Edunes Online Education
University: Rabindranath Tagore University (RTU), Hojai, Assam
Course: B.Sc. Botany (Honours)
Subject: Morphology, Embryology & Anatomy of Angiosperms (BOT-MAJOR-2)
Edunes Online Education
MEGASPOROGENESIS — Beginning of Female Gametophyte Development
Megasporogenesis is the process by which the
Megaspore Mother Cell (MMC) undergoes meiosis
to produce haploid megaspores inside the ovule.
🧠 Core Concept:
Microsporogenesis → Forms pollen (male side) Megasporogenesis → Forms megaspore (female side)
This is the starting point of female gametophyte formation.
Microsporogenesis → Forms pollen (male side) Megasporogenesis → Forms megaspore (female side)
This is the starting point of female gametophyte formation.
Location:
Occurs inside the nucellus of the ovule.
Occurs inside the nucellus of the ovule.
📌 Ovule → Nucellus → MMC → Meiosis → Megaspores.
2️⃣ FORMATION OF MEGASPORE MOTHER CELL (MMC)
Inside the nucellus, a hypodermal archesporial cell differentiates.
🧠 Developmental Thinking:
Not every cell becomes reproductive.
One specialized cell enlarges and commits to meiosis.
Not every cell becomes reproductive.
One specialized cell enlarges and commits to meiosis.
The archesporial cell:
• Enlarges • Becomes dense in cytoplasm • Develops a prominent nucleus
It directly functions as:
Megaspore Mother Cell (MMC) — Diploid (2n)
• Enlarges • Becomes dense in cytoplasm • Develops a prominent nucleus
It directly functions as:
Megaspore Mother Cell (MMC) — Diploid (2n)
In most angiosperms:
Only one MMC develops per ovule.
Only one MMC develops per ovule.
📌 One ovule → One MMC → One functional megaspore (usually).
🔹 Characteristics of MMC
| Feature | Description |
|---|---|
| Ploidy | Diploid (2n) |
| Size | Large cell |
| Cytoplasm | Dense |
| Nucleus | Prominent |
| Position | Towards micropylar region |
🧠 Why large and dense?
Because it is preparing for meiosis — intense chromosomal activity requires high metabolic activity.
Because it is preparing for meiosis — intense chromosomal activity requires high metabolic activity.
3️⃣ MEIOSIS IN MEGASPOROGENESIS
The MMC undergoes two meiotic divisions.
🧠 Mathematical Thinking:
2n → n (Reduction occurs in Meiosis I)
2n → n (Reduction occurs in Meiosis I)
🔹 Meiosis I (Reduction Division)
2n → n + n
Homologous chromosomes separate.
Chromosome number becomes half.
2n → n + n
Homologous chromosomes separate.
Chromosome number becomes half.
🔹 Meiosis II
n → n + n
Sister chromatids separate.
n → n + n
Sister chromatids separate.
Final Result:
Four haploid (n) megaspores formed.
Four haploid (n) megaspores formed.
📌 Meiosis = One reduction + One equational division.
4️⃣ MEGASPORE TETRAD FORMATION
After meiosis:
1 MMC (2n) → 4 Megaspores (n)
1 MMC (2n) → 4 Megaspores (n)
🧠 Structural Arrangement:
The four megaspores are arranged in a straight line.
This arrangement is called a Linear Tetrad.
The four megaspores are arranged in a straight line.
This arrangement is called a Linear Tetrad.
Visualization:
● ↓ ● ↓ ● ↓ ●
● ↓ ● ↓ ● ↓ ●
📌 Tetrad = Group of four.
Linear tetrad = Four cells in one line.
Linear tetrad = Four cells in one line.
Important:
Usually only one megaspore becomes functional; others degenerate (next stage of development).
Usually only one megaspore becomes functional; others degenerate (next stage of development).
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