Occur rare type of conjugation and seen in Homothallic/Monoecious species. It is of two type direct and indirect.
Indirect: On the lateral side of filament above the septum the two adjacent cells gives papillate out growth. Cytase is secrete by the cells the partition wall get dissolved forming conjugation pasage due to the activity of enzyme cytase. Simultaneously the protoplast of the adjacent develops into the gamete which is Isogametes. The cells are referred as Gametangia.
After sometime one gamete out of two gamete shows motility and determined as male gamete. The male gamete passes through conjugation tube and enters the female gametangia and fuses with female gamete to form zygote then forms thick walled Zygospore.
Direct: (Primitive type of conjugation)
During the time of conjugation few attached species shows.
Two adjacent cells which are going to conjugate. The lower cell get enlarged and form barrel shaped structure termed as Female gamete.
The upper cell (male gametangia) forms male gamete which are some what elongated cylindrical and pointed lower end and here conjugation tube are not forming. The male gamete with pointed end directly pierces the septum and forms a hole in septum and passes through the hole into female GAmetangia where it fuse with female gamete to form gametes. The zygote after developing thick wall develops into zygospore.
Zygospore:
Zygospore represents the only diploid stage in the lifecycle of spirogyra and it is yellowish brown ellipsoidal structure having diploid nucleus and covered by three layered wall : - the outer exospore/exine
- The middle mesospore/mesine
- th innermost Endospore/ intine
The zygospore due to the thick wall can withstand long period of dry and dessication. They are liberated after the destruction of mother cell or mother filament and they remain at the bottom of water body. With the coming of rainy season,the zygospores start to germinate. Before germination,the diploid nucleus divides meiotically and form four haploid nuclei. Out of which three degenerate and one remains at functional nucleus. At the time of germination the exospore and mesospore rupture and the endospore along with functional nucleus comes out as single cell germinating tube which immediately divides into two celled structure. The basal cell forms the rhizoidal cell and the upper cell by repeated division and growth develops into new spirogyra filament.