5. Parthenogenesis:

This is the ability of the egg to develop to a new individual without being fertilized by a male gamete. It can be considered a special kind of asexual reproduction since the progeny comes only from the mother. Parthenogenesis occurs in some worms, Crustaceans, and Insects. Honey bee reproduces this way as the queen produces some eggs that develop without fertilization forming drones and some others develop after fertilization to queen and workers, according to the type of food provided. Drones are haploid (N), the queen and workers are diploid (2N).

In some other cases, the eggs may result from mitotic division, where they develop to diploid (2n) individuals from the beginning as in case of the Aphid insect.

Parthenogenesis has been induced artificially by egg activation as in case of frogs and sea stars through it exposure to heat, electric shock, irradiation, some salts, agitation, or pricking with a needle. These stimuli lead to duplication of their chromosomes and so develop without fertilization to individual that are totally identical with the mother. By similar stimuli, early embryonic stages of rabbits were obtained from their eggs.

6. Tissue culture:

Scientists carried out experiments of tissue culture on Plants and Animals. They grow their tissues in a semi-natural nutrient medium and follow their differentiation and progress till full organisms are obtained. In an exciting experiment a scientist separated small pieces of Carrot plant into conical flasks containing Coconut milk, which comprises the whole plant hormones and nutrient elements. They began to grow and develop into a full Carrot plant. He separated single cells from the same plant and cultivated them by the same method and obtained full plants. Similarly, Tobacco plants were obtained from isolated cells of Tobacco leaves that have been treated by the same method. These experiments confirmed that any somatic cell in the plant comprises the whole genetic information that can be translated a whole developed organism if cultivated in a proper nutrient medium containing plant hormones with certain ratio. These methods are now applied in propagating rare plants or desirable strains or more resistant to diseases. Selected tissues for such culture can be preserved in liquid Nitrogen for a long period while remaining alive till cultivation. Hence, scientists foresee hopes on the progress of these techniques nearly in order to solve food problems, and to shorten the time needed for growing desirable crops.

Second: Sexual Reproduction:

Sexual reproduction necessitates the presence of 2 individuals, a male and a female for the production of sexual gametes. Gametes should meet for fusion during fertilization. At mating male gamete meets with the proper female gamete and fuse to form the zygote. Zygote then divides and grows to the embryo, then to the young and the adult that comprises parents’ features. So, the young receives from both parents even a minute part of their nuclear substance, and thus becomes a blend from both. Conversely, the young in the Asexual reproduction receives nuclear substance from only one parent and so becomes a copy from it. However, sexual reproduction is more time, and energy consuming than the Asexual reproduction, since it occurs after elapse of certain age and preparation. In some cases, the parent should prepare the proper nest or burrow before mating. They may alternate in guarding the eggs, and protection of the young till they grow larger. Some forms even face more hardships in order to protect their young, as they keep their embryos within their bodies till birth. These young may remain with their parents in a social life for more protection and learning more about their behaviour. Beside, production of new individuals is limited here to half the number of organisms, females only but not the males. In Asexual reproduction, all the individuals multiply. In spite of all the previous, Sexual reproduction provide the descendants with continuous innovations in their genetic contents that enable them to continue in the face of environmental variation.

The sexual reproduction depends on meiotic division, when forming gametes, during which the number of chromosomes is reduced to its half, so cells resulting from this division are haploid (N). During fertilization, the male gamete (N) fuses with the female gamete (N) to restore the original chromosomal number (2N), which differs according to the species.

Types of sexual reproduction:

1. Conjugation:

It takes place in some primitive organisms such as some Protozoa, Algae, and Fungi where reproduction takes place by mitosis at suitable conditions, but they turn to reproduce sexually by conjugation if subjected to drought, or a change in the water temperature or purity.

Conjugation in Spirogyra:

Spirogyra is common in the green scum of standing water, where its filaments float. Each filament is formed of one row of cells. This Alga turns to conjugate on advent of unsuitable conditions. Conjugations in Spirogyra are of two types:

1. Scalariform conjugation in Spirogyra:

1. When two filaments contact each other, a protrusion grows inwards between opposite pairs of cells.

2. The walls inbetween disappear forming a conjugation tube.

3. The protoplasm of each cell of one filament rolls up into a sphere and moves across the conjugation tube to fuse with the protoplasm of the cells of the opposite filament forming a zygote (2N).

4. Zygote becomes coated by a thick wall that protects it along unsuitable conditions and is then called zygospore (2N).

5. The zygospore remains dormant till the surrounding conditions improve then it germinates to a new filament.

2. Lateral conjugation:

This conjugation occurs between the adjacent cells of the same filament. The protoplasm of one cell moves to the adjacent cell through an opening in the wall inbetween them.

N.B.:

Cells of the algal filament are haploid (N), and after conjugation, the zygote (2N) is formed, then the zygospore (2N) forms. It divides by meiosis before germination of the new filament leading to the haploidity of the algal cells once more.

