Asexual reproduction

One parent is enough

In asexual reproduction, a single parent makes new individuals on its own. No sex cells, or gametes, are made, and no partner is needed. The young are very like the parent, apart from small variations.

It is fast and simple, so many small organisms and plants use it. Here are the main ways.

Fission

A single cell divides into new cells. This is fission.

• Binary fission. The cell splits into two daughter cells. Amoeba can split in any plane. Leishmania, which causes kala-azar, has a whip-like part at one end and always splits along a fixed plane.
• Multiple fission. The cell splits into many daughter cells at once. The malaria parasite, Plasmodium, does this.

In binary fission the nucleus divides first, then the cytoplasm pinches in, and two cells move apart. Each daughter can grow and divide again. So the number doubles with each round.

Fragmentation and regeneration

• Fragmentation. A filament of the green alga Spirogyra simply breaks into pieces. Each piece grows into a new filament.
• Regeneration. If Planaria, a flatworm, is cut into pieces, each piece can grow into a whole worm. Special cells divide and make the missing parts. Hydra can do this too.

Most animals cannot regenerate a whole body, so regeneration is not their usual way to reproduce.

Budding

In *Hydra*, a small bud grows out from the side of the body, where cells divide again and again. The bud grows tentacles. When it is ready, it breaks away and lives as a new Hydra.

Spore formation

Bread mould, *Rhizopus*, grows threads called hyphae over the bread. Upright threads end in round cases called sporangia. Each sporangium holds many tiny spores.

Spores have thick walls that protect them until they land on a moist surface. Then each spore can grow into a new mould. Most spores never find food, so the mould makes very many.

Vegetative propagation

Many plants grow new plants from a root, stem or leaf, with no seeds.

• Natural examples. The leaf of Bryophyllum grows small buds along its edges. A potato tuber sprouts from its eyes, which are buds.
• By people. Sugarcane, roses and grapes are grown from cuttings of stem. Gardeners also use layering and grafting.

It has real uses:

• the plant bears flowers and fruits sooner than one grown from seed;
• plants that have lost the power to make seeds, such as banana, orange, rose and jasmine, can still be grown;
• all the new plants are like the parent, so a good variety keeps its qualities.

Try it in Python

The program follows one Amoeba that divides every 40 minutes, hour by hour.

Practise with new numbers

Common mistakes

• "Fission makes one new cell." In binary fission one cell becomes two new cells, and the parent no longer exists as itself.
• "Fragmentation and regeneration are the same." In fragmentation the body breaks up on its own. Regeneration is making a whole body from a cut piece, using special cells.
• "A bud is a seed." A bud of Hydra is a growing outgrowth of the body. Seeds come from sexual reproduction.
• "Spores are seeds of the mould." Spores are single cells made without sex, protected by a thick wall.
• "Vegetative propagation gives new varieties." It gives copies of the parent. New varieties come from variation.

Remember these

• Asexual reproduction needs one parent and no gametes. The young are very like the parent.
• Fission: Amoeba and Leishmania split into two, Plasmodium into many.
• Spirogyra fragments, Planaria regenerates, Hydra buds, and Rhizopus makes spores in sporangia.
• Vegetative propagation grows plants from roots, stems or leaves. It is quick and keeps a variety true.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How does the binary fission of Leishmania differ from that of Amoeba?

2. A thread of Spirogyra breaks into four pieces in a pond. How many new filaments can grow, and what is this called?

3. Why does Rhizopus make so many spores?

4. Banana plants make no seeds. How do farmers grow new ones?

5. Give two benefits of growing roses from cuttings rather than from seeds.