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Double fertilization

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**Historical Discovery and Observations**:
– Sergei Nawaschin and Léon Guignard independently discovered double fertilization.
– Initial observations were made using Lilium martagon and Fritillaria tenella.
– Development of electron microscopy clarified aspects like the lack of cell walls in male gametes and the proximity of plasma membranes to surrounding cell membranes.

**Variations of Double Fertilization**:
– Gymnosperms like Ephedra and Gnetum exhibit rudimentary double fertilization processes.
– Examples include Ephedra nevadensis where the first sperm fertilizes the egg cell and the second fertilizes an additional egg nucleus.
– Gnetum gnemon shows two sperm nuclei joining with free egg nuclei to form zygotes, leading to the synthesis of a supernumerary embryo that is later aborted.

**In Vitro Studies and Techniques**:
– In vitro double fertilization is used to study molecular interactions in flowering plants.
– Techniques like test-tube fertilization have been developed to overcome obstacles like the confinement of sperm in pollen tubes and eggs in embryo sacs.
– Controlled fusion of egg and sperm has been achieved with plants like poppies, allowing for the observation of pollen germination and double fertilization in vitro.

**Structures and Functions Related to Double Fertilization**:
– Megagametophyte, originating from a megaspore mother cell, participates in double fertilization in angiosperms.
– It contains various cells like the egg cell, synergids, polar nuclei, and antipodal cells.
– Microgametophyte develops within pollen grains in anthers.

**Evidence, Studies, and Significance**:
– Studies have documented male nuclei migration inside female gametes and identified genes involved in the process.
– Evidence of double fertilization in Gnetales has been reported, with comparative molecular research on genomes like Gnetum gnemon.
– The rejection of the Anthophyte hypothesis suggests that double fertilization may be a product of convergent evolution.
Double fertilization is a key process in the sexual reproduction of flowering plants, ensuring the development of both the embryo and endosperm.

Double fertilization or Double fertilisation (see spelling differences) is a complex fertilization mechanism of 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 through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle opening in the ovule. The pollen tube proceeds to release the two sperm in the embryo sacs.

The parts of a flower
Double fertilization
Double fertilization in Arabidopsis

The cells of an embryo sac of an unfertilized ovule are 8 in number and arranged in the form of 3+2+3 (from top to bottom) i.e. 3 antipodal cells, 2 polar central cells, 2 synergids & 1 egg cell. 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, the process being called syngamy, while the other sperm and the two haploid polar nuclei of the large central cell of the megagametophyte form a triploid nucleus (triple fusion). Some plants may form polyploid nuclei. The large cell of the gametophyte will then develop into the endosperm, 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 sperm.

In a study conducted in 2008 of the plant Arabidopsis thaliana, 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 imaging. Some of the genes involved in the migration and fusion process have also been determined.

Evidence of double fertilization in Gnetales, which are non-flowering seed plants, has been reported.

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