetative cells with basin-shaped to more or less reticulate chloroplast. Sexual reproduction is isogamous.
It is a fresh-water alga being abundantly found in rain-water, pools, ponds and similar other places. In the Arctic and Alpine regions the alga is commonly known as ‘red snow’ as it imparts red colour to the snow. Structurally, the plant body is very similar to Chlamydomonas for being unicellular, uninucleate, anteriorly placed biflagellate, and broadly ellipsoid to ovoid ‘in shape with several contractile vacuoles (Fig. 29A).
It is characterized by the presence of a space between the protoplast and the cell wall. This space is filled with a watery gelatinous substance and is traversed by numerous delicate cytoplasmic strands extending from the protoplast to the cell wall.
The chloroplast is basin-shaped to reticulate containing pyrenoids. Usually, the cells are coloured bright-red due to the presence of the pigment haematochrome in the cell sap. For long time the genus was named Haematococcus for having haematochrome in the protoplast. The cell wall is thick and is differentiated into a superficial strongly thickened layer and an extensive gelatinous zone.
The alga reproduces both asexually and sexually. Asexual reproduction may take place just by the division of the free-swimming cells into two to four daughter cells (Fig. 29B to E). But more frequently during asexual reproduction the cells withdraw their flagella and come to rest. Each cell increases in diameter, contents of which divide longitudinally into two to sixteen parts.
All of them develop wall and are confined for some time within the mother cell. Ultimately they are liberated out by the rupture of the mother cell wall and develop into new individuals.
Sexual reproduction is isogamous. The process is directly related with the prevailing unfavourable condition of the substratum. The flagella fall off and the cells come to rest, contents of which divide to form gametes much smaller in size and larger in number than the cells that are produced asexually.
After remaining confined within the mother cell, the gametes are liberated out and ultimately unite in pairs resulting in the formation of a zygote (Fig. 29F to I). After a period of rest the diploid nucleus of the zygote, on return of favourable conditions divides twice, first division of which is reductional, to form four haploid cells (Fig. 29J).
These haploid cells are set free by the rupture of the zygote wall, become flagellated and develop into new Sphaerella cells (Fig. 29K).
Depending on conditions of the substratum, the vegetative cells may form large akinetes which on germination can give rise to zoospores, hypnospores, or gametes.
1. Unicellular biflagellate plant body with characteristic space between the protoplast and the cell wall which is filled with a watery gelatinous substance.
2. Basin-shaped to reticulate chloroplast.
3. Cells with several contractile vacuoles.
4. Sexual reproduction isogamous.
Family # 3. Volvocaceae:
The Volvocaceae include forms in which flagellate cells are grouped into colonies (coenobia) of cells being multiple of two. The colonies may be flat, disc-shaped or spherical.
All are fresh-water. Asexual reproduction is either by the division of ordinary vegetative cells or by the development of special vegetative cells—gonidia. Sexual reproduction ranges from isogamy, anisogamy to advanced oogamy with the development of antherozoids and egg.
This alga is abundant in fresh-water ponds and ditches. Typical chlamydo- monas-cells are closely packed together to form spherical to ellipsoidal coenobium (Fig. 30A). The coenobium is composed of relatively definite number of cells ranging from 4 to 32, which are enclosed in a homogeneous gelatinous matrix with an outer watery sheath.
In a coenobium cells are so compact that they are often angular as a result of mutual pressure. The coenobia are hollow with the cells forming a single layer in the peripheral gelatinous matrix. The two flagella of each cell pass out through specially differentiated canals of the gelatinous matrix. The coenobia exhibit a marked polarity with the constituent cells showing a definite orientation.
Asexual reproduction is by the formation of daughter coenobia, prior to which the parent coenobium ceases to be motile and sinks to the bottom of the pool. Then each cell produces by several divisions, daughter coenobia (Fig. 30B to D). All the cells of the parent coenobium behave alike in reproduction. The daughter cells become inverted and assume a spherical shape and ultimately each cell produces two flagella.
The daughter coenobia are ultimately liberated by the disintegration of the parent envelope (Fig. 30E). Sexual reproduction is anisogamous. Divisions leading to the formation of gametes are identical with those of asexual reproduction process. But the fusing gametes are always unequal in size with the larger one more sluggish than the smaller counterpart.
Gametes from different coenobia only fuse. Fusion may be terminal or lateral (Fig. 30F). The quadriflagellate zygote remains motile for a while before it loses its flagella and secretes a wall (Fig. 30G & H).
Only one of the four resulting cells of the germinating zygote normally survives forming a swarmer (Fig. 30 I & J). After swarming for a short time the swarmer withdraws its flagella and secretes a gelatinous envelope, and divides and re-divides to form a new coenobium (Fig. 30K).
Some Indian species:
Pandorina morum( Mull.) Bory.; P. morumf. major Iyeng.
