Double fertilization in angiosperms

The flower as an organ of sexual reproduction

The main distinguishing feature of angiosperms is the presence of a flower, a generative organ that ensures sexual reproduction. The key reproductive structures of the flower are stamens and pistils.

• The stamen is the male part of the flower, consisting of the anther and the filament.
• The pistil is the female part, formed by the stigma, style, and ovary.

According to the presence of reproductive structures, flowers are classified as follows: bisexual flowers contain both stamens and pistils; unisexual flowers are divided into male (stamens only) and female (pistils only).

Gametophytes of angiosperms

In flowering plants, the gametophytic generations are highly reduced. The male gametophyte is the pollen grain, and the female gametophyte is the embryo sac. Their formation occurs in different parts of the flower and is termed microsporogenesis (male) and megasporogenesis (female, referred to as macrosporogenesis in the lecture).

Megasporogenesis and formation of the embryo sac

The process takes place within the ovary of the pistil, inside the ovule.

1. A diploid mother cell (2n) undergoes meiosis, forming four haploid cells (n).
2. Three megaspores degenerate, leaving one functional haploid cell.
3. This cell undergoes mitosis three times, resulting in the formation of an eight-nucleate embryo sac.
4. Two polar nuclei fuse to form the central diploid cell (2n) located in the middle of the sac.
5. Of the remaining six haploid cells, one becomes the egg cell, while the other five perform auxiliary functions (synergids and antipodal cells).

Microsporogenesis and formation of the pollen grain

Microsporogenesis takes place in the anthers of the stamens.

• The diploid mother cell undergoes meiosis, yielding four haploid microspores.
• Each microspore divides by mitosis, forming a mature pollen grain consisting of two cells: the vegetative cell and the generative cell.
• The generative cell divides mitotically once more, forming two sperm cells.

Thus, a mature pollen grain contains a vegetative cell and two sperm cells—the male gametes of angiosperms, which lack flagella.

Mechanism of double fertilization

Double fertilization was discovered by S. G. Nawaschin in 1898 and is a characteristic feature of angiosperms.

1. The pollen grain lands on the pistil stigma (pollination).
2. The vegetative cell germinates, forming a pollen tube that grows through the style toward the ovary.
3. Two sperm cells travel through the pollen tube to the embryo sac.
4. The sperm cells penetrate the interior of the ovule through the micropyle.
5. The first sperm cell (n) fuses with the egg cell (n), forming a diploid zygote (2n) from which the embryo develops.
6. The second sperm cell (n) fuses with the central diploid cell (2n), forming a triploid cell (3n) — the basis of the future endosperm.

Formation of the seed and fruit

Following double fertilization, seed development takes place:

• The plant embryo develops from the zygote.
• The endosperm—a nutritive tissue that nourishes the embryo—develops from the triploid cell.
• The seed coat develops from the integuments of the ovule.
• The walls of the pistil ovary develop into the pericarp, forming the fruit.

The biological significance of double fertilization lies in the fact that the endosperm forms simultaneously with the embryo and only upon successful fertilization, which conserves the plant's resources.

Key points

• Double fertilization is a distinguishing feature of angiosperms, discovered by S. G. Nawaschin.
• The male gametophyte is the pollen grain (vegetative cell + 2 sperm cells), and the female is the eight-nucleate embryo sac.
• Megasporogenesis takes place in the ovary of the pistil, and microsporogenesis takes place in the anthers of the stamens.
• One sperm cell (n) + egg cell (n) → zygote (2n) → embryo.
• The second sperm cell (n) + central cell (2n) → triploid cell (3n) → endosperm.
• The endosperm is triploid and performs a nutritive function for the embryo.
• The seed coat develops from the ovule integuments, and the fruit develops from the ovary walls.
• The pollen tube ensures the delivery of sperm cells to the embryo sac through the micropyle.