Spirogyra

1.0General Characteristic of Spirogyra 

Spirogyra is green algae found in freshwater ponds, pools, and lakes. It is also known as water silk or pond silk due to its slippery feel. Its thallus is a long thread-like cylindrical, up to several centimeters long, like a filament that is unbranched. 

The basal cells of these filaments function as the attachment organ. The cell wall in Spirogyra is a single piece. The cell wall is bi-layered; the inner layer is cellulose, and the outer layer is pectose. The outermost portion of pectose changes into pectin, the latter dissolves in water to form a slimy sheath.

A large central vacuole is filled with cell sap, often rich in tannins and other chemical compounds in solution. Their chloroplast is spiral or ribbon-shaped, which may be loosely or tightly coiled. They vary in number in different species, from one to as many as sixteen in each cell. Inside the spiral chloroplast, a series of small, spherical round bodies of pyrenoids are present.

2.0Classification of Spirogyra

3.0Diagram of Spirogyra

4.0Reproduction in Spirogyra

Spirogyra Reproduction are of following types:

Vegetative Reproduction 

• Fragmentation: When the conditions are favourable, Spirogyra reproduces by fragmentation. Fragmentation occurs by dissociating the filament at the cross wall into one or more segments called fragments. Each fragment develops into a new filament. The following methods can be done by mechanical injury or by dissolution of the middle lamella of the end wall due to a change in the physical properties of water.

Asexual Reproduction

Asexual Reproduction in Spirogyra is following types

• Aplanospore: During unfavourable conditions, aplanospore are formed; Only a few species form aplanospore. Zoospores are entirely absent in Spirogyra. 
• Akinetes: Akinetes are also found in some species of Spirogyra. Akinetes develop a thick wall around the cell. 
• Azygospore/parthenogenesis: sometimes, the cells begin to form gametes, but these gametes fail to fuse; sometimes, they secrete cell walls around them and behave like a spore. They germinate and form new filaments. 

Sexual Reproduction

Sexual Reproduction is an isogamous type. It is accomplished by Conjugation, where the entire content of two similar cells is fused. It is of two types :

• Scalariform Conjugation: It takes place between two opposite cells of neighbouring filaments. They form a structure like a ladder and are also known as H-shape conjugation. A tube develops, known as a conjugation tube, and the protoplast of the conjugating cell forms a single gamete.  The male gamete detached from the parental cell wall and fused with the female gamete to form a zygospore. This results in an empty cell and a cell containing a zygospore. 
• Lateral Conjugation: in lateral Conjugation, neighbouring cells of the same filaments behave like male and female gametes, respectively. A conjugation tube is formed between these two cells.

Lateral conjugation is of two types, indirect and direct. 

• Indirect lateral conjugation:  in this type of Conjugation, adjacent cells of the same filament form a conjugating tube, and these two cells behave like male and female gametes, respectively. In this case, the same filament has a cell containing a zygospore and an empty cell beside this. 
• Direct lateral conjugation: this type of Conjugation generally takes place between the two cells next to the basal cell, which usually functions as an attaching organ. The cell next to the basal cell becomes swollen and behaves like a female gamete, and the upper cell, which is cylindrical in shape, remains unchanged. The male and female gametes fused in the female cell to form a zygospore. 

Zygospore

The zygospore is formed from the fusion of two gametes, resulting in a united protoplasmic mass. During the contraction phase, water loss from the contractile vacuoles causes the zygospore to shrink away from the cell wall, assuming a rounded or oval shape. A new thick, multi-layered brown wall is formed, consisting of:

(i) Exospore: The outer thin layer, primarily cellulose with pectin, is sometimes sculptured and soluble in a mixture of cellulose and pectinase.

(ii) Mesospore: The thickest middle layer, resistant to enzymes due to sporopollenin content, typically with a dehiscence line and varying surface texture.

(iii) Endospore: The thin inner layer, cellulose in nature, contributes to the zygospore's resistance to unfavorable conditions.

Maturation of Zygospore 

During zygospore maturation, they are released as the cell walls of the female cells decay. They sink to the bottom of the pool. Their thick, resistant walls enable them to withstand desiccation. Before germination, the diploid zygospore nucleus undergoes zygotic meiosis, comprising two divisions: reduction division followed by mitosis. This process results in four haploid nuclei within the zygospore protoplast, with three nuclei typically aborting. Cytokinesis does not occur. The functional haploid nucleus initiates germination, leading to the development of a single filament. In either case, the zygospores are liberated by the destruction or decay of the cell walls of the female cells. They sink to the bottom of the pool. Their thick, resistant walls enable them to withstand desiccation. 

Prior to germination, the diploid zygospore nucleus undergoes meiosis. It is known as zygotic meiosis. At the end of meiosis, the zygospore protoplast contains four haploid nuclei. Three of these abort. There is no cytokinesis. The surviving haploid nucleus is functional. The zygospore with the surviving haploid nucleus germinates to give rise to a single filament. Upon germination, the tough outer layers of the zygospore break open, and a small green tube grows out. Inside this tube, a bright green colour appears. 

5.0Life cycle of Spirogyra