What Is Double Fertilization?

What Is Double Fertilization?

Flowering plants also known as angiosperms, use an intricate and complex fertilisation method called double fertilisation. A female gametophyte, also known as the embryo sac, is joined to two male gametes in this procedure. It all starts when a pollen grain sticks to the stigma of the carpel, a flower’s female reproductive component. The pollen grain absorbs moisture and germinates, generating a pollen tube that continues down through the style toward the ovary. The pollen tube’s tip subsequently enters the ovary and passes through the ovule’s micropyle opening. Subsequently, the two sperm in the megagametophyte are released by the pollen tube.

An unfertilized ovule has eight cells that are arranged in a particular pattern from top to bottom, that is three antipodal cells, two polar central cells, two synergids, and one egg cell. One sperm fertilises the egg cell, while the other sperm joins the two polar nuclei of the megagametophyte’s huge central cell. Syngamy is the process by which haploid sperm and haploid eggs join to produce a diploid zygote, whereas the other sperm and the two haploid polar nuclei of the megagametophyte’s huge central cell join to make a triploid nucleus, also known as triple fusion. Polyploid nuclei can occur in some plants. The gametophyte’s big cell will eventually turn into endosperm, a nutrient-rich tissue that will sustain the growing embryo. The ovary, which surrounds the ovules, develops into the fruit, which protects and possibly disperses the seeds. Mitosis produces the two maternal nuclei that contribute to endosperm from the same single meiotic product that produced the egg. The contribution of the mother to the genetic makeup of triploid endosperm is double that of the embryo. The migration of male nuclei into the female gamete, in fusion with the female nuclei, was observed for the first time utilising vivo imaging in a 2008 research of the plant Arabidopsis thaliana. Some of the genes involved in migration and fusion have been identified as well. There has been evidence of multiple fertilisation in Gnetales, which are non-flowering seed plants.

Gymnosperms with double fertilisation:

In the sexual reproduction of an order of gymnosperms known as Gnetales, a significantly more primitive form of double fertilisation occurs. This occurrence has been recorded in Ephedra and Gnetum, a subgroup of Gnetophytes. A single binucleate sperm cell is placed into the egg cell in Ephedra nevadensis. The second sperm nucleus is directed to fertilise an extra egg nucleus located in the egg cytoplasm after the initial fertilisation event. This second ventral canal nucleus is usually found to be functionally worthless in most other seed plants. In the female gametophyte of Gnetumgnemon, there are several free egg nuclei in the female cytoplasm. Female cytoplasm and free nuclei travel to surround the pollen tube once the pollen tube penetrates the adult female gametophyte. Two sperm nuclei are released from the binucleate sperm cell, which combine with free egg nuclei to generate two viable zygotes, a feature shared by the Ephedra and Gnetum families. The second fertilisation event produces an extra diploid embryo in both families. This extra embryo is later terminated, leaving only one mature embryo to be produced. Because the female gametophyte is responsible for nutritional provision, the extra fertilisation result in Ephedra does not nourish the primary embryo. In gymnosperms, the more primitive mechanism of double fertilisation results in two diploid nuclei contained within the same egg cell. The angiosperm situation, on the other hand, results in the separation of the egg cell and endosperm. Gnetophytes are more closely related to conifers than angiosperms, according to comparative molecular research on the genome of gnemon. The anthophyte hypothesis, which claims that gnetales and angiosperms are sister taxa, has been debunked, leading to suspicion that double fertilisation is a product of convergent evolution that emerged independently among gnetophytes and angiosperms.

Brief History:

Sergei Nawaschin in Kyiv, Russian Empire, and Léon Guignard in France discovered double fertilisation more than a century ago. The finding was made by each of them independently of the other. The first observations of double fertilisation were done with the traditional light microscope on Lilium martagon and Fritillaria tenella. Due to the limits of the light microscope, many concerns about the mechanism of double fertilisation remained unresolved. Many of these questions were solved, however, with the introduction of the electron microscope. The observations of W. Jensen’s group revealed that male gametes do not have cell walls and that the gamete’s plasma membrane is close to the plasma membrane of the cell that surrounds them inside the pollen grain.

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