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Difficulty: MediumKingdom Fungi: Moulds, Yeasts, and Mushrooms

Arrange the following events in the correct chronological sequence during sexual reproduction (zygospore formation) in the bread mould (*Rhizopus*), from the initial contact of hyphae to the formation of a mature zygospore.

  1. 1Two compatible hyphal strains (+ and -) grow towards each other and produce specialized outgrowths called progametangia.
  2. 2Septa (cross-walls) form behind the swollen tips to isolate multinucleate gametangia from the supporting suspensor cells.
  3. 3The intervening walls dissolve, allowing plasmogamy and karyogamy to occur between the paired nuclei.
  4. 4The resulting cell enlarges and secretes a thick, dark, resistant wall to become a dormant zygospore.

Answer

The correct chronological order of zygospore formation in Rhizopus is: First, compatible (+ and -) hyphae produce progametangia outgrowths; Second, septa form to isolate multinucleate gametangia; Third, contacting walls dissolve leading to cytoplasmic and nuclear fusion (plasmogamy and karyogamy); Fourth, a thick, dark wall is secreted to form a mature zygospore.
Sexual reproduction in Rhizopus proceeds sequentially: (1) attraction and growth of (+ and -) progametangia toward each other, (2) formation of septa to isolate gametangia from suspensors, (3) dissolution of common walls leading to plasmogamy and karyogamy, and (4) wall thickening to form a mature, resistant zygospore.

Step-by-Step Solution

1
Identify the initiating event of sexual conjugation in Rhizopus.
Two compatible mating strains (+ and - hyphae) come into proximity and form specialized outgrowths called progametangia.
Chemical attractants (hormones) cause opposite mating types to grow toward one another.
2
Determine the cellular isolation phase.
Septa form behind the swollen tips of the progametangia to delimit distinct gametangia.
This separates the reproductive multinucleate protoplasm from the vegetative suspensors.
3
Trace the fusion phase (syngamy).
The touching walls of the gametangia break down, facilitating plasmogamy (cytoplasmic fusion) followed by karyogamy (nuclear fusion).
Cellular fusion combines cytoplasm and pairs of + and - haploid nuclei to form diploid nuclei within the shared zygote.
4
Identify the final structural maturation step.
The enlarged zygote secretes a thick, black, warty cell wall around itself, becoming a resistant zygospore.
The zygospore enters dormancy to withstand adverse environmental conditions until favorable conditions return.

Key Concept

Sexual reproduction and zygospore formation in Zygomycota (Rhizopus)
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Topics

Kingdom Fungi: Moulds, Yeasts, and Mushrooms

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