Read this article to learn about Spirogyra: Life Cycle of Spirogyra and Germination of Zygospore !
Kingdom – Plantae
Sub Kingdom -Thallophyta
Spirogyra is a cosmopolitan, freshwater, filamentous green alga, represented by about 300 species. Most of them are green floating in stagnant ponds and pools, a few species are attached and some occur in running water (S. adnata). It was discovered by Link. Spirogyra is commonly known as pond silk, water silk, pond scum or mermaid’s trees because of their bright green silky appearance.
The plant body of Spirogyra is an un-branched filamentous thallus (Fig. 5.5), measuring about 1 mt. in length. The Spirogyra filament is very slimy due to the presence of mucilage sheath that lines the whole filament. The mucilage often holds the filaments together in a matted blanket.
Each un-branched filament of Spirogyra consists of a number of elongated cylindrical cells of similar type joined end to end. The terminal cell is dome-shaped. In attached species, the lower non-green cell is called holdfast or hapteron (with irregular lobes, e.g., S.fluviatilis). Presence of non-green rhizoids (branches) reported by Jao (1936) in two species, S. rhizobra and S. rhizopus.
Each cell of Spirogyra filament is cylindrical and consists of 2 parts: cell wall and protoplast. The cell wall surrounds the protoplast, is protective and consists of two layers i.e. inner cellulose layer and outer pectose layer. The pectose layer dissolves in water to form a slimy mucilage sheath covering the whole filament. The transverse wall (cross wall) between adjacent cells has middle lamella (septum). The cross wall maybe plane, replicate or colligate. In replicate type, the middle lamella forms two ring like ingrowths while in colligate type H-shaped pieces develop near middle lamella.
The protoplast is differentiated into many parts, from outside to inside; they are plasma membrane, primordial utricle (=peripheral cytoplasm), large central vacuole and a nucleus. The primordial utricle contains 1-16 spirally arranged ribbon-shaped chloroplasts along with various organelles. The margin of chloroplast may be smooth or serrated. In the chloroplast, a row of pyrenoids present at regular intervals. Each paranoid consists of a central protein core surrounded by a starch sheath.
A single nucleus present at the central cytoplasmic mass that held in the center of vacuole. The nucleus held in position by radiating. The cytoplasmic strands traverse through the vacuole to reach the primordial utricle. Except holdfast, all the cells of filament can divide and elongate to increase the length of Spirogyra filament.
Life cycle of Spirogyra:
In Spirogyra, 3 types of life cycle can be seen – vegetative and sexual cycles are most common. But asexual cycle occurs only occasionally. Life cycle of Spirogyra is haploidic where the haploid vegetative filament represents a prolonged gametophyte generation and the brief sporophyte phase is represented by diploid zygospore (zygote). In the life cycle, the thalloid vegetative filaments multiples by vegetative and sexual reproductions are most common while asexual reproduction occur only occasionally.
Further, the vegetative reproduction is comparatively shorter than the other types:
A. Vegetative cycle:
During the growing season, the vegetative filament of Spirogyra undergoes fragmentation, so that each fragment independently develops into a new filament by repeated cell division and elongation. In favorable conditions, fragmentation is a most common method of multiplication. The fragmentation is caused by (i) mechanical injury (ii) dissolution of middle lamella (Hi) development of H-shaped pieces (e.g. in S. colligates).
B. Asexual cycles:
Asexual life cycle is less common and reported only in a few Spirogyra sp. Asexual cycles involve the formation of aplanospores, akinetest, azygospores (Fig. 5.6).
Under un-favourable conditions, the protoplast of each vegetative cell shrinks and develops a wall around it to form an aplanospore. Each non-motile aplanospore germinates to form a new filament. For example, S. articulate, S. mirabilis etc. In S. aplanospora the life cycle involves the formation of aplanospores only.
