Alterations of Generations- Types & Explanation

Alterations of Generations

Introduction

There is a huge diversity in the biological world. Every organism has evolved its own mechanism through which it multiplies and produces its offspring. Many factors such as the organism’s habitat, its internal physiology, etc., are collectively responsible for how it reproduces.  

Reproduction is a biological process in which an organism gives rise to its young ones, which are similar to itself. The offspring then grows and matures into an adult and, in turn, produce new offspring. Thus, we can say that it is a continuous cycle of birth, growth and death.  

Reproduction enables the continuity of the species, one generation after another. We have already studied how genetic variation is created and inherited during reproduction. 

Explanation

Based on whether one or two organisms are participating in the process of reproduction, it is of two types.  

• Asexual reproduction: offspring produced by a single parent with or without gamete formation. 
• Sexual reproduction: Offsprings are produced by the fusion of male and female gametes derived from either one or two parents. 

Asexual reproduction 

Asexual reproduction is when a single parent is capable of producing offspring. As a result, the offspring produced are usually genetically identical to one another and their parent.  

The advantage of asexual reproduction is that it’s generally a more rapid process than sexual reproduction. Finding a mate, which can be dangerous and time-consuming, is not required in asexual reproduction. The disadvantage is that asexual reproduction does not give rise to the same degree of genetic variation that results from sexual reproduction. This suggests that a reproductive strategy that depends only on asexual reproduction can leave populations vulnerable to environmental changes. 

Asexual reproduction is generally seen in single-celled organisms and in plants and animals which have relatively simple organizations. 

Asexual reproduction occurs in multiple ways, including binary fission, budding, regeneration, vegetative propagation, reproduction by spores and parthenogenesis. 

• Binary fission is one such mode of asexual reproduction that occurs in unicellular organisms. One parent cell replicates its genetic material during binary fission and then divides into two identical daughter cells (e.g., Amoeba, Paramecium). 
• Spore formation: During unfavorable conditions, amoeba forms a cyst and divides by multiple fission. When favorable conditions return, the cyst breaks and spores are released. All spores give rise to individual organisms. 
• Budding: It is another type of asexual reproduction that happens in some unicellular and basic multicellular organisms. During budding, the offspring is linked to the parent during the first few months of its life. Eventually, the offspring separates from the mother and lives a life of its own. 
• Budding is similar to binary fission in unicellular organisms. The only difference is that during budding, an immature daughter cell sprouts from a mature parent cell and grows to maturity