Double fertilisation and the endosperm

Two fusions in one sac

The pollen tube has grown through the style and has entered the ovule through the micropyle. It reaches one of the two synergids and enters it through the filiform apparatus. The tip of the tube bursts, and it releases the two male gametes into the cytoplasm of the synergid.

Now two things happen.

1. One male gamete moves to the egg cell and fuses with its nucleus. This fusion of a male gamete with an egg is called syngamy. The result is a diploid cell, the zygote.
2. The other male gamete moves to the two polar nuclei in the central cell and fuses with them. Three haploid nuclei fuse, so this is called triple fusion. The result is a triploid nucleus, the primary endosperm nucleus (PEN).

Because two kinds of fusion, syngamy and triple fusion, take place in the same embryo sac, the event is called double fertilisation. NCERT calls it an event unique to flowering plants.

After the triple fusion, the central cell is the primary endosperm cell. It grows into the endosperm. The zygote grows into the embryo.

Counting the chromosomes

Take a plant with 24 chromosomes in its root-tip cells.

The zygote is diploid, with $2n$ chromosomes. The PEN is triploid, with $3n$. Every cell of the endosperm comes from the PEN by mitosis, so every endosperm cell is triploid.

The three nuclei of the PEN are not all different. The two polar nuclei come from the same megaspore, so they are exactly alike. The egg is also exactly like them. The two male gametes come from the same generative cell, so they are alike too. We use this in the counting of genes later.

The endosperm

The endosperm is the food store for the embryo. It develops before the embryo does. Most zygotes wait until a certain amount of endosperm has formed. In this way the embryo is sure to get food.

The PEN divides again and again by mitosis, and no walls form at first. The nuclei lie free in the cytoplasm. This stage is called the free-nuclear endosperm. After a time, cell walls form between the nuclei, and the endosperm becomes cellular.

The coconut shows both stages. The tender coconut water is the free-nuclear endosperm, with thousands of nuclei. The white kernel around it is the cellular endosperm.

The endosperm has two fates.

• It may be used up by the growing embryo before the seed is ripe. This happens in pea, groundnut and beans.
• It may stay in the ripe seed and be used when the seed germinates. This happens in castor, coconut, wheat and maize.

Counting the genes of the endosperm

The chromosome numbers are not all that you can count. You can also count the genes. Suppose a female plant is $Aa$ and the male plant is $Aa$.

• The embryo sac may come from a megaspore with $A$ or with $a$. Then the egg and the two polar nuclei all carry the same allele.
• The male gamete carries $A$ or $a$. Both male gametes carry the same one.

So a seed has the embryo made of the egg and the male gamete, and the endosperm made of two copies of the female allele and one male allele. The four possible seeds are listed below. Each has a chance of one in four.

• Female allele $A$, male allele $A$: embryo $AA$, endosperm $AAA$.
• Female allele $A$, male allele $a$: embryo $Aa$, endosperm $AAa$.
• Female allele $a$, male allele $A$: embryo $Aa$, endosperm $Aaa$.
• Female allele $a$, male allele $a$: embryo $aa$, endosperm $aaa$.

You should find that the zygote has the same number of chromosomes as a root-tip cell, and the endosperm nucleus has one and a half times as many.

Try it in Python

The program counts the chromosomes in the nuclei of a double fertilisation.

Practise with new numbers

Common mistakes

• Calling the endosperm diploid. It is triploid, because it comes from three haploid nuclei.
• Saying that the zygote is made of three nuclei. It is made of two: a male gamete and an egg.
• Saying that both male gametes fuse with the egg. One fuses with the egg, and the other with the polar nuclei.
• Mixing up the coconut parts. The coconut water is the free-nuclear stage, and the white kernel is the cellular stage.
• Thinking that the embryo grows first. The endosperm starts first.

Remember these

• The tube releases two male gametes into a synergid. One fuses with the egg (syngamy), and the other with the two polar nuclei (triple fusion).
• Zygote: $2n$. Primary endosperm nucleus: $3n$. Double fertilisation is unique to flowering plants.
• The endosperm starts before the embryo. It is free-nuclear first and cellular later.
• The endosperm is used up in pea, groundnut and beans, but it stays in castor, coconut, wheat and maize.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Where are the two male gametes released?

2. What are syngamy and triple fusion, and what does each make?

3. A plant has 18 chromosomes in its root-tip cells. How many are in its zygote, and how many in its endosperm cells?

4. What are the two stages of endosperm development, and where do you see both in the coconut?

5. Why does the embryo start to grow only after some endosperm has formed?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 1, Sexual Reproduction in Flowering Plants.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.
• Urry L. A. et al. (2017). Campbell Biology, 11th edition, Pearson, chapter 38.