Double fertilization is a complex fertilization mechanism that has evolved in flowering plants (angiosperms). This process involves the joining of a female gametophyte (megagametophyte, also called the 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 grain then takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary

Ovary (plants)

In the flowering plants, an ovary is a part of the female reproductive organ of the flower or gynoecium. Specifically, it is the part of the pistil which holds the ovule and is located above or below or at the point of connection with the base of the petals and sepals...

 through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle

Micropyle

A micropyle is small opening in the surface of an ovule, through which the pollen tube penetrates, often visible as a small pore in the ripe seed....

 opening in the ovule. The pollen tube proceeds to release the two sperm in the megagametophyte.

One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the megagametophyte. The haploid sperm and haploid egg combine to form a diploid zygote, while the other sperm and the two haploid polar nuclei of the large central cell of the megagametophyte form a triploid nucleus (some plants may form polyploid nuclei). The large cell of the gametophyte will then develop into the endosperm

Endosperm

Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...

, a nutrient-rich tissue which provides nourishment to the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which protects the seeds and may function to disperse them.

The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm, arise by mitosis from the same single meiotic product that gave rise to the egg. The maternal contribution to the genetic constitution of the triploid endosperm is double that of the embryo.

In a recent study done of the plant Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

, the migration of male nuclei inside the female gamete, in fusion with the female nuclei, has been documented for the first time using in vivo

In vivo

In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...

 imaging. Some of the genes involved in the migration and fusion process have also been determined.

Brief history

Double fertilization was discovered more than a century ago by Sergius Nawaschin in St. Petersburg, Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

, and Léon Guignard in France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

. Each made the discovery independently of the other. Lilium martagon

Lilium martagon

Lilium martagon is a species of lily. It has a widespread native region extending from central Europe east through northern Asia to Mongolia and Korea. Several subspecies have been named. Horticulturally it is in Division 9, It is stem-rooting, growing between 1m and 2m tall...

and Fritillaria tenella were used in the first observations of double fertilization, which were made using the classical light microscope. 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

Electron microscope

An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

, many of the questions were answered. Most notably, the observations made by the group of W. Jensen showed that the male gametes did not have any cell walls and that the plasma membrane of the gametes is close to the plasma membrane of the cell that surrounds them inside the pollen grain.

In vitro double fertilization

In vitro double fertilization is often used to study the molecular interactions as well as other aspects of gamete fusion in flowering plants. One of the major obstacles in developing an in vitro double fertilization between male and female gametes is the confinement of the sperm in the pollen tube and the egg in the embryo sac. A controlled fusion of the egg and sperm has already been achieved with poppy plants. Pollen germination, pollen tube entry, and double fertilization processes 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”.

Megagametophyte

The female gametophyte

Gametophyte

A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

, or megagametophyte, used in double fertilization is the embryo sac. This develops within an ovule

Ovule

Ovule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...

, enclosed by the ovary at the base of a carpel. Surrounding the embryo sac are two integuments, which form an opening called the micropyle. The embryo sac, which is primarily haploid, originates from the diploid megaspore

Megaspore

Megaspores, also called macrospores, are a type of spore that is present in heterosporous plants. These types of plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female gametophyte, which is fertilized by sperm produced...

 mother cell within the ovule. The megasporocyte undergoes a meiotic cell division, producing four haploid megaspores. The next sequence of events varies, depending on the particular species. However, in most angiosperm species, only one of the four resulting megaspores survives. This megaspore

Megaspore

Megaspores, also called macrospores, are a type of spore that is present in heterosporous plants. These types of plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female gametophyte, which is fertilized by sperm produced...

 undergoes three rounds of mitotic division, resulting in one large cell with eight haploid nuclei. Membranes then divide this mass into a muticellular female gametophyte

Gametophyte

A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

, called the embryo sac. The lower end of the embryo sac consists of the haploid egg cell positioned in the middle of two other haploid cells, called synergids. The synergids function in the attraction and guidance of the pollen tube to the embryo sac through the micropyle. At the upper end of the embryo sac are three antipodal cells. The remaining two nuclei, called polar nuclei, share the cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 of the large central cell of the embryo sac, rather than being partitioned into separate cells.

Microgametophyte

The male gametophyte

Gametophyte

A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

s, or microgametophytes, used in double fertilization are pollen

Pollen

Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...

 grains. They develop within the microsporangia, or pollen sacs, of anthers at the tips of the stamens. Each microsporangia contains diploid microspore

Microspore

In biology, a microspore is a small spore as contrasted to the larger megaspore.In botany, microspores develop into male gametophytes, whereas megaspores develop into female gametophytes. The combination of megaspores and microspores is found only in heterosporous organisms...

 mother cells, or microsporocytes. Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually develop into a pollen grain. A microspore

Microspore

In biology, a microspore is a small spore as contrasted to the larger megaspore.In botany, microspores develop into male gametophytes, whereas megaspores develop into female gametophytes. The combination of megaspores and microspores is found only in heterosporous organisms...

 undergoes mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

 and cytokinesis in order to produce two separate cells, the generative cell and the tube cell. These two cells in addition to the spore wall make up an immature pollen grain. As the male gametophyte matures, the generative cell passes into the tube cell, and the tube cell produces the pollen tube. Once the pollen grain has matured, the anthers break open, releasing the pollen. The pollen is carried to the stigma of another flower, by wind or animal pollinators, and deposited on the pistil. As the pollen grain germinates, the pollen tube elongates, and the generative cell undergoes mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

, producing two sperm cells. The pollen tube extends down the long style of the carpel and into the ovary, where its sperm cells are released near the embryo sac. Double fertilization proceeds from here.