Double fertilization is a complex fertilization mechanism of flowering plants ( angiosperms ). This process involves the joining of a female gametophyte ( also called megagametophyte embryo sac ) with two male gametes (sperm). It begins when a pollen grain adheres to the stigma of the carpel , the female reproductive structure of a flower. The pollen grains then absorb moisture and begin to germinate , forming a pollen tube that extends down through the style to the ovary ., The tip of the pollen tube then enters the ovary and enters the ovule through the opening of the micropyle. The pollen tube proceeds to release two spermatozoa into the megagametophyte.
The cells of a fertilized ovule are 8 in number and arranged as 3+2+3 (from top to bottom), i.e. 3 antipodal cells, 2 polar central cells, 2 synapses and 1 egg cell. One sperm fertilizes the egg cell and the other sperm attaches to the two polar nuclei of the large central cell of the megagametophyte . The fusion of the haploid sperm and one haploid egg is the diploid zygote, the process being called syngamy , while the polar nucleus of the other sperm and the large central cell of the two haploid megagametophyte form a triploid nucleus ( triple fusion ). Some plants can form polyploid nuclei. large cell of gametophyte thenwill develop into the endosperm , a nutrient-rich tissue that provides nutrition to the developing embryo. The ovary , surrounding the spores, develops into the fruit, which protects the seeds and may act by dispersing them. [1]
The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm are produced by mitosis from the same single meiotic product that gave rise to the egg. The maternal contribution to the genetic formation of the triploid endosperm is twice that of the embryo. In a 2008 study of the Arabidopsis thaliana plant, the migration of male nuclei inside female gametes in fusion with female nuclei is documented for the first time using in vivo imagingSome genes involved in the migration and fusion process have also been determined. [2] Evidence of double fertilization has been reported in Gnetales , which are non-flowering seed plants.
Brief history
Double fertilization was discovered more than a century ago by Sergei Navashin in Kiev, [4] in the Russian Empire and by Leon Guignard in France. Each independently discovered the other. [5] Lilium martagon and Fritillaria tenellaThe first observations of double fertilization, which were made using the classical light microscope, were used. Due to the limitations of the light microscope, there were many unanswered questions regarding the process of double fertilization. However, with the development of the electron microscope, many questions were answered. Most notably, observations made by W. Jensen’s group showed that the male gamete did not have any cell walls and that the plasma membrane of gametes is close to the plasma membrane of the cell that surrounds them inside the pollen grain. . [6]
Double fertilization in gymnosperms
A more elementary form of double fertilization occurs in the sexual reproduction of an order of gymnosperms commonly known as Gnetales. [3] Notably, this phenomenon has been documented in both Ephedra and Gnetum , a subset of Gnetophytes. [7] In Ephedra nevadensis , a single binucleate sperm cell is deposited in the egg cell. After the initial fertilization event, the second sperm nucleus is diverted to fertilize an additional egg nucleus found in the egg’s cytoplasm. In most other seed plants, this second ‘ventral canal nucleus’ is normally found to be functionally dysfunctional. [8] in Gnetum gnemon, many free egg nuclei present in the female cytoplasm inside the female gametophyte. After the entry of the mature female gametophyte by the pollen tube, the female cytoplasm and free nucleus surround the pollen tube. The dinuclei are two sperm nuclei released from the sperm cell which then fuse with the free egg nucleus to form two viable zygotes, a homologous feature between the families Ephedra and Gnaetum .
In both families, the second fertilization event produces an additional diploid embryo. This extraterrestrial embryo is later aborted, leaving only one mature embryo to be synthesized. [10] ephedraIn additional fertilization the product does not nourish the primary embryo, as the female gametophyte is responsible for the provision of nutrients. [9] The more primitive process of double fertilization in gymnosperms results in two diploid nuclei in the same egg cell. This is different from the case of angiosperms, which result in the separation of the egg cell and endosperm. [11] G. Comparative molecular research on the genomes of gnomons has shown that gnophytes are more closely related to conifers than to angiosperms. [12] [13] [14]Rejection of the anthophyte hypothesis, which identifies gnatales and angiosperms are sister taxa, leads to speculation that the process of double fertilization is a product of convergent evolution and arose independently between gnatophytes and angiosperms. [15]
In Vitro Double Fertilization
In vitro double fertilization is often used to study molecular interactions as well as other aspects of gamete fusion in flowering plants. One of the major obstacles to developing in vitro double fertilization between male and female gametes is the binding of the sperm in the pollen tube and the egg in the embryo sac. Controlled fusion of egg and sperm with poppy seeds has already been achieved. [16] Pollen germination, pollen tube entry, and the double fertilization process have all been observed to proceed normally. In fact, this technique has already been used to obtain seeds in various flowering plants and was named “test-tube fertilization”. [17]
Related structures and functions
Megagametophyte
The female gametophyte, the megagametophyte, which participates in double fertilization in angiosperms which is haploid, is called embryo sac. It develops within an ovule, which is surrounded by the ovary at the base of a carpel. Surrounding the megagametophyte are (one or) two integuments, which are called an early micropyle. The megagametophyte, which is usually haploid, derives from the (usually diploid) megaspore mother cell, also called the megasporocyte.
The next sequence of events varies depending on the specific species, but the following events occur in most species. The megasporocyte undergoes a meiosis, forming four haploid megaspores. Of the four resulting megaspores, only one survives. This megaspore undergoes three rounds of mitotic division, resulting in eight haploid nuclei in eight cells (the central cell has two nuclei, called polar nuclei). At the lower end of the embryo sac is a haploid egg cell located between two other haploid cells, which are called synergists. Synergids function in the attraction and guidance of the pollen tube to the megagametophyte through the micropyle. At the upper end of the megagametophyte there are three exponent cells.
Microgametophyte
Male gametophytes, or microgametophytes, which participate in double fertilization, are contained within the pollen grains. They develop within the microsporangia, or pollen sacs, of anthers on the stamens. Each microsporangium contains diploid microspore mother cells or microsporocytes. Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually develop into a pollen grain. A microspore undergoes mitosis and cytokinesis to produce two separate cells, the germ cell and the tube cell. In addition to the spore wall, these two cells form an immature pollen grain.
As the male gametophyte matures, the germ cell moves to the tube cell, and the germ cell undergoes mitosis, forming two sperm cells. Once the pollen grains are mature, the anthers open, Leaves pollen. Pollen is carried by wind or animal pollinators to the pistil of another flower, and deposited on the stigma. As the pollen grain germinates, the tube cell produces the pollen tube, which expands and spreads into the long style of carpel and the ovary, where its sperm cells are released into the megagametophyte. From here double fertilization takes place.
