Reproduction In Unicellular Organisms And Invertebrates SS1 Biology Lesson Note

1. REPRODUCTION IN AMOEBA

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

In binary fission 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.

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

2. REPRODUCTION IN PARAMECIUM

Paramecium reproduces asexually by binary fission and sexually by conjugation.

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

Sexual reproduction is by conjugation of two individuals of different lines of descent. Stages in conjugation include:

Two matured parameciums come together and get fused by their oral grooves. The micronucleus divides twice by meiosis and four nuclei are formed in each conjugant

The smaller micronuclei are exchanged between the two conjugants. The migratory micronucleus fuses with the stationary micronucleus in each conjugant to form a zygote

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

3. REPRODUCTION IN SPIROGYRA

Spirogyra reproduces asexually by fragmentation and sexually by conjugation. 

1. In fragmentation, when a filament reaches a certain length, parts of it break away and grow into new filaments
2. In 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 the other acts as the female.

The male gamete passes through the conjugation tube to meet the female gamete and fuses 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.

4. REPRODUCTION IN EARTHWORMS

Earthworms are hermaphrodites i. e. 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 to be engaged in copulation 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 the segments 9-15 of one worm are opposite the clitellum (segments 32-37) of the other and are held firmly by chaetae during copulation. 

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

5. REPRODUCTION IN COCKROACH

Sexual reproduction takes place in cockroaches and fertilization is internal. Male and female cockroaches mate and the male introduces sperm into the genital opening of the female. The sperm are then stored in the sperm pouch until the eggs are released from the two ovaries. As the eggs are released, they are fertilized by the stored sperm. Fertilized eggs are laid (about 10–16 eggs) 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. After 30 – 100 days, the eggs hatch into nymphs which are wingless, small and whitish.

 The nymphs feed, grow, and become brown, 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 – 20 months to develop from eggs to imagos. Metamorphosis is incomplete.

6. REPRODUCTION IN HOUSEFLY

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

2 – 7 batches of eggs (100 – 150 eggs in a batch) are laid by the female housefly in a moist dirty environment. The eggs hatch into white larvae in about 8 – 24 hours.

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 segmented body lie spiny pads for movement. It has two pairs of spiracles for breathing. The larva moults several times and lasts for about 5 – 14 days after which it moves to a dry place to begin the pupal stage.

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

The adult housefly called 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 the wings are dry. 

7. REPRODUCTION IN SNAIL 

Reproduction in land snails is hermaphroditic and fertilization internal. The female snail has a fertilization pouch for sperm to travel into. The snails will transfer their spermatophores to a place called epiphallus. The epiphallus is part of the sperm duct to the penis to help put the spermatophores into place by using their flagellum. From here, sperm is transported to the bursa duct where fertilization takes place. During snail development, there is a 180o 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.

ASSIGNMENT

1. Conjugation occurs in the following organisms except     
2. spirogyra     B. paramecium     C. mucor     D. hydra 

2. The possession of the male and female reproductive parts by a single organism is termed     
3. oviparous     B. meiosis     C. hermaphroditism    D. parthenogenesis 

3. Internal fertilization takes place in the following organisms except     
4. earthworm     B. tapeworm     C. butterfly     D. toad

4. The zygote of spirogyra secretes a resistant wall around itself known as 
5. membrane B. cyst C. clot D. zygospore

5. Paramecium reproduces asexually by 
6. binary fission B. meiosis C. mitosis D. budding

6. Describe sexual reproduction in tapeworms.

7. What are the advantages and disadvantages of sexual reproduction?