The Need for Reproduction and Asexual Modes

Introduction: Why Does Life Continue?

Look around you — a sapling sprouting near the school gate, a sparrow guarding her tiny eggs in a nest, a stray kitten following its mother. Every living thing you see was once produced by another living thing of its own kind. No frog has ever come from a stone, no rose has ever appeared from a piece of glass. Living organisms do something that non-living things can never do — they make new organisms similar to themselves. This biological process is called reproduction.

An individual lizard, mango tree or human being will eventually grow old, weaken and die. If no new lizards, mango trees or humans were being produced, every species would disappear from the Earth in a single generation. So the importance of reproduction is not for the parent — it is for the species as a whole. In this chapter we will explore how organisms reproduce, how variations slip into offspring, and why this small leak of variation has shaped the entire history of life.

11.1 Why Is Reproduction Important?

Reproduction matters for two interlinked reasons — the survival of the species, and the gradual change of life forms over long periods of time.

(a) Continuity of the species

Every individual has a fixed lifespan. A house fly may live a few weeks, a dog about ten to fifteen years, a peepal tree a few centuries. Reproduction creates fresh individuals to replace those that die, so the population never falls to zero. Without reproduction, even the strongest, most well-adapted species would vanish in one generation.

(b) Source of variation and evolution

When parents produce offspring, the offspring are similar to the parents but rarely identical. Small differences — slightly different beak length, slightly thicker fur, slightly more salt-tolerant cells — appear among them. These differences are called variations. When the environment changes — a new disease appears, the climate cools, food becomes scarce — some variants survive better than others. Over many generations this leads to evolution and to new species.

(c) DNA copying — the heart of reproduction

Inside every cell of an organism lies a long molecule called DNA (deoxyribonucleic acid). DNA carries the instructions for building the body and running its life processes. Before a cell divides, it must first make a copy of its DNA so that each daughter cell receives a full set of instructions. This DNA copying is the basic event of reproduction.

The copying machinery is amazingly accurate, but it is not perfect. Once in a while a wrong "letter" gets inserted, deleted or substituted. Most of these mistakes are harmless or even harmful, but a few may turn out to be useful. These small accidental changes are the raw material of variation. So variation is not extra — it is built into the very process of reproduction.

11.2 Two Broad Modes of Reproduction

Across the living world, reproduction occurs in two broad ways:

• Asexual reproduction — a single parent gives rise to offspring without the involvement of gametes or fertilization. The offspring are genetically nearly identical copies of the parent (clones).
• Sexual reproduction — two parents contribute special reproductive cells (gametes) that fuse to form a new individual. The offspring carry a mixed set of genes from both parents.

This part focuses on asexual modes; sexual reproduction is taken up in Part 2.

11.3 Modes of Asexual Reproduction

Asexual reproduction is common among bacteria, single-celled organisms, simple animals, fungi and many plants. It is fast, requires no partner and is energy-efficient — but the offspring are very similar to the parent and to each other, so the population is more vulnerable to a sudden change in the environment.

A. Binary Fission (Amoeba, bacteria)

In binary fission, the parent splits into two equal halves. In an Amoeba the nucleus divides first; the cytoplasm then constricts in the middle and pinches off, producing two daughter Amoebae. The plane of division can be in any direction. Bacteria, paramecium and many other unicellular organisms reproduce this way.

B. Multiple Fission (Plasmodium)

In multiple fission, the parent cell divides simultaneously into many daughter cells. The malarial parasite Plasmodium uses multiple fission inside human red blood cells. The single Plasmodium cell first develops many nuclei without dividing; then the cytoplasm splits around each nucleus to release a swarm of daughter cells at once. This strategy is useful when the organism faces unfavourable conditions — many copies are produced quickly, and at least some are likely to survive.

C. Fragmentation (Spirogyra)

The filamentous green alga Spirogyra simply breaks into smaller pieces when it grows. Each fragment then grows into a new filament. This kind of asexual reproduction by accidental breakage is called fragmentation. It works well only in simple, multicellular organisms that have similar cells along their length.

D. Regeneration (Hydra, Planaria)

If a Hydra or a flatworm Planaria is cut into pieces, each piece can grow back the missing parts and become a complete organism. This ability is called regeneration. Specialised cells in the cut piece divide rapidly and reorganise themselves into the right organs in the right places. Regeneration, however, is not the usual way these animals reproduce in nature — they normally reproduce by other methods such as budding.

E. Budding (yeast, Hydra)

In budding, a small outgrowth called a bud develops on the body of the parent. The bud grows in size and finally detaches as a new individual. In the single-celled fungus yeast, a small bulge appears on the cell, the nucleus divides, one nucleus enters the bulge, and the bud separates. In Hydra, several buds may grow simultaneously from the body of the parent before falling off.

F. Spore Formation (Rhizopus)

The bread mould Rhizopus grows as fine threads (hyphae) on damp bread. Some hyphae grow upward and produce tiny knob-like sporangia at their tips. Inside each sporangium, hundreds of microscopic spores are formed. When the sporangium bursts, the spores are released into the air. Each spore that lands on a moist surface germinates into a new mould colony. Spores have tough protective walls so they can survive heat, cold and dryness for long periods — a great advantage in the unpredictable environments fungi live in.

G. Vegetative Propagation

Many flowering plants can produce new plants from their vegetative parts — roots, stems or leaves — without using seeds. This kind of asexual reproduction is called vegetative propagation. Important examples are:

• Potato eyes (stem): The "eyes" on a potato are tiny buds on the underground stem (tuber). Each eye can sprout into a new potato plant.
• Bryophyllum leaves: The notches along the margin of a Bryophyllum leaf carry small adventitious buds. Placed on moist soil, each bud grows into a new plantlet.
• Sugarcane stem cuttings: A piece of sugarcane stem with at least one node, planted in soil, produces roots and shoots and grows into a new plant.
• Rose, hibiscus, money plant — stem cuttings; ginger and turmeric — underground rhizomes; onion — bulbs.

Vegetative propagation is widely used by farmers and gardeners because: (i) the new plants are genetically identical to the parent, so a desirable variety can be exactly preserved; (ii) plants like banana that do not produce viable seeds can still be grown; (iii) flowering and fruiting begin earlier than in seed-grown plants.

11.4 Activity — Growing a Plant from a Leaf

Quick Recap