Sexual reproduction in Spirogyra takes place by conjugation, i.e.the union of two non-motile gametes. Sexual reproduction in Spirogyra can be considered as isogamous type as there is no any differences in the shape of gametes which occurs by the conjugation of non flagellated gametes. However, one of the gametes can be regarded as male as it migrates and another as female as it remains stationary. So they can be regarded as morphologically isogamous but physiologically anisogamous.
There are major two types of conjugation known in case of Spirogyra and they are:
- Scalariform Conjugation
- Lateral Conjugation
Scalariform Conjugation is most common method of sexual reproduction in Spirogyra that takes place in heterothallic or dioecious (having sexes that reside in different individuals) species. This type of conjugation is reported in the species like S. tuwensis, S. cylindrica etc.
- During the time of conjugation both the filaments of different strains lie parallel facing each other.
- The close contact induces the secretion of more mucillage which holds the filament more firmly.
- All the cells of both filaments produce a small dome shaped protuberances in their longitudinal walls facing each other. These small tubular protuberances is known as transverse tubes or papillae.
- The papillae grow in size and come in contact with each other through their anterior end. Upon their contact, the papillae gets dissolved by a enzyme called cytase and forms a common tube like passage known as conjugation tube.
- The cells of both filaments connected by conjugation tubes are called gametengia one of which function as male gametengium and other as female gametengium.
- Simultaneously, the protoplasmic content of gametengia contract from the cell wall by losing water and metamorphose into a naked, non motile and non ciliated gametes.
- The protoplast of male gamete moves towards the female gamete in another filament through conjugation tube . The protoplast of two gametes fuse to form a zygote.
- The zygote secrete a thick wall around itself and known as zygospores.
- As a result, the zygospores are formed in series in one filament and another filament remain empty.
- In conjugating filaments each pair undergoes the same process as mentioned above. Hence the structure looks like ladder steps due to the presence of number of conjugation tubes between them.So, this process is named as scalariform conjugation. (GK: escalar: ladder)