Double fertilisation

What is double fertilisation?

The explanation and chromosome counts below use NCERT’s typical Polygonum-type embryo sac in a diploid plant. Other embryo-sac types can give different endosperm ploidies.

In plain words: a pollen tube brings two male gametes into the embryo sac, and both are used. One fertilises the egg and starts the next generation, the embryo; the other joins the two polar nuclei and starts the food store for that embryo, the endosperm.

Where the two male gametes go

After the pollen tube has grown through the style and entered the ovule, it enters one of the synergids through the filiform apparatus (page 17). Then (page 18):

• the pollen tube releases the two male gametes into the cytoplasm of the synergid;
• one male gamete moves towards the egg cell and fuses with its nucleus;
• the other male gamete moves towards the two polar nuclei in the central cell and fuses with them.

NCERT calls the combination of embryo-forming and endosperm-forming fertilisation unique to flowering plants (page 18); give this in an NCERT-based answer. Beyond the NCERT text: two fertilisation events also occur in some Ephedra and Gnetum, but they do not produce angiosperm-type triploid endosperm (Friedman, Science, 1990; Friedman and Carmichael, International Journal of Plant Sciences, 1996). Compare gymnosperm reproduction.

Syngamy

One male gamete fuses with the nucleus of the egg cell, which completes syngamy. This forms a diploid cell, the zygote (page 18).

• male gamete (n) + egg (n) → zygote (2n)
• The zygote develops into the embryo (page 18).

Triple fusion and the primary endosperm nucleus

The other male gamete fuses with the two polar nuclei in the central cell to give a triploid primary endosperm nucleus (PEN) (page 18).

As this involves the fusion of three haploid nuclei it is termed triple fusion.

NCERT Biology (Class 12), Section 1.3, page 18

• male gamete (n) + two polar nuclei (n + n) → PEN (3n)
• After triple fusion, the central cell becomes the primary endosperm cell (PEC), which develops into the endosperm (page 18).
• In NCERT Figure 1.13 (a) (page 18), the synergids and the antipodal cells left in the fertilised embryo sac are labelled as degenerating.

In short (NCERT page 18):

• Syngamy: 1 male gamete + egg → zygote (2n) → embryo.
• Triple fusion: 1 male gamete + 2 polar nuclei → PEN (3n) → endosperm.

Five haploid nuclear contributions take part in the typical pattern: two sperm nuclei, one egg nucleus and two polar nuclei (reasoned from NCERT page 18). The polar nuclei may have already fused into a diploid secondary nucleus before the second sperm arrives; the resulting PEN is still 3n.

Solved examples

Common mistakes to avoid

Quick revision summary

• The pollen tube enters a synergid and releases two male gametes.
• Syngamy: one male gamete fuses with the egg nucleus, giving the diploid zygote, which develops into the embryo.
• Triple fusion: the other male gamete fuses with the two polar nuclei, giving the triploid primary endosperm nucleus; the central cell becomes the primary endosperm cell and develops into the endosperm.
• The egg-plus-central-cell fertilisations give embryo and endosperm, the characteristic angiosperm pattern of double fertilisation.

Questions students ask

What is the difference between syngamy and triple fusion?

Syngamy and triple fusion are the two fusions of double fertilisation, one for each of the two male gametes that a pollen tube brings into the embryo sac (NCERT page 18). In syngamy a male gamete fuses with the nucleus of the egg cell, two haploid nuclei in all, and the product is the diploid zygote; in triple fusion the other male gamete fuses with the two polar nuclei of the central cell, three haploid nuclei, and the product is the triploid primary endosperm nucleus (page 18). The zygote develops into the embryo, whereas triple fusion gives rise to the endosperm, the food tissue of the developing seed (pages 18–19).

What is the difference between fertilisation and double fertilisation?

Fertilisation in the ordinary sense is a single fusion of a male and a female gamete that forms a diploid zygote, as in humans, where NCERT calls “the process of fusion of a sperm with an ovum” fertilisation (NCERT Chapter 2, page 35; see human fertilisation). Double fertilisation, in flowering plants, is that fusion, called syngamy, plus a second one in the same embryo sac, triple fusion, which forms no embryo but the triploid primary endosperm nucleus from which the endosperm grows (NCERT page 18). So syngamy alone is fertilisation in the usual sense; only the two fusions together make double fertilisation.

Why is double fertilisation important?

Double fertilisation is important because it starts both new parts of the seed together: syngamy gives the zygote, which develops into the embryo, and triple fusion gives the primary endosperm nucleus, whose tissue fills with reserve food that nourishes the developing embryo (NCERT pages 18–19). In NCERT’s typical sexual pathway the endosperm therefore forms only in an ovule that has been fertilised (our reasoning from page 18), and in seeds where it persists it is used up during germination (page 19).

Is double fertilisation found only in flowering plants?

Double fertilisation as NCERT describes it, syngamy together with triple fusion, is found only in flowering plants: NCERT calls it “an event unique to flowering plants” (page 18). A gymnosperm’s pollen tube discharges its male gametes near the archegonia of the female gametophyte (NCERT Class 11, Chapter 3, page 33; see gymnosperm fertilisation), and, by our reasoning, there are no polar nuclei for a second male gamete to join. Beyond the NCERT text, a 1990 study found a regular second fusion in Ephedra, a non-flowering seed plant, but there the second sperm nucleus joins the egg’s ventral canal nucleus (Friedman, Science, 1990).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Into which cell does the pollen tube release the two male gametes?

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   Into the cytoplasm of one of the synergids (NCERT page 18).

2. Q2. What does the central cell become after triple fusion, and what does it develop into?

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   The primary endosperm cell (PEC), which develops into the endosperm (NCERT page 18).

3. Q3. In a plant with 2n = 20, how many chromosomes are there in the primary endosperm nucleus?

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   30: the PEN is triploid, 3 × 10 (NCERT page 18).