WHAT IS DOUBLE FERTILIZATION? (DESCRIPTION) - BIOLOGY - 2025

The double fertilization is a process that occurs in plants, whereby one of gametic nuclei of pollen grain (male gametophyte) fertilizes reproductive cell female (egg cell) and another fertile another cell, which will be different if it is an angiosperm or a gnetal.

In angiosperms the second nucleus of the gametophyte fuses with the two polar nuclei present in the central cell of the pollen sac to later develop in the endosperm. In gnetals, on the other hand, the second nucleus of the male gametophyte fuses with the nucleus of the ventral canal to produce a second embryo.

Fertilization in plants. 1) sporophyte; 2) the anther; 3) ovum; 4) ovary; 5) microspore stem cells; 6) megaspore stem cells; 7) microspore; 8) megaspores; 9) pollen grain (male gametophyte); 10) embryo sac (mature female gametophyte); 11) seed; 12) endosperm (3n); 13) mature seed; I) diploid (2n); II) haploid (n); III) meiosis; IV) Mitosis; V) Pollination; VI) double fertilization; Taken and edited from: TheLAW14.

Botanists originally believed that double fertilization was an exclusive phenomenon of angiosperms, however the process was also later described for plants of the gnetal group.

History

The botanist of Russian origin Sergey Gavrilovich Nawashin working with angiosperms of the Lilium martagon and Fritillaria tenella species was the first to observe the double fertilization process. This phenomenon was later demonstrated by the famous Polish-German botanist Eduard Strasburger.

After this discovery, various botanists reported abnormal double fertilization events in different groups of gymnosperms (Thuja, Abies, Pseudotsuga, among others). The products of the second fertilization could degenerate, produce free nuclei or also give rise to additional embryos.

Later it was shown that double fertilization was a normal event in plants of the gnetal group, but in these, unlike in angiosperms, the second fertilization always produces additional embryos and not endosperms.

Description

In angiosperms

In most angiosperms the precursor cell of the megaspores (megasporocyte) produces four haploid megaspores by meiosis, of which only one develops to produce a megagametophyte, while the others degenerate.

The megagametophyte produces eight nuclei, of which two (polar nuclei) migrate towards the central zone of the megagametophyte or embryo sac, thus giving rise to a binucleate cell.

The rest of the nuclei are arranged in groups of three on the periphery, one of which will form the ovocell, the two adjacent ones will form the synergies, while the remaining three located at the opposite end will form the antipodes.

For its part, the male gametophyte (pollen grain) produces three nuclei; two spermatic and one vegetative. When the pollen grain comes into contact with the stigma, it germinates and produces a pollen tube which grows through the style, attracted by substances produced by the synergists.

The two sperm nuclei then migrate through the pollen tube to perform double fertilization. One of the sperm nuclei will fuse to the nucleus of the egg cell and will form a zygote that will give rise to the embryo, while the other will fuse with the two nuclei of the central cell giving rise to the endosperm.

The endosperm

The endosperm is a triploid tissue formed from the fusion of the central cell of the embryo sac (binucleate) with one of the two sperm nuclei of the pollen grain. Endosperm cells are rich in starch granules embedded in a protein matrix and their function is to supply nutritional material to the developing embryo.

There is no consensus among scientists about the evolutionary origin of the endosperm in angiosperms. Some authors maintain that it is an extra embryo whose development was transformed into nutritional tissue for the benefit of the other embryo.

Other authors argue that the endosperm is not the product of sexual reproduction, but rather is a vegetative phase of the development of the embryo sac, as occurs in gymnosperms. Both hypotheses have their detractors and defenders.

In Gnetales

The only gymnosperm plants in which there is proven double fertilization belong to the genera Gnetum and Ephedra (Gnatales). However, in none of them does the endosperm originate as a result of this double fertilization.

Ephedra

In at least two species of Ephedra the female gametes form in archegonia within monosporic gametophytes. The spermatic tube for its part contains four nuclei, two of which are reproductive.

These sperm nuclei are released in the megagametophyte and one of them will fuse with the nucleus of the egg cell, while the other will fuse with the nucleus of the ventral canal.

The result is two zygotes that will develop into genetically identical viable embryos, because the two female nuclei were formed within a monosporic gametophyte, while the two sperm nuclei from the same pollen tube are also genetically identical.

Two or more archegonia can form in each female gametophyte, resulting in multiple double fertilization events occurring simultaneously within a gametophyte.

Gnetum

The double fertilization in Gnetum presents important differences when compared to the double fertilization that occurs in Ephedra. In Gnetum, unlike in Ephedra, the female gametophytes are tetrasporic and not monosporic.

Another difference is that in Gnetum neither archegonia nor predestined ovocells are formed. Due to the loss of oocells, numerous female gametophytic nuclei are fertilizable; in this way, the two sperm nuclei from a pollen tube can fertilize any two female nuclei.

As in Ephedra, the double fertilization process in Gnetum will produce two viable zygotes, but in this the zygotes will not be genetically identical due to the tetrasporic nature of the female gametophyte. In Gnetum, multiple double fertilization events can also occur, if there are enough pollen grains.

Gnetal plant Ephedra viridis. Taken and edited from: Dcrjsr.

Double fertilization and evolution in plants

The discovery of double fertilization in Gnetum and Ephedra supports the evolutionary hypothesis according to which this process originated in a common ancestor of Gnetales and angiosperms, which is why it would be a synapomorphy (shared derivative character) that would allow them to be grouped in the clade of anthophytes. (monophyletic).

Synapomorphies are derived characters that are shared by two or more species or taxa and that therefore may show some degree of kinship. The ancestral character (plesiomorphic) in that case would be simple fertilization.

In this way, the Gnetales could be a basal group within the clade of anthophytes in which double fertilization gives rise to two viable zygotes, while the appearance of the endosperm as a product of double fertilization would be a unique synapomorphy within angiosperms..

References

1. Endosperm. On Wikipedia. Recovered from en.wikipedia.org.
2. Double fecundation. On Wikipedia. Recovered from en.wikipedia.org.
3. C. Lyre. What is a monophyletic group? Recovered from lifeder.com.
4. MW Nabors (2004). Introduction to Botany. Pearson Education, Inc.
5. JS Carmichael & WE Friedman (1996). Double fertilization in Gnetum gnemon (Gnetaceae): Its bearing on the evolution of sexual reproduction within the Gnetales and the Anthophyte clade. American Journal of Botany.
6. WE Friedman (1990). Sexual reproduction in Ephedra nevadensis (Ephedraceae): further evidence of double fertilization in a non-flowering seed plant. American Journal of Botany.
7. Unit 24: Fertilization and embryogenesis. 24.3. Angiospermae. In Morphology of vascular plants. Recovered from biologia.edu.ar.