Asexual Reproduction

Reproduction and Life Span

Every living organism exists for a finite period between its birth and its natural death — this is called its life span. Life spans vary enormously across the living world: some organisms such as certain insects live only a few days, while long-lived trees can survive for thousands of years. Interestingly, life span is not simply linked to body size — two birds of similar size, such as a crow and a parrot, can have very different life spans, and a mango tree lives for a much shorter time than a peepal tree. Figure 1.1 presents an activity grid of common organisms — an elephant, rose, dog, butterfly, crow, banana tree, cow, parrot, crocodile, horse, fruit fly, rice plant, tortoise and banyan tree — with some life spans given (a butterfly, 1–2 weeks; a crow, about 15 years; a parrot, up to 140 years; a crocodile, about 60 years; a tortoise, 100–150 years) and space left for the reader to fill in the rest.

Whatever an organism's life span, death is inevitable for every individual — except, notably, single-celled organisms, which do not undergo a natural death in the same sense because the parent cell simply divides to become the next generation. Despite this individual mortality, life on Earth has continued unbroken because organisms reproduce: reproduction is the biological process by which an organism produces offspring similar to itself. The offspring in turn grow, mature and reproduce, creating an unending cycle of birth, growth and death that keeps a species going generation after generation.

A short profile box in the chapter honours the botanist Panchanan Maheshwari, who built up the Department of Botany at the University of Delhi into a major centre for plant embryology and tissue-culture research and led the writing of the first NCERT Higher Secondary Biology textbooks.

Two Basic Patterns of Reproduction

Depending on how many parents are involved, reproduction is grouped into two broad types:

• Asexual reproduction — a single parent produces offspring, with or without the formation of gametes.
• Sexual reproduction — two parents of opposite sex participate, and the process involves the fusion of male and female gametes.

Asexual Reproduction

In asexual reproduction a single parent alone gives rise to offspring. Because no gamete fusion (and hence no mixing of genetic material from two sources) is involved, the offspring are genetically and morphologically identical to the parent and to one another — such a group of identical individuals is called a clone.

Asexual reproduction is the dominant mode in single-celled organisms and in plants and animals with a relatively simple body organisation. In many Protists and Monerans, the parent cell itself divides by mitosis to directly form two new individuals, so cell division is the act of reproduction. Figure 1.2 illustrates two such mechanisms: budding in yeast, where an unequal division produces a small bud attached to the parent cell that eventually separates and matures into a new yeast cell, and binary fission in Amoeba, where a cell divides into two equal-sized daughter cells that each grow into an adult.

Some organisms have additional strategies for surviving harsh conditions. Under unfavourable conditions, Amoeba withdraws its pseudopodia and secretes a hard, three-layered protective covering around itself — a process called encystation. When conditions improve, the encysted Amoeba undergoes multiple fission inside the cyst to produce many tiny amoebae; the cyst wall then ruptures and releases them into the surroundings, a process known as sporulation.

Asexual Reproductive Structures

Fungi and simple plants such as algae reproduce using specialised asexual structures, shown in Figure 1.3: microscopic, motile zoospores (as in Chlamydomonas), conidia (spore-bearing structures produced at the tips of branching stalks in Penicillium), buds that grow out from the body wall and develop tentacles of their own (as in Hydra), and gemmules — internal packets of cells surrounded by a spicule-studded wall (as in sponges).

Vegetative Propagation

Plants often reproduce asexually through structures that develop from their vegetative (non-reproductive) parts — this is called vegetative propagation, and the structures involved (runners, rhizomes, suckers, tubers, offsets, bulbs) are called vegetative propagules, shown in Figure 1.4. Familiar examples include the "eyes" (notch-like buds) of a potato tuber that sprout into new plants, the rhizome of ginger that produces buds and adventitious roots at its nodes, the bulbil of Agave, the adventitious buds that arise at the notches along a Bryophyllum leaf margin, and the runner-borne offset of water hyacinth. In each case new plantlets typically arise at the nodes of modified stems once these come into contact with damp soil or water — a fact gardeners and farmers exploit for the commercial propagation of crops such as potato, sugarcane, banana, ginger and dahlia. Because no second parent is involved, vegetative propagation is a form of asexual reproduction, and its offspring are also clones of the parent plant. The invasive weed water hyacinth is a striking example of how effective vegetative propagation can be: it spreads across a water body at a phenomenal rate, choking oxygen from the water and threatening fish and other aquatic life.

Fragmentation

In some simple animals, if the body accidentally or naturally breaks into pieces, each fragment is capable of growing into a complete new adult — this is called fragmentation, seen for example in Hydra.

It is worth noting that asexual reproduction is characteristic of organisms with a simple body plan, such as algae and fungi, which often switch to sexual reproduction just before conditions turn unfavourable, because sexual reproduction generates genetic variation that improves a population's chances of surviving change. Higher plants commonly show both vegetative (asexual) and sexual reproduction, whereas most animals reproduce only sexually.