Lesson 1: Sexual and asexual reproduction

Video Lesson:

Competence

After the successful completion of this section, the student will be able to:

• Describe reproduction.
• Define sexual and asexual reproduction.
• Describe the sexual and asexual reproduction.
• Mention types of sexual and asexual reproduction.
• Explain each types of asexual reproduction.
• Explain vegetative propagation and their types.

Brainstorming Questions

Before starting this section, ask yourself this question: “What do I know about sexual and asexual reproduction (recall what you have learned
in previous grades) and what do I want to learn from this section”?

Key terms:

Chromosome, hypha, mitosis, mycelium, spore

• Reproduction is one of the unique characteristics of life. The ability of organisms to reproduce to form their own kind is the one characteristic that best distinguishes living things from nonliving matter. Two modes of reproduction are recognized: asexual and sexual. In asexual reproduction, there is only one parent and with no special reproductive organs or cells. Each organism is capable of producing identical copies of it as soon as it becomes an adult. Sexual reproduction as a rule involves two parents, each of which contributes special germ cells (egg or sperm) that in union (fertilization) develop into a new individual.

Asexual reproduction

• Asexual reproduction is the production of individuals without gametes (eggs or sperm). It includes a number of distinct processes, all without involving sex or a second parent. Asexual reproduction appears in bacteria and unicellular eukaryotes and in many invertebrates, fungi and plants. However, asexual reproduction is absent among vertebrates. The basic forms of asexual reproduction are fission (binary and multiple), budding, and fragmentation. For example, a small piece of stem planted in the soil may form roots and grow into a complete plant.
• Asexual reproduction has couple of advantages – no mate is needed; no gametes are needed; all the good characteristics of the parent are passed on to the offspring; and offspring will grow in the same favorable environment as the parent. Plants that reproduce asexually usually store large amounts of food that allow survival. The disadvantages are there is little variation created, so adaptation to a changing environment (evolution) is unlikely. If the parent has no resistance to a particular disease, none of the offspring will have resistance. Lack of dispersal can lead to competition for nutrients, water and light.

Types of asexual reproduction

Fission

• In fission, the organism divides into two (binary fission) or more (multiple fission) equal parts. Binary fission is common among bacteria, algae and protozoa. In binary fission, the body of the unicellular parent divides by mitosis into two approximately equal parts, each of which grows into an individual similar to the parent. In bacteria, the cell simply divides into two and each new cell becomes an independent organism. However, before a bacterium divides, the bacterial nucleus is replicated (copied) to produce two identical copies so that the daughter cells receive one copy each. Alternatively, the nucleus of the orgasms divide repeatedly and each daughter nucleus breaks away together with a small portion of the cytoplasm, resulting in the production of many daughter cells. This is common among some parasitic protozoa, for example, malarial parasites. Also, some invertebrates reproduce through fission.

Fragmentation

• Fragmentation is one of the most common modes of asexual reproduction involving the breakdown of a parent organism into parts that develop into whole organism. Fragmentation is observed in fungi, plants, animals and algae. For Example, Spirogyra, the filamentous green-algae undergoes fragmentation which results in many filaments. Each filament grows into matured filament Also a multicellular animal (e.g., worms) breaks into two or more parts, with each fragment capable of becoming a complete individual. Many invertebrates can reproduce asexually by simply breaking into two parts and then regenerating the missing parts of the fragments.

Budding

• Another common type of asexual reproduction is budding. In this mode of reproduction, the organism divides into two unequal parts. It is common in fungal species and invertebrates. During the process, a bulge forms on the side of the cell, the nucleus divides mitotically, and the bud ultimately detaches itself from the mother cell. For example, in some fungi such as yeasts, a bud develops on the surface of either the yeast cell or the hypha, with the cytoplasm of the bud being continuous with that of the parent cell. The nucleus of the parent cell then divides; one of the daughter nuclei migrates into the bud, and the other remains in the parent cell. Budding also occurs commonly in some invertebrate animals such as hydra.

Sporulation

• The majority of fungi, however, reproduce asexually by the formation of spores (i.e., sporulation). Spores are dispersed often by air currents and if they reach a suitable situation, they grow new hyphae. The hyphae develop into a mycelium. At the tip of the hyphae, a swelling or sporangium spore sac forms. Penicillium and Mucor are examples of mould fungi that grow on decaying food or vegetable matter.

