Asexual Reproduction

Reproduction is a fundamental biological process enabling organisms to produce new individuals similar to themselves, ensuring the continuity of species across generations.

The life span of an organism is the duration from its birth to its natural death. Life spans vary greatly among different species and are not necessarily correlated with organism size.

All individual organisms eventually die, except for single-celled organisms which do not experience natural death in the same way as multicellular ones. Reproduction is the mechanism that allows plant and animal species to persist over long periods.

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Types In Unicellular Organisms

Asexual reproduction involves a single parent producing offspring. The resulting offspring are genetically and morphologically identical to each other and to the parent organism. Such individuals are referred to as clones.

This mode of reproduction is prevalent in single-celled organisms and those with relatively simple body organisation.

In unicellular organisms like Protists and Monerans, cell division itself is the reproductive process. The parent cell divides to form new individuals through mitosis.

Examples include:

• Binary fission: The parent cell divides into two equal halves, each developing into a new adult. Seen in organisms like Amoeba and Paramecium.
• Budding: Division is unequal, producing small buds that initially remain attached to the parent cell. These buds eventually detach and mature into new individuals. Observed in organisms like Yeast.

Under unfavorable conditions, Amoeba can undergo encystation, where it withdraws its pseudopodia and forms a protective three-layered cyst around itself.

Upon the return of favorable conditions, the encysted Amoeba performs multiple fission within the cyst, producing numerous tiny amoebae (pseudopodiospores). The cyst wall then ruptures, releasing the spores to develop into new individuals. This process is known as sporulation.

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Asexual Reproductive Structures

Some organisms, particularly members of Kingdom Fungi and simple plants like algae, reproduce asexually using specialised structures.

Common examples of these structures are:

• Zoospores: Microscopic, motile structures, typically found in algae like Chlamydomonas.
• Conidia: Non-motile spores produced externally, as seen in Penicillium.
• Buds: Outgrowths on the parent body that develop into new individuals, for example, in Hydra.
• Gemmules: Internal buds containing a mass of archaeocytes within a protective coat, found in sponges.

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Vegetative Propagation In Plants

In plants, asexual reproduction is often referred to as vegetative reproduction or vegetative propagation.

This method involves specific parts of the plant body, called vegetative propagules, detaching from the parent and growing into a new individual. Since it involves a single parent and no gamete fusion, it is considered a form of asexual reproduction. Offspring produced vegetatively are also genetically identical clones.

Examples of vegetative propagules include:

• Runner: Horizontal stems growing along the surface of the soil (e.g., Grasses).
• Rhizome: Underground stems that grow horizontally (e.g., Ginger, Banana).
• Sucker: Shoots arising from the base of the stem (e.g., Mint).
• Tuber: Swollen underground stems that store food, containing 'eyes' (buds) from which new plants grow (e.g., Potato).
• Offset: Short, thick runners found in aquatic plants (e.g., Water Hyacinth, Pistia).
• Bulb: Underground structure with fleshy leaves, where a new plant grows from the central bud (e.g., Onion).

New plantlets often arise from the nodes present on modified stems (like rhizomes or tubers). These nodes possess buds and can produce roots and shoots when they come into contact with moist soil or water.

For example, in Bryophyllum, adventitious buds emerge from the notches along the margin of the leaves, which can detach and grow into new plants.

Fragmentation is another asexual method where the body of an organism breaks into pieces, and each fragment develops into a complete adult. This is seen in organisms like Hydra and some algae.

The ability of plants to propagate vegetatively is widely used by gardeners and farmers for the commercial propagation of many species.

A notable example is the Water Hyacinth (Eichhornia crassipes), often called the 'Terror of Bengal' due to its rapid vegetative spread through offsets. This invasive weed quickly covers water bodies, depleting oxygen and harming aquatic life. It was introduced to India for its aesthetic appeal but became a significant ecological problem.

While simple organisms like algae and fungi primarily use asexual reproduction, they may switch to sexual reproduction under adverse environmental conditions, as this method often provides better chances of survival.

Higher plants can exhibit both asexual (vegetative) and sexual reproduction, whereas most animals primarily rely on sexual reproduction.

Sexual Reproduction

Sexual reproduction is a more complex process compared to asexual reproduction. It typically involves the participation of two parents of opposite sexes (male and female). A key characteristic is the formation and fusion of gametes.

This fusion of male and female gametes results in the formation of a zygote, which then develops into a new organism. Because it combines genetic material from two parents, sexual reproduction leads to offspring that are not genetically identical to either parent or to each other, contributing to genetic variation.

