Sexual Reproduction

After the emergence of eukaryotic cells, sexual reproduction was an early evolutionary advance. Its evolutionary success can be seen in the reality that almost all eukaryotes reproduce sexually. An organism that can create progeny through asexual budding, fragmentation or asexual eggs has a clear advantage. These reproduction procedures do not require the presence of another creature of the opposite sex. There’s no need to waste time looking for or attracting a mate. That energy could be put to better use by creating more offspring. Indeed, some species that live alone have kept their ability to reproduce asexually. As these asexual communities exclusively contain females, each individual is capable of reproducing, and males in sexual populations are the other half of the population who do not produce progeny. As a result, an asexual population can increase twice as rapidly as a sexual population. But this benefit of asexual reproduction is also a drawback of sexual reproduction as it will increase the number of species that reproduce asexually.

Sexual reproduction in plants

Plant reproduction is producing new progeny in plants, which can be done sexually or asexually. By fusing gametes during sexual reproduction, new plants are produced that are genetically distinct from either parent. Without the union of gametes, asexual reproduction generates new individuals. Unless mutations occur, the clonal plants are genetically identical to the parent plant. The anther is the pollen-bearing portion of the stamen, which forms the male sex organs and the female sex organs called pistils are found in the centre of the flowers. The pollen must be transferred to the stigma, a component of the pistil by pollination. Self-pollination and cross-pollination are both options for plants. When a plant’s pollen fertilises its ovules, this is known as self-pollination. Cross-pollination occurs when the wind or animals carry pollen from one plant to fertilise the ovules of another plant. The sexual parts of plants are present in the flowers.

•   The male sexual part is called the Androecium 
•   Female reproductive part is called the gynoecium.

Androecium: The male reproductive part

It consists of stamens. Stamens are the male reproductive systems of angiosperms. Stamens are made up of two parts: a filament and an anther, which work together to produce pollen through meiosis and then disperse it. Anther generates pollen, and a rod filament, which delivers nutrients and water to the anther and organises it to promote pollen dispersal. A typical angiosperm anther is bilobed, In the transverse section of an anther, the bilobed structure of the anther is extremely noticeable. The anther is a four-sided (tetragonal) structure with two microsporangia in each lobe and four microsporangia in the corners. The microsporangia continue to grow and evolve into pollen sacs. They extend the entire length of an anther and are densely packed with pollen grains. Other cell types assist with pollen maturation, protection, and release, while male sporogenous cells divide and go through meiosis within the anther to form microspores, which become pollen grains.

Pollen grains

Pollen grains are normally spherical and have 25-50 micrometres diameter. It features a two-layered wall that stands out. The exine, or hard outer coat, is formed of sporopollenin, one of the most durable organic materials known. It can tolerate high temperatures as well as strong acids and alkalis. So far, no enzyme has been discovered that can digest sporopollenin. Exine of pollen grains has large openings called germ pores, devoid of sporopollenin. Due to the presence of sporopollenin, pollen grains are preserved as fossils. A remarkable assortment of patterns and motifs may be found in the display. The intine is the innermost wall of the pollen grain. It’s a cellulose and pectin-based thin and continuous layer. A plasma membrane surrounds the cytoplasm of pollen grains. The vegetative and generative cells are found in the mature pollen grain. The vegetative cell is more significant, has a huge irregularly shaped nucleus, and a substantial food reserve. The generative cell is tiny and floats in the vegetative cell’s cytoplasm. It has thick cytoplasm and a nucleus and is spindle-shaped. Pollen grains are shed by nearly 60% of angiosperms at this 2-celled stage. Before pollen grains, the generative cell divides mitotically to produce two male gametes in the surviving species.

Gamete formation in Male plants:

The little generative nucleus and the larger vegetative nucleus separate the pollen grain into two pieces. Two male nuclei are produced by the generative cell, while the pollen tube is produced by the vegetative cell. Four pairs of fertile cells called sporogenous cells are visible when the microgametophyte is first created inside the pollen grain. The tapetum, a wall of sterile cells that gives sustenance to the cell and eventually becomes the pollen grain’s cell wall, surrounds these cells. These sporogenous cells mature into diploid microspore mother cells in the end. These microspore mother cells, also known as microsporocytes, then go through meiosis and divide into four haploid microspore cells. These newly formed microspore cells(Pollen grain) subsequently undergo mitosis, resulting in the formation of a tube cell and a generative cell. The generative cell subsequently goes through another round of mitosis to produce two male gametes, sperm.

Gynoecium: The female reproductive part

The gynoecium looks like a carpel or clump of fused carpels in the flower’s middle part. The gynoecium develops into a fruit after fertilisation, covering and nurturing the developing seeds while also assisting in dispersal. The gynoecium has a variety of specialised tissues. Carpels are a collection of pistils. One or more pistils can be found in the gynoecium. One or more carpels may be bonded together (fused). Carpels and pistils have three parts: a stigma (the sticky apex of the pistil that acts as a pollen receptor), a style, and an ovary. The stigma, style, and ovary can be made up of parts fused from numerous carpels. Plant ovaries are ovule-holding parts of the gynoecium. Rod-like features characterise the style. It is found at the bottom of the ovary, between the ovary and the stigma. Some plants have no styles in their pistils. The stigma is the pollen receptor at the apex of the carpel. Stigmas can exist independently or as a group called the “stigmatic area.” Megasporangia ovules in the ovary produce megaspores, which mature into female gametophytes after meiosis.

Only one of the megaspores is functional in most flowering plants, while the other three degenerate. Only the female gametophyte (embryo sac) develops from the functional megaspore. Monosporic development refers to producing an embryo sac from a single megaspore. They are used to make egg cells. When these egg cells come into contact with pollen, they fertilise and generate zygotes. Seeds are formed from these zygotes.

Formation of gametes in females

Angiosperms, to create female gametes go through two steps of gametogenesis:

Megasporogenesis:

The ovules are found in several lobes within the ovary. A megaspore mother cell develops from a cell in the ovule. The mother cell of a megaspore is diploid. Meiosis occurs in this megaspore mother cell, resulting in the formation of four haploid megaspores. Three megaspores degenerate, leaving each ovule with only one megaspore.

Mega-gametogenesis:

This megaspore nucleus is now beginning to divide mitotically, resulting in the formation of eight nuclei. Six nuclei migrate to opposite poles (3 each), leaving two nuclei in the middle. Polar nuclei are the nuclei that remain at the centre. The secondary nucleus is formed when these polar nuclei merge. An embryo sac develops from the megaspore. Three cells are grouped at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell.

Conclusion

Sexual reproduction occurs in nearly all eukaryotes. One of the benefits of sexual reproduction that has made it so effective appears to be the variation incorporated into reproductive cells during meiosis, in sexual life cycles, meiosis and fertilisation alternate. Meiosis creates gametes, genetically distinct reproductive cells with half the number of chromosomes as the parent cell. The union of haploid gametes from two persons, known as fertilisation, returns the diploid condition. As a result, they are sexually reproducing organisms that cycle between haploid and diploid stages. However, the methods for producing reproductive cells and the time between meiosis and fertilisation differ significantly. There are three types of life cycles: diploid-dominant (shown by most mammals), haploid-dominant (shown by all fungi and some algae), and alternation (shown by all fungi and some algae).