REPRODUCTION IN UNICELLULAR ORGANISMS AND INVERTEBRATES

REPRODUCTION IN AMOEBA

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

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 becomes favourable, the cyst burst; each nucleus surrounded by a part of the cytoplasm of the parent. In this way, very small amoebae are formed.

REPRODUCTION IN PARAMECIUM

Paramecium reproduces asexually by binary fission and sexually by conjugation.

Binary fission occurs under favourable conditions, the micronucleus divides into two equal 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 paramecium 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 migatory 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 divide to form four paramecia each having a meganucleus and a micronucleus.

Conjugation in paramecium

EVALUATION

Describe binary fission in amoeba.
Describe sexual reproduction in paramecium.

REPRODUCTION IN SPIROGYRA

Spirogyra reproduces asexually by fragmentation and sexually by conjugation.

In fragmentation, when a filament reaches a certain length, parts of it break away and grow into new filaments

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 act 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 form a zygospore.
After a period of rest and favourable condition, the outer coats burst and a young filament grows out.

REPRODUCTION IN EARTHWORM

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 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 worm separate. After 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.

EVALUATION

Describe conjugation in a named alga.
What is hermaphroditism?
Describe the process of copulation in earthworm.

REPRODUCTION IN COCKROACH

Sexual reproduction takes place in cockroaches and fertilization is internal. Male and female cockroaches mates and the male introduces sperm into the genital opening of the female. The sperms 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 deposit it in a safe dark place. After 30 – 100 days, the eggs hatch into nymphs which are wingless, small and whitish in colour.

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 pad and later develop into full grown wings. Cockroaches require 11 – 20 months to develop from eggs to imagos. Metamorphosis is incomplete.

REPRODUCTION IN HOUSEFLY

Adult male and female mates and within two to three days fertilized eggs are laid. The laying of eggs takes place in the day light. 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 maggot has a segmented body. The head bears a pair of hook 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.

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 travelled to the bursa duct where fertilization takes place. During snail development, there is a 180^o 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.

GENERAL EVALUATION/REVISIONAL QUESTIONS

What is conjugation?
Describe conjugation in a named fungi.
Differentiate between complete and incomplete metamorphosis.
Describe sexual reproduction in tapeworm.
Mention five structural parts of a earthworm involved in reproduction.
Differentiate between metamorphosis in housefly and cockroach.
Describe fertilization in snails

WEEKEND ASSIGNMENT

Conjugation occurs in the following organisms except spirogyra B. paramecium C. mucor D. hydra
The possession of the male and female reproductive parts by a single organism is termed oviparous B. meiosis C. hermaphroditism D. parthenogenesis
Internal fertilization takes place in the following organisms except earthworm
tapeworm C. butterfly D. toad
The zygote of spirogyra secretes a resistant wall around itself known as A. membrane
cyst C. clot D. zygospore
Paramecium reproduces asexually by A. binary fission B. meiosis C. mitosis D. budding

THEORY

Describe sexual reproduction in tapeworm.
What are the advantages and disadvantages of sexual reproduction?