Despite the diversity in external form and internal structure among sexually reproducing organisms (plants, animals, fungi), they share a common pattern of reproductive events, following a specific sequence.

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Pre-Fertilisation Events

These events occur before the actual fusion of gametes in sexual reproduction. The two main pre-fertilisation events are:

1. Gametogenesis: The process of forming gametes.
2. Gamete Transfer: The process of bringing the male and female gametes together.

Life Phases

All organisms must reach a certain level of growth and maturity before they can reproduce sexually. This period is called the juvenile phase.

• In plants, the juvenile phase is known as the vegetative phase.
• The length of the juvenile/vegetative phase varies widely between different species.
• The transition from juvenile/vegetative phase to the reproductive phase is often marked by distinct changes (e.g., flowering in plants).
• After the reproductive phase, organisms enter the senescent phase or old age, characterised by a decline in metabolic functions, eventually leading to death.

In annual and biennial plants, these three phases are clearly defined. However, in perennial plants, distinguishing these phases can be challenging.

Some plants exhibit unusual flowering patterns, like Bamboo which flowers only once in its lifetime (often after decades) before dying, or Strobilanthus kunthiana (Neelakuranji) which flowers once every 12 years.

In animals, the reproductive phase is also marked by physiological and morphological changes leading to reproductive activity. The duration of this phase is variable.

Mammals show cyclical changes during their reproductive phase. Non-primate mammals exhibit the oestrus cycle, while primates (monkeys, apes, humans) have the menstrual cycle.

Some mammals reproduce only during specific favourable seasons and are called seasonal breeders. Others are reproductively active throughout their reproductive phase and are known as continuous breeders.

The transitions between these life phases are regulated by hormones in both plants and animals. Hormones, in conjunction with environmental factors, control reproductive processes.

Gametogenesis

This is the process of forming haploid gametes (male and female) from diploid or haploid parent cells.

• Homogametes (Isogametes): In some algae, the male and female gametes are morphologically very similar and cannot be easily distinguished. Example: Cladophora.
• Heterogametes: In most sexually reproducing organisms, the male and female gametes are morphologically distinct. The male gamete is called the antherozoid or sperm, and the female gamete is called the egg or ovum. Examples: Fucus (an alga), Homo sapiens (humans).

Sexuality in Organisms

Organisms can be classified based on whether male and female reproductive structures are present in the same individual or different individuals.

• Bisexual organisms: Possess both male and female reproductive structures.
  • In plants, this is termed homothallic or monoecious. Flowering plants may have both male (staminate) and female (pistillate) flowers on the same plant (monoecious), e.g., Cucurbits, Coconut.
  • In animals, bisexual individuals are called hermaphrodites, e.g., Earthworm, Sponge, Tapeworm, Leech.
• Unisexual organisms: Male and female reproductive structures are in separate individuals.
  • In plants, this is termed heterothallic or dioecious. Flowering plants have male flowers on one plant and female flowers on a different plant (dioecious), e.g., Papaya, Date palm.
  • In animals, individuals are either male or female, e.g., Cockroach.

Cell Division During Gamete Formation

Gametes are always haploid (n), containing half the number of chromosomes as the parent cell in diploid organisms.

• In organisms with a haploid parent body (e.g., Monera, Fungi, Algae, Bryophytes), gametes are produced by mitotic division. Meiosis does not occur during gamete formation in these organisms but may occur in the zygote.
• In organisms with a diploid parent body (e.g., Pteridophytes, Gymnosperms, Angiosperms, most animals including humans), gametes are produced by meiotic division. Specialised diploid cells called meiocytes (gamete mother cells) undergo meiosis to form haploid gametes.

Meiosis is a reductional division that reduces the chromosome number from diploid (2n) to haploid (n). Thus, meiosis and gametogenesis are always linked in diploid organisms.

The chromosome number in gametes (n) is exactly half the chromosome number in meiocytes (2n).

Name of organism	Chromosome number in meiocyte (2n)	Chromosome number in gamete (n)
Human beings	46	23
House fly	12	6
Rat	42	21
Dog	78	39
Cat	38	19
Fruit fly	8	4
Ophioglossum (a fern)	1260	630
Apple	34	17
Rice	24	12
Maize	20	10
Potato	48	24
Butterfly	380	190
Onion	16	8

Gamete Transfer

After formation, gametes must be brought together for fusion.