Key Terms

• Chromosome: the hereditary material that carry the biological information.
• Hypha (pl. hyphae): the long filamentous branches found in fungi.
• Mitosis: type of cell division that produce two identical daughter cells.
• Mycelium: the vegetative part of a fungus, consisting of a network of fine white filaments (hyphae).
• Spore: reproductive cell.

• Vegetative propagation is a method of asexual reproduction in plants where structures with lateral meristems such as roots, stems, bud, and leaves give rise to new self-supporting individual. The following are types of vegetative reproduction.
• Natural Vegetative propagation

1.1 Stolons (runners)

• Runners originate from auxiliary bud in a lower portion of plant and grow along the surface of the soil. In the cultivated strawberry, for example, leaves, flowers, and roots are produced at every other node on the runner. Just beyond each second node, the tip of the runner turns up and becomes thickened. This thickened portion produces first adventitious roots and then a new shoot that continues the runner. Thus a complete plant may develop and take root at the node, nourished for a time by food sent from the parent plant through the stolon. Eventually, the stolon dries up and withers, leaving an independent daughter plant growing a short distance away from the parent.

1.2 Rhizome

• In many plants, horizontal shoots arise from lateral buds near the stem base and grow under the ground. Such underground horizontal stems are called rhizomes. At the nodes of the rhizome are buds, which may develop to produce shoots above the ground. The shoots become independent plants when the connecting rhizome dies. Many grasses propagate by rhizomes; the couch grass is a good example.

1.3 Corms

• Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter.

1.4 Tubers

• Tubers are modified stems that may store starch, as seen in the potato (Solanum sp.). Tubers arise as swollen ends of stolon’s, and contain many adventitious or unusual buds. If the tubers are left in the ground or transplanted, the buds will produce shoots, using food stored in the tuber.

1.5 Bulb

• A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem.

1. Artificial Vegetative propagation
2. Artificial methods of asexual reproduction are frequently used to give rise to new and sometimes novel, plants. They include grafting, cutting, layering, marcotting and micro propagation.

2.1 Grafting

• Grafting is an artificial method of asexual reproduction used to produce plants combining favorable stem and root characteristics. The stem of the plant to be grafted is known as the scion. The root is called the stock.

Cutting

• Plants such as hibiscus and Aibika are propagated through stem cuttings where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start producing a root even when placed only in water. For example, leaves of crotons or tanget will root if kept undisturbed in water for several weeks.

Layering or runner

• A method in which a stem attached to the plant is bent and covered with soil. Young stems that can be bent easily without any injury are the preferred plant for this method.

Parthenogenesis

• Some species of animals (e.g., bees) are able to reproduce asexually. Among common honeybees (Apis mellifera), a queen bee might lay 2,000 eggs per day. Nearly all of these eggs are fertilized by sperm the queen has received during one of her nuptial fights, and each one of these eggs will develop into one of the worker bees of the colony every one of them a female. A queen can, however, choose to let some of her eggs go unfertilized; no sperm from a male ever fuses with these eggs, yet bees develop within them and hatch from them. Since egg and sperm do not come together in this process, this is not sexual reproduction. Instead, each of these bees has been derived through parthenogenesis: a form of asexual reproduction in which an unfertilized egg develops into an adult organism. Among the honeybees, all the bees derived through parthenogenesis are males these are the few drones of a bee colony. Whip tail lizard (Aspidoscelis uniparens) produces females by employing parthenogenesis. All the members of this species are female, and all reproduction in the species comes through parthenogenesis. Thus, each new lizard develops solely from one of her mother’s eggs, meaning that each is a clone of her mother.

Sexual reproduction in Humans

• Sexual reproduction involves the production of sex cells. It almost always involves two parent organisms. These sex cells are called gametes and they are made in reproductive organs. The gametes are produced through meiosis. Sexual reproduction starts with the union of sperm and an egg in a process called fertilization. This can occur either inside (internal fertilization) or outside (external fertilization) the body of the female. Fertilization results in the formation of a single cell called a zygote. The zygote then grows into a new individual. The female gametes are always larger than the male gametes and are not mobile.