Double Fertilization and Triple Fusion

Union of the second male gamete with two polar nuclei to form endosperm, along with fertilization, is called double fertilization.

Double fertilisation is a process in which both male gametes released from a pollen grain participate in fertilisation. One fuses with the egg, and the other fuses with the polar nuclei - both at the same time, inside the embryo sac.

Double fertilization

The Two Events:

Syngamy is the fusion of one male gamete (n) with the egg cell (n). This is also called true fertilisation or generative fertilisation.

• The male gamete moves from the synergid towards the egg cell and fuses with its nucleus.
• This produces a Zygote (2n) - diploid, meaning it has two sets of chromosomes.
• The zygote undergoes repeated mitotic divisions and develops into the embryo.
• The embryo eventually grows into the new plant

Triple Fusion is the fusion of the second male gamete (n) with the two polar nuclei (n + n) present in the central cell of the embryo sac.

• The second male gamete migrates to the central cell of the embryo sac.
• It fuses with the two polar nuclei(or with the secondary nucleus, if the two polar nuclei have already fused together).
• Since three haploid nuclei fuse in total (1 male gamete + 2 polar nuclei), it is called Triple Fusion.
• This produces the Primary Endosperm Nucleus (PEN) — triploid (3n).
• PEN → Primary Endosperm Cell (PEC) → divides repeatedly → forms Endosperm (nutritive food tissue for the embryo)

(a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN); (b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)