Question

Fertilisation is internal in spirogyra. Where would you get zygospores, if 3 filaments are conjugated by scalariform conjugation?
A. Lateral filaments
B. Central filaments
C. Central filament and both lateral filaments
D. Either central or lateral filaments

Answer 1

Hint: Spirogyra undergoes reproduction via vegetative, asexual, or sexual means; while the life cycle of spirogyra is haplontic (the dominant stage is free-living haploid).

Complete answer:
Spirogyra is free-floating green algae that are present in the freshwater habitat i.e. ponds, lakes, etc. They are commonly known as pond-silk or water-silk. Genus spirogyra is named after their spiral chloroplast present in the cell of algae, and they are classified under phylum Chlorophyta due to the presence of chlorophyll. They are photosynthetic and contribute to the total carbon dioxide fixation carried out. They increase oxygen levels in their own habitat and many aquatic organisms feed on them. Reproduction in spirogyra occurs via vegetative, asexual, or by sexual means. Vegetative reproduction is done by fragmentation, while asexual reproduction occurs through the formation of azygospores or aplanospores. While sexual reproduction in spirogyra is isogamous (i.e. male and female gametes are of similar size) occurs by conjugation which is of two types i.e. Scalariform conjugation, and Lateral conjugation.
1. Scalariform conjugation: In scalariform conjugation, spirogyra two filaments either central or lateral filaments come together and lie side by side and align themselves laterally and connect through the branch-like-conjugation tubes. This conjugation tube grows from each of the filaments and meets in the centre to form a gap in which zygote moves and each zygote will grow in a filament. This structure form looks like a ladder because of this it is named scalariform or H-shape conjugation.
2. Lateral conjugation: In lateral conjugation, spirogyra adjacent cells function as male gamete and female gamete. A conjugation tube is formed between cells of the same filament.

Hence, the correct answer is option (D).

Note: Zygote in spirogyra is known as Zygospores, they are diploid and formed by the fusion of the male and female gametes, and show internal fertilisation. These zygospores are the only diploid stage in the life cycle of spirogyra.