(ii) Formation of akinetes:
Under unfavorable conditions, the vegetative cells of S. farlowii develop thick-walled cells called akinetes. On the return of favourable conditions each akinete germinate into a new filament.
(iii) Formation of azygospores or parthemspores:
In the life cycle of S. varians and S. groenlandica, sometimes the gametes fail to fuse and each get enclosed by thick cell wall to become azygospore or parthenospore. Each azygospore germinates in to a new filament.
C. Sexual cycle:
In Spirogyra, the sexual reproduction involves a cycle alternation between a haploid vegetative filament (gametophyte plant) and a diploid zygospore, towards the end of growing season the Spirogyra filament produce aplanogametes in vegetative cells called gametangia The gametes fuse to form a zygospore (2n). At the time of germination, the zygopsore nucleus undergoes meiosis to produce 4 haploid nuclei but only one of them survives. Thus, a zygospore gives rise to a haploid new filament.
Therefore, in the haplontic life cycle of Spirogyra no true alternation of distinct plants observed. Rather, alternation of chromosome numbers from haploid to diploid and back to haploid conditions can be noticed.
Sexual reproduction of Spirogyra involves conjugation, which may be defined as a primitive type of isogamy called aplanogametic isogamy. In this type of isogamy, the entire protoplasmic contents of vegetative cells (viz., Gametangia) function as gametes. It always involves the gametangial contact. Conjugation is of two type—Scalariform and Lateral conjugation.
(i) Scalariform conjugation:
It is most common method in most heterothallic species of Spirogyra. It takes place in the night when two filaments come close and lie opposite and parallel to each other. Now, opposite cells develop outgrowths or protuberances called conjugation processes. At the point of contact, the tips of these processes dissolve (bycytase) to form conjugation tube between opposite cells. This resembles r ladder. Hence, the conjugation is called scalariform (Gr. scala = ladder; forma = shape) or H-shape conjugation.
When the conjugation tube is forming, the protoplasts of conjugating cells (gametangia) recede, round up and function as gametes. The male gametes transfer through the conjugation tube into the opposite cell (female gametangia) and fuse with female gamete to form zygote (2n). The zygote secretes a thick wall to become zygospore. Therefore, in the late stage of conjugation, male gametangia remain empty and the female gametangia contain zygospores (2n) (Fig. 5.7).
(ii) Lateral conjugation:
It is of rare occurrence and involves the fusion of gametes from two adjacent cells of the same filament in monoecious or homothallic species. It is of two types: Indirect and Direct Lateral conjugation.
(a) Indirect lateral conjugation:
Two outgrowths emerge on both side of a transverse septum of two adjunct cells and after some growth the two protuberances unite to form a conjugation tube. Of the two adjacent cells, one behaves as male gametangium whiles the other as female gametangium. The content (now behaving as gamete) from male gametangium passes through conjugation tube and enters into female gametangium .By the fusion of male and female gametes a diploid zygospore is formed. Thus, in each second cell of a filament, a zygospore is formed, e.g., S. tenuissima, S. affinis (Fig. 5.8).
(b) Direct lateral conjugation:
In this type of conjugation, the male gametangium after passing through an aperture in the transverse septum of adjoining gametangium enters the female gametangium and fuses with the female gamete, and a diploid zygospore is formed, e.g., S .jogensis(fig. 5.8).
Germination of Zygospore (Fig. 5.9):
Zygospore is the only diploid phase in the sexual life cycle. The decay of female gametangia causes the liberation of zygospore which remains dormant in the pond bottom till the favorable conditions return. Zygospore wall is thick and differentiated in to 3-layers. Outer exosporium (cellulose), middle mesosporium (cellulose & chitin) and inner endosporium (cellulose).
On return of favourable condition the diploid nucleus of zygospore undergo meiosis to produce 4 haploid nuclei out of which 3 degenerate. The zygospore with one haploid nucleus gradually enlarges and burst open to release a germ tube. The repeated transverse divisions of germ tube form a new filament.