Reproduction in Unicellular Organisms and Invertebrates – SS1 Biology Lesson Note (Third Term)

This topic describes how Amoeba, Paramecium, Spirogyra, earthworms, cockroaches, houseflies and snails reproduce. Amoeba reproduces asexually, while Paramecium and Spirogyra reproduce both asexually and sexually (by conjugation). Earthworms, cockroaches, houseflies and snails reproduce sexually. The cockroach shows incomplete metamorphosis and the housefly shows complete metamorphosis.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe binary fission and multiple fission in Amoeba;
• describe binary fission and the stages of conjugation in Paramecium;
• explain fragmentation and conjugation in Spirogyra;
• describe copulation and cocoon formation in earthworms;
• outline the life cycles of the cockroach and the housefly and state the type of metamorphosis in each;
• state how reproduction takes place in land snails.

How does Amoeba reproduce?

Amoeba reproduces asexually by binary fission and multiple fission (sporulation) during adverse conditions.

Binary fission in Amoeba

When an amoeba reaches full size, it stops moving and divides into two equal parts, starting from the nucleus. This is followed by the division of the cytoplasm, after which two daughter amoebae are formed.

Multiple fission in Amoeba

The amoeba becomes rounded and secretes a cyst around itself. Inside the cyst, the nucleus divides several times. When conditions become favourable, the cyst bursts and each nucleus is surrounded by a part of the cytoplasm of the parent. In this way, very small amoebae are formed.

How does Paramecium reproduce?

Paramecium reproduces asexually by binary fission and sexually by conjugation.

Binary fission in Paramecium

Binary fission occurs under favourable conditions. The micronucleus divides into two halves by mitosis and each half moves to the opposite side of the cell. The meganucleus elongates and the cytoplasm constricts, after which two young paramecia are produced.

Conjugation in Paramecium

Sexual reproduction is by conjugation of two individuals of different lines of descent. The stages in conjugation are:

1. Two mature paramecia come together and fuse by their oral grooves. The micronucleus divides twice by meiosis and four nuclei are formed in each conjugant.
2. The smaller micronuclei are exchanged between the two conjugants. The migratory micronucleus fuses with the stationary micronucleus in each conjugant to form a zygote.
3. The zygote in each conjugant divides thrice to form eight nuclei. The ex-conjugant with four meganuclei and four micronuclei divides to form four paramecia, each having a meganucleus and a micronucleus.

Conjugation in Paramecium

How does Spirogyra reproduce?

Spirogyra reproduces asexually by fragmentation and sexually by conjugation.

Fragmentation

When a filament reaches a certain length, parts of it break away and grow into new filaments.

Conjugation

The cells of two filaments come to lie side by side and a conjugation tube is formed between them. The cells in one filament act as the male gamete while those in the other act as the female.

The male gamete passes through the conjugation tube to meet the female gamete and fuses with it to form a zygote.

The zygote secretes a resistant wall around itself and forms a zygospore. After a period of rest and favourable conditions, the outer coats burst and a young filament grows out.

How do earthworms reproduce?

Earthworms are hermaphrodites, that is, each has both male and female sex organs and therefore produces both male and female gametes. Reproduction is by sexual means.

Process of copulation

Two worms that are to copulate come to lie close together with their ventral surfaces touching. Copulation takes place at night outside the burrows. The reproductive organs of earthworms are anteriorly located. The worms lie in such a position that segments 9 to 15 of one worm are opposite the clitellum (segments 32 to 37) of the other, and they are held firmly by chaetae during copulation.

After copulation, the two worms separate. After a few days, eggs are laid and fertilised in a cocoon secreted by the clitellum. The embryo develops inside the cocoon, and one worm hatches from a batch of eggs in one cocoon.

How do cockroaches reproduce?

Mating and egg laying

Sexual reproduction takes place in cockroaches and fertilisation is internal. Male and female cockroaches mate and the male introduces sperm into the genital opening of the female. The sperm are stored in the sperm pouch until the eggs are released from the two ovaries. As the eggs are released, they are fertilised by the stored sperm.

Fertilised eggs (about 10 to 16) are laid in a horny egg case (ootheca), which the female carries in her abdominal pouch for some time and later deposits in a safe, dark place.

Nymphs and adults

After 30 to 100 days, the eggs hatch into nymphs, which are wingless, small and whitish. The nymphs feed, grow and become brown. They moult about 13 times to become adults.

In the process of moulting, the wings first appear as wing pads and later develop into full-grown wings. Cockroaches require 11 to 20 months to develop from eggs to imagos. Metamorphosis is incomplete.

How do houseflies reproduce?

Adult male and female houseflies mate and, within two to three days, fertilised eggs are laid. The laying of eggs takes place in the daylight. The housefly undergoes complete metamorphosis.

Eggs

The female lays 2 to 7 batches of eggs (100 to 150 eggs in a batch) in a moist, dirty environment. The eggs hatch into white larvae in about 8 to 24 hours.

Larva

The larva, called the maggot, has a segmented body. The head bears a pair of hooks for tearing food and drawing the larva along. On the ventral surface of the body lie spiny pads for movement. It has two pairs of spiracles for breathing.

The larva moults several times and lasts for about 5 to 14 days, after which it moves to a dry place to begin the pupal stage.

Pupa

The maggot shortens and its skin becomes hard and brown, forming the pupal case (puparium). The pupa does not feed or move. Internal re-organisation takes place at this stage. In about 3 to 10 days, the young adult hatches out of the puparium.

Adult

The adult housefly, called the imago, emerges from the puparium using a sac-like organ (ptilinum) to break it open. It moves to the surface of the dirt and flies away when its wings are dry.

How do land snails reproduce?

Reproduction in land snails is hermaphroditic and fertilisation is internal. The female snail has a fertilisation pouch for sperm to travel into. The snails transfer their spermatophores to a place called the epiphallus. The epiphallus is part of the sperm duct to the penis and helps put the spermatophores into place by using their flagellum. From here, sperm is transported to the bursa duct, where fertilisation takes place.

During snail development, there is a 180° twist of the visceral mass that brings the anus and the mantle cavity forward to a position above the head. This process is known as torsion.

Evaluation

1. Name the two types of asexual reproduction in Amoeba.
2. List the stages of conjugation in Paramecium.
3. Describe how a zygospore is formed in Spirogyra.
4. Why is the earthworm described as a hermaphrodite?
5. Name the stages in the life cycle of the housefly in order.

Assignment

1. Conjugation occurs in the following organisms except
   A. spirogyra B. paramecium C. mucor D. hydra
2. The possession of the male and female reproductive parts by a single organism is termed
   A. oviparous B. meiosis C. hermaphroditism D. parthenogenesis
3. Internal fertilisation takes place in the following organisms except
   A. earthworm B. tapeworm C. butterfly D. toad
4. The zygote of spirogyra secretes a resistant wall around itself known as
   A. membrane B. cyst C. clot D. zygospore
5. Paramecium reproduces asexually by
   A. binary fission B. meiosis C. mitosis D. budding
6. Describe sexual reproduction in tapeworms.
7. What are the advantages and disadvantages of sexual reproduction?