2. Reproduction by sexual gametes:

Higher plants and animals reproduce by means of sexual gametes that are differentiated into 2 kinds : male gametes and female gametes. Both are derived from meiosis that takes place in gonads.

1. Characteristics of male gametes:

1. Have the ability for locomotion.

2. Are adapted to locomotion, as the lose much of its cytoplasm.

3. Their bodies become pointed and provided with a locomotary tail or flagellum to help transport the genetic material to the female gamete during fertilization.

4. Since some of them are subjected to loss, 4 male gametes are produced from each original cell.

2. Characteristics of female gametes:

1. They are formed in the female ovary, and usually remain stationary within the female body till fertilization.

2. They are spherical.

3. They are enriched with food, where they are produced in few numbers.

Means of transportation of male gametes to meet female gametes:

1. Male gametes may be transported to the female via water, as in aquatic animals like bony fishes and toads where both the male and the female shed their gametes in water where external fertilization occurs and the embryo develops in water.

2. In terrestrial animals, fertilization takes place internally. Sperms should be introduced to the eggs (ova) that remain within the female body till fertilization.

N.B.:

Fertilization:

Fertilization is the fusion of the male gamete nucleus (N) with that of the female (N) to form the zygote (2N) that resumes its diploid nature, and passes towards embryonic development by means of mitosis.

Alternation of Generations:

Some plant and animal species can breed both Asexually and Sexually in an alternation of generations during their life cycles. They gain from both methods their advantages of rapid production and genetic diversity. These enable them to disperse widely and to conform to the environmental fluctuations. This may associated with variation in chromosome number of these generations. This phenomenon can be shown by the following examples:

1. Life Cycle of Plasmodium (Malaria Parasite):

This is a sporozoan parasite that belongs to Protozoa that infects both Man and female of Anopheles mosquito. Its life cycle starts when the infected mosquito bites the Human skin.

1. The female Anopheles mosquito pours in the Human blood minute spindle shaped Sporozoites.

2. Sporozoites move towards the liver, where they spend an incubation period during which they make 2 cycles of Asexual reproduction where the nuclei divide by Schizogony giving many merozoites.

3. Merozoites migrate to blood infecting the red blood cells, where they pass several cycles of Schizogony.

4. They produce huge numbers of merozoites that are released together every 2 days with the destruction of the infected red blood cells. Meanwhile, symptoms of Malaria fever appear on the patient (as heat, chill, and sweating).

5. Merozoites give rise to gametocytes, those migrate to the mosquito with patient’s blood.

6. Gametocytes develop to gametes in the mosquito’s stomach where they fuse into the zygote (2N) that transforms to the Ookinete (2N) that penetrates into the stomach`s wall, and grows to the Oocyst (2N).

7. The nucleus of the Oocyst divides by meiosis, this process is known as Sporogony producing numerous Sporozoites (N) that liberate and move towards the mosquito’s salivary glands to be ready for human infection.

So, the life of Plasmodium includes Sexual generation that reproduces by gametes fusion in mosquito followed by Asexual generation that reproduces by Sporogony in (mosquito) and by Schizogony (in Man).

2. Life Cycle of a Fern Plant (Ploypodium):

From the common ferns, Polypodium, which is known in plant nurseries as an ornamental plant, and Adiantum, which grows on well edges and shaded streams. The life cycle of Polypodium starts by the Sporophyte (2N).

1. Sporophyte carries the leaves, on their lower surfaces there are sori. Sori contain numerous spore cells (2N) which divide by meiosis giving spores (N).

2. On maturation of these spores, they are released and carried by winds to far distances.

3. Upon reaching a wet soil, spores germinate, forming several cells, that develop to a flat heart shaped body called the Gametophyte (N).

4. Gametophyte grows over the soil.

5. Rhizoids arise from the lower surface extremity of the gametophyte, that penetrate into the soil to absorb water and salts. The genital organs grow from the anterior region of the same surface of the Gametophyte. The male genital organs are called Antheridia, while the female genital organs are called Archegonia.

6. On maturation of the antheridia, the male gametes (ciliated sperms (N)) liberate and swim over soil water to reach the mature Archegonium for fertilizing its egg (N) forming the zygote (2N).

7. The zygote divides and differentiates to a new Sporophyte (2N), that grows over the Gametophyte, on which it depends for sometime till developing its own roots, stem, and leaves.

8. The Gametophyte degenerates, while the Sporophyte grows to repeat the life cycle. So, the Sporophyte (2N) that reproduces Asexually alternates with the Gametophyte (N) that reproduces Sexually in the life cycle of ferns. By such way, it represents a typical example of the phenomenon of alternation of generations in the living organisms.

3. Life Cycle of Schistosoma (Bilharzia):

The life cycle of Schistosoma is an example of alternation of generations in the Animal Kingdom. Adult worms live in Man, and reproduce sexually in his body. The parasite also uses snails as intermediate hosts, where Asexual reproduction occurs. Moreover, both types of reproduction enable the parasite to face the life of parasitism in a better way.