Double Fertilisation

Introduction

Double fertilization is a compound mechanism for the fertilization of flowers (angiosperms). This 41 involves the union of two male gametes (sperm) with a female gamete (also called mega-gamete, or embryonic sac). Double Fertilization is defined as “A complex process in which one female gamete and two male gametes fuse for fruit formation.” 

Process of Double Fertilisation 

It starts when the pollen grains attach to the pistil’s stigma, the female flower’s reproductive structure. The pollen grains then absorb moisture and start germinating, forming pollen tubes that descend through the stem to the ovary. The spike of the pollen tube then enters the ovary and pierces through the egg’s micro pellet cavity. The pollen tube continuously releases two mega-gamete spermatozoa. The number of unfertilized egg cells is eight, and they are arranged in the form of 3 + 2 + 3 (up to down). That is three anti-leg cells, two polar enterocytes, two synergies, and one egg cell. One sperm fertilizes the egg cell, and the other sperm connects to the two polar nuclei of the large central cell of the giant gamete mother cell. Ploidy sperm and haploid egg combine to form a diploid zygote. This process is called singami, and the two haploid polar nuclei of other sperm and large central cells of mega-gametophyte form triploid nuclei (triple fusion). 

Some plants can produce polyploid nuclei. Next, large gametophyte cells grow into the endosperm. The endosperm is a nutrient-rich tissue that nourishes developing embryos. The ovules surrounding the ovules grow into fruits that can protect the seeds and potentially distribute them. The two nuclei (polar nuclei) of the parent cell that contribute to endosperm are the result of mitosis of the same single meiotic product from which the egg is derived. The maternal contribution to the genetic makeup of the triploid endosperm is twice that of the embryo.

Process of Double Fertilisation in Plants 

This is the process of fertilization in which a female gametophyte attaches with two male gametes. This process involves the pollen grain getting stuck to the carpel of female reproductive structure of a flower. This pollen grain when stuck takes the moisture and extends as a pollen tube down till ovary through the style part in a flower. The pollen tube then entends till the micropyle which extends up to opening the ovule. The pollen tube releases the two sperms in the megagametophyte.  This whole process of one female gametophyte attaching with two male gametes is termed as double fertilization. 

Double fertilisation of Angiosperms 

Double fertilization in angiosperm is a kind of process in which the reproduction takes place within the same flower. This flower acts as a reproductive part and includes male as well as female reproductive parts within it. Thus both the sperm as well as the egg both are located in the same flower. It is through the in plant pollination that the pollen grains reach through the stigma via style. In the double fertilization one egg fuses with two male sperms and thus it proves to be a complex process of fertilization.  the other with a bipolar nucleus to produce a diploid (2n) zygote and a triploid (3n) primary endosperm nucleus (PEN), respectively. 

Double Fertilisation of Gymnosperms 

A much more basic form of double fertilization occurs during the sexual reproduction of gymnosperms, commonly known as Gnetales. In particular, this event has been recorded in both Ephedra and Gnetum, which are subsets of Gnetophytes. The ventral canal nucleus is usually functionally useless in most other seed plants. In Gnetum gnemon, numerous free oocytes are present in the female cytoplasm inside the female gametophyte. 

After the mature female gamete penetrates the pollen tube, the female cytoplasm and free nucleus migrate to surround the pollen tube. Two sperm nuclei are released from the heteronuclear sperm cell and then combine with the free nucleus of the egg to form two viable zygotes, a homologous property of ephedra and centipedes. In both families, the second fertilization produces additional diploid embryos. These extra embryos are later discarded, and only one mature embryo is synthesized. The secondary fertilization product of ephedra does not nourish the primary embryo. This is because the female gamete is responsible for providing nutrients. The more primitive process of double fertilization in gymnosperms leads to the formation of two diploid nuclei surrounded by one egg. This is in contrast to the angiosperm state, where the egg and endosperm separate. Comparative molecular studies of the genome have shown that genetic plants are more closely related to conifers than to angiosperms. Rejection of the antophyte hypothesis that hominids and angiosperms are sister taxa leads to the assumption that the double fertilization process was the product of convergent evolution and occurred independently between females and angiosperms.

Double fertilization, in which triploid endosperm and diploid zygote are formed, is a distinctive feature of flowering plants. Gymnosperms (e.g. pines), non-seed organ plants (e.g. ferns), and non-organ plants (e.g. mosses) do not have this double fertilization. 

In many flowering plants, endosperm nutrients are transferred to the embryonic leaves of the developing plant called cotyledons. After germination, the leaves of the plant begin photosynthesis, providing the seedlings with their source of energy.

Conclusion 

The process of double fertilization is explained in detail in which one female gamete fuses with two male sperms to produce a fruit. The process as well as the double fertilization in angiosperm and gymnosperm is explained.  Both angiosperm and gymnosperm are centered towards the goal of reproduction but the methods as well as the process in very different in angiosperms double fertilization a flower acts as a reproductive part and includes both the male and female gametes within it while in gymnosperm it is regarded as a sexual reproduction and more so takes place within a matured plant.