Breeding of Vegetable, Spice and Tuber Crops

I.Sexual reproduction

• Multiplication of plants through embryos which have been developed by fusion of male and female gametes is known as sexual reproduction. All the seed propagating species belong to this group.

Sporogenesis

• Production of microspores and megaspores is known as sporogenesis.In anthers, microspores are formed through microsporogensis and in ovules, the megaspores are formed through megasporogenesis.

Microsporogenesis

• The sporophytic cells in the pollen sacs of anther which undergo meiotic division to form haploid microspores are called microspore mother cells (MMC) or pollen mother cell (PMC) and the process is called microsporogenesis. Each PMC produces four microspores and each microspore after thickening of the wall transforms into pollen grain.

Megasporogenesis

• A single sporophytic cell inside the ovule, which undergoes meiotic division to form haploid megaspore, is called megaspore mother cell (MMC) and the process is called megasporogenesis. Each MMC produces four megaspores out of which three degenerate resulting in a single functional megaspore.

Gametogenesis

• The production of male and female gametes in the microspores and megaspores is known as gametogenesis.

Microgametogenesis

• Microgametogenesis is the production of male gametes or sperm. On maturation of the pollen, the microspore nucleus divides mitotically to produce a generative and a vegetative or tube nucleus. The pollen is generally released in this binucleate stage.The reach of pollen over the stigma is called pollination.
  


• After the pollination, the pollen germinates. The pollen tube enters the stigma and travels down the style. The generative nucleus at this phase undergoes another mitotic division to produce two male gametes or sperm nuclei. The pollen along with the pollen tube possessing a pair of sperm nuclei is called microgametophyte. The pollen tube enters the embryo sac through micropyle and discharges the two sperm nuclei.

Megagametogenesis

• The nucleus of the functional megaspore undergoes three mitotic divisions to produce eight or more nuclei.The exact number of nuclei and their arrangement varies from one species to another. The megaspore nucleus divides thrice to produce eight nuclei. Three of these nuclei move to one pole and produce a central egg cell and two synergid cells on either side. Another three nuclei migrate to the opposite pole to develop into three antipodal cells.


• The two nuclei remaining in the centre, the polar nuclei, fuse to form the secondary nucleus. The megaspore thus develops into a mature female gametophyte called megagametophyte or embryo sac. The development of embryo sac from a megaspore is known as megagametogeneis. The embryo sac generally contains one egg cell, two synergids with the apparent function of guiding the sperm nucleus towards the egg cell and three antipodals, which form the prothalamus cells and one diploid secondary nucleus.

Fertilization

• The fusion of one of the two sperms with the egg cell producing a diploid zygote is known as fertilization. The fusion of the remaining sperm with the secondary nucleus leading to the formation of a triploid primary endosperm nucleus is termed as triple fusion. The primary endosperm nucleus after several mitotic divisions develops into mature endosperm (consists of triploid cells) nourishes the developing embryo which is a diploid.
 

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