Asexual reproduction, Bryophytes, fern, alternative generation (A-level biology)

Patterns of reproduction

Asexual reproduction

This is a type of reproduction that does not involve the fusion of gametes or a combination of genetic materials from different individuals.

Types of asexual reproduction

• Fission: The organism divides into two or more equal sized parts, e.g. binary fission in amoeba
• Spore formation: Spores are unicellular bodies formed by cell division in a parent organism. Having become detached from the parent, they develop directly or indirectly into a new individual, provided environmental conditions are suitable.

Generally, spores are very small which enables them to be distributed by animals.

• Budding: this is a method of reproduction where an organism develops an outgrowth which when detached from a parent become self-supporting e.g. yeast and hydra
• Fragmentation: This is where an organism is broken into two or more pieces, each of which grows into a new individual. As a means of reproduction, fragmentation depends on the organism having good power if regeneration. E.g. spirogyra.
• Vegetative reproduction: is a form of asexual reproduction in plants where part of the body become detached and develop into a new self-supporting individual
• Parthenogenesis: This is the development of a new offspring from an unfertilised egg. Haploid parthenogenesis (e.g. in pineapple, the egg is produced by meiosis whereas in diploid parthenogenesis the egg is produced by mitosis; e.g. production of wingless aphids.

Advantages of asexual reproduction

• producing more offspring than sexual reproduction mode in a given time
• good qualities are retained in the offspring

The disadvantage of Asexual reproduction

• may lead to the accumulation of  recessive genes in a population
• no genetic variability

Sexual reproduction

In its broadest sense, sexual reproduction is any process in which genetic materials is transferred from one cell to another. It generally involves the fusion of gametes.

Cross-fertilization and self-fertilization

• Cross-fertilization- gametes come from two separate individuals.
• Self-fertilization – both gametes come from the same individual.

Advantage of sexual reproduction

• allows for the mixing of genetic material (genetic variability).

Evolution of reproductive methods

Fundamental to sexual reproduction is the method by which gametes are brought together. In external fertilization, the union of gametes occurs outside the body whereas, in internal fertilization, the union of gametes occurs inside the body. Generally, internal fertilization necessitates the use of some kind of intromittent organ (e.g. penis) to introduce the sperms into the female body.

Internal fertilization has two advantages over external fertilization

• It is a surer method: there are fewer chances of gametes being wasted
• A fertilized egg can be enclosed with a protective covering before it leaves the female body (Oviparity) e.g. eggshells or develop within the female (Vivi parity)

Many organisms desert their offsprings as soon as they have been produced as fertilized eggs. Others provided some sort of parental care. E.g. man. By and large the more parental care, the fewer the number offsprings produced.

An organism’s life cycle is the sequence of events from fertilization in one generation to fertilization in the next generation. The life cycle of human and most animals follow the plant below:

The life cycle

Alternative generations

Alternative generation is a life cycle the diploid and haploid generation follow one another and the two-generation are dissimilar or the alternation of a sexual phase and an asexual phase in the life cycle of an organism. However, diploid generation is better adapted due to two pairs of a gene that harmful gene due to mutation may be masked and there is greater opportunity for variation whereas the haploid has one gene

Advantage of alternative generation

Each generation occupies a different niche reducing intraspecific competition.

Plants and some algae have a two-generation of life cycle called the alternative generation that involves meiosis.

1. The sporophyte, the diploid generation, produced spores by meiosis. Spore develops into haploid generation.
2. The gametophytes, the haploid generations produce gametes by mitosis that unite to form a diploid organism

The two generations are dissimilar and one is dominant over the other. The dominant generation is larger and exists for a long period of time.

The life cycle of a moss

The gametophyte is dominant in bryophyte such as moss

1. The leafy soot bears separate male and female gametangia: the antheridia and archegonia.
2. Flagellate sperms are produced in antheridia and these swim in external water to archegonia that contain a single egg.
3. When the egg is fertilized, the developing embryo is retained within the archegonia.
4. The mature sporophyte growing on top of the gametophyte produces spores by meiosis that when they spread by wind and find favorable conditions germinate into anew gametophyte.

The life cycle of Bryophytes

The similarities and differences between bryophytes and pteridophytes

• In both the diploid sporophyte produces spores and the haploid gametophyte produce gametes
• In both female gametes are non-motile while male gametes are motile
• In both spores are produced by meiosis
• In both the sporophytes grow out of the gametophyte
• In both gametes are produced by mitosis
• In both spores germinate into the gametophyte
• In both zygote grows into a sporophyte
• In both Sporophyte produce spores whereas gametophyte produce

Differences between moss and fern

The life cycle of a moss	The life cycle of a fern
Gametophyte is dominant Sporophyte is dependent Spores first germinates into a thread-like protonema which produces buds that grow into a gametophyte Transfer of sperms from antheridia to archegonium is by rain splash   Gametophyte long-lasting   The sporophyte is very temporary and photosynthesize to a limited extent	Gametophyte is dependent Sporophyte is dominant Spores germinate directly into a gametophyte     Sperms swim in moist environments from the antheridia to an egg at the base of the archegonium. Gametophyte temporary organ Sporophyte long-lasting and is photosynthetically active

 

The importance of alternative generation to the life of moss and fern (3marks)

• Spores can survive harsh conditions and only germinate when conditions are favorable
• Formation of spore result in variation
• It ensures the rapid multiplication of plant species as spores are usually produced in large numbers.
• Enable the different generations to occupy different ecological niches and allow the survival of the plant.
• Interdependence between the gametocyte and sporophytes generations ensures that both generations exist at any given time. This prevents the extinction of the plant

Reasons why mosses are restricted in a wet environment

• The body is not covered with a cuticle, thus require water to prevent desiccation
• Water is required to enable male gametes to swim to the eggs to allow fertilization.
• Lack of true roots, and thus need to absorb water by osmosis.
• They lack a vascular system, therefore, require water to parts by osmosis

Reasons why ferns are more adapted to terrestrial life than mosses.

• Ferns have rhizomes (horizontal underground stems) which act as organs of perennation, allowing the ferns to survive unfavorable climatic conditions, unlike mosses.
• Ferns produce more spores than mosses. This increases their chance of reproduction and colonization of habitat compared to mosses
• The lignified xylem offers more support to the fern
• Ferns have broader leaves that offer a large surface area for trapping light for photosynthesis
• Sporophyte generation of ferns is nutritionally independent while that of the mosses depend on the gametophyte for nutrition
• Ferns have true stems to support the leaves to obtain enough light for photosynthesis.
• Leaves of ferns have cuticle to prevent water loss and desiccations.

Significance of alternative generation

• Rapid multiplication through the production of spores
• Spore survive harsh conditions
• Spore formation by meiosis leads to variation
• The random fusion of gametes by fertilization causes variation
• Gametophytes and sporophytes occupy different ecological niches reducing competition

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Asexual reproductions, Bryophytes and ferns

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