• Typically, the male gamete is motile, and the female gamete is stationary (except in some fungi and algae where both are motile).
• A medium is required for the transfer of male gametes. In simple plants (algae, bryophytes, pteridophytes), water serves as the medium.
• To increase the chances of fertilisation, a large number of male gametes are produced to compensate for those that fail to reach the female gametes.

In seed plants, pollen grains contain the male gametes, and the ovule contains the egg.

• Pollen grains from the anthers must be transferred to the stigma.
• In bisexual, self-pollinating plants (e.g., Peas), transfer is easy as anthers and stigma are close.
• In cross-pollinating plants (including dioecious species), a specialised process called pollination facilitates pollen transfer to the stigma.
• After landing on the stigma, pollen grains germinate and form pollen tubes that grow towards the ovule, carrying the male gametes.

In unisexual animals, specific mechanisms (like copulation) have evolved to ensure gamete transfer and contact between individuals of opposite sexes.

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Fertilisation

Fertilisation (or syngamy) is the process of fusion of the male and female gametes, resulting in the formation of a diploid cell called the zygote.

In some organisms, like rotifers, honeybees, some lizards, and birds (e.g., turkey), the female gamete develops into a new organism without undergoing fertilisation. This phenomenon is known as parthenogenesis.

Where Syngamy Occurs

• External Fertilisation: Syngamy takes place outside the body of the organism, typically in an aquatic medium (water). Observed in most aquatic organisms like many algae, fishes, and amphibians.
  • Organisms exhibiting external fertilisation often show synchrony in gamete release between sexes.
  • They release a very large number of gametes into the water to maximise the chances of fusion.
  • Disadvantage: The offspring are vulnerable to predators and environmental threats, leading to low survival rates to adulthood.
• Internal Fertilisation: Syngamy occurs inside the body of the female organism. Found in terrestrial organisms like fungi, higher animals (reptiles, birds, mammals), and most plants (bryophytes, pteridophytes, gymnosperms, angiosperms).
  • The egg is formed and remains inside the female body.
  • The motile male gametes must reach the egg for fusion.
  • Compared to external fertilisation, the number of eggs produced is significantly lower, while the number of male gametes produced is still large.
  • In seed plants with non-motile male gametes, they are transported to the egg via pollen tubes.

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Post-Fertilisation Events

These are the events that occur after the formation of the zygote in sexual reproduction.

The Zygote

• The formation of a diploid zygote is a universal characteristic of sexual reproduction in all organisms.
• Its location depends on the type of fertilisation: external in external fertilisation (usually water), internal in internal fertilisation (inside the female body).
• The subsequent development of the zygote varies depending on the organism's life cycle and environment.
• In fungi and algae, the zygote often forms a thick, resistant wall that protects it from desiccation and damage, undergoing a period of rest before germination.
• In organisms with a haplontic life cycle, the zygote divides by meiosis to form haploid spores that develop into haploid individuals.
• In organisms with diplontic or haplo-diplontic life cycles, the zygote divides by mitosis.

The zygote is the vital link that ensures the continuity of the species from one generation to the next. Every sexually reproducing organism begins life as this single diploid cell.

Embryogenesis

Embryogenesis is the process of development of the embryo from the zygote.

This process involves two key cellular events:

1. Cell Division (Mitosis): Increases the number of cells in the developing embryo.
2. Cell Differentiation: Groups of cells undergo modifications to form specialised tissues and organs, eventually forming a complete organism.

Development in Animals

Animals are classified as oviparous or viviparous based on whether the zygote develops inside or outside the female body.

• Oviparous Animals: Zygote development occurs outside the female body. They lay fertilised (sometimes unfertilised, in parthenogenesis) eggs, often covered by a protective shell (like a hard calcareous shell in reptiles and birds). The eggs are laid in a safe environment, and young ones hatch out after incubation. Example: Reptiles, Birds.
• Viviparous Animals: Zygote development occurs inside the female body. The zygote develops into a young individual within the mother's body, and after reaching a certain growth stage, the young ones are delivered (born). Example: Majority of Mammals (including humans).

Viviparous organisms generally have a higher chance of offspring survival compared to oviparous ones due to better embryonic care and protection inside the mother's body.

Development in Flowering Plants

In flowering plants, fertilisation occurs inside the ovule.

• After fertilisation, floral parts like sepals, petals, and stamens typically wither and fall off. However, the pistil remains attached to the plant.
• The zygote develops into the embryo.
• The ovules mature into seeds.
• The ovary develops into the fruit.
• The fruit develops a protective thick wall called the pericarp.

After dispersal, seeds germinate under favourable conditions to produce new plants, carrying the embryo (progenitor of the next generation) within them.

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