Lateral conjugation is another common method of sexual reproduction in Spirogyra. It takes place in homothallic or monoecious (having male and female reproductive organs in the same individual) species of Spirogyra. lateral conjugation takes place between the adjacent cells of the same filament. The zygospores are usually formed in alternate cells. Hence lateral conjugation takes place is also called chain conjugation and it takes place in two ways:
- Indirect lateral conjugation
- Direct lateral conjugation
Indirect lateral conjugation
- In some species like S. affinis, S. sahnii, S. ampla indirect lateral conjugation takes place between two adjacent cells of the same filament among which one behaves as male gametengium and other as female gametengium.
- It begins with the development of papillae on the lateral sides of the septum.
- The common wall gets partially dissolved by enzymatic action and forms a passage or a lateral conjugation tube between the cells.
- The upper cell behaves as a male gametengium and lower one behaves as female gametengium.
- The contents of one cell which behaves as a male gametengium becomes male gamete and moves towards the female gametengium which behaves as a female gamete through conjugation tube.
- The protoplasm of both gametes fuse to form a diploid zygospore.
Direct lateral conjugation
- Direct lateral conjugation is observed in S. jogensis.
- This type of conjugation also takes place between the adjacent cells among which the upper cells act as male gametengium and lower cell act as female gametengium.
- The male cell and female cell become distinct as female cell swells up to become barrel shaped while the male cell remains smaller and cylindrical.
- The protoplasm of the conjugating cells contract and secrete a thick wall around itself to become gametes.
- The contents of male gamete become somewhat conical, elongated or rod shaped and pierces the septum.
- A pore like opening this develops in the septum through which male gamete passes and fuses with female gamete to form zygospores.
Germination of Zygospores
Zygospore is formed by the fusion of male gamete and female gamete. It is cylindrical, circular and ellipsoidal structure and dark brown in colour. It is composed of three layers I.e. exine, mesine and intine.
Prior to germination, the diploid nucleus of zygospores divide meiotically to form four nuclei. Three of them disintegrate leaving only one functional nucleus. The zygospore having single haploid nucleus germinates to form a germ tube. The germinating tubes divide and give two cells- lower hyaline rhizodal cell and upper green divides repeatedly to form new filament of Spirogyra.