1. The coenobia are oblong to subspherical and are comparatively of small size.
2. Coenobium of typical Chlamydomonas-cells.
3. Anisogamous sexual reproduction.
Commonly found in fresh-water pools and ponds, particularly during the rainy season. Spherical or ellipsoidal coenobia are distinguishable into anterior and posterior portions.
There may be sixteen to sixty-four biflagellate, globose or somewhat pear- shaped cells present in the coenobium, which are not closely packed and are sometimes arranged in well-marked transverse series distributed in both anterior and posterior tiers (Fig. 31 A). Cells with usual Chlamydomonad-structure are embedded in a mucus investment in the coenobium (Fig. 31B).
Almost all the cells of the coenobium give rise to new daughter coenobia with the exception of a few anterior cells which are much larger and do not reproduce. The process of daughter coenobium formation is same as that of Pandorina. Sexual reproduction is advanced anisogamy indicating close approach to oogamy with the formation of typical anterozoids and non-motile female gamete—ovum (Fig. 31E).
Coenobia are monoecious or dioecious. The anterior cells of a monoecious coenobium give rise to the anterozoids and the posterior ones form ova. But in heterothallic species there are distinct male and female coenobia which develop male and female gametes respectively.
The vegetative cells of the coenobium divide and revived forming anterozoids (Fig. 31C & D). Whereas, during female gamete formation, the vegetative cells arc directly converted into female gametes—ova without undergoing any division. The zygote remains within the parent coenobium until it disintegrates.
Zygote germinates to give rise to a single swarmer which swims about for a time and then, in the same manner as a vegetative cell, divides and redivides to form a new coenobium.
Some Indian species:
Eudorina elegans Ehrb.; E. indica Iyeng.
1. The coenobia are ellipsoidal and larger than in Pandorina.
2. Anisogamous sexual reproduction between an immobile large ovum and an elongated spindle-shaped biflagellate antherozoid exhibiting oogamous condition.
This genus represents the most elaborate development of coenobial form, showing considerable advancement from the more simple and primitive Volvocales. The coenobia of Volvox, occurring in temporary and permanent fresh-water pools are spherical and are about the size of a small pinhead.
Each coenobium is a hollow sphere of mucilage with a single layer of very numerous, even as many as fifty thousand cells (Fig. 32A), arranged in a single layer joined together by fine strands of cytoplasm,(Fig. 32B).
The individual cells have much the same structure as Chlamydomonas (Fig. 32C). The whole coenobium moves and rotates by means of flagella of individual cells. Most of the cells in a coenobium are vegetative and do not have the capacity to form new individuals.
From the point of view of reproduction, Volvox differs essentially from Pandorina and Eudorina in that, certain cells in a coenobium only take part in reproduction.
Of the many cells of a Volvox coenobium, only a few differentiate as asexual reproductive cells growing larger with the disappearance of flagella, and thickening or with an increase in number of the cytoplasmic strands in the vicinity of them; these cells are called gonidia (sing, gonidium) (Fig. 32A & E).
A gonidium divides and redivides producing a number of daughter cells (Fig. 33A to E), all of which are held together as a new colony. But since a number of gonidia divide at the same time several new colonies are formed simultaneously. After division ceases, the young colony turns itself inside out by inversion through a small pore, or opening—the phialopore (Fig. 33F to H).
The individual cells then develop flagella. The new coenobium may or may not escape from the parent coenobium immediately after development (Fig. 32D).
Sexual reproduction is oogamous. There are both homothallic and heterothallic species of Volvox. Both kinds of gametes, in small numbers, are developed especially from cells of the posterior half of the coenobia. Cells producing female gametes or ova enlarge considerably without undergoing any division and they lose their flagella during development.
Cells forming male gametes or antherozoids divide successively giving rise to numerous spindle-shaped, biflagellate anterozoids (Fig. 34A to C’).
The female gametes are non-motile, much larger than the male gametes, and are surrounded by a gelatinous sheath (Fig. 34D). The antherozoids swim to the egg, enter through the gelatinous sheath, and finally fertilization is accomplished (Fig. 34E). The resulting zygote develops a thick, often stellate wall, and undergoes a resting period (Fig. 34F).
The first division of the zygotic nucleus is meiotic. A single zoospore is produced which forms a new coenobium (Fig. 35).
Volvox represents the apex of this motile coenobial line for having an elaborate mechanism of coenobial development and oogamous sexual reproduction.
Some Indian species:
Volvox africanus West.ƒ. minor Iyeng.; V. aurens Ehrb.; V. globator (L) Ehrb.; V. merrille Shaw; V. prolijicus Iyeng.; V. rousseleti West var. lucknowensis Iyeng.
1. Biflagellate pear-shaped cells arranged in a coenobium of large number of cells.
2. Mature coenobium composed of vegetative cells, colony forming cells or gonidia and anterozoid mother cells and ovum mother cells.
3. Asexual reproduction by gonidia which produce daughter coenobia by an elaborate process.
4. Sexual reproduction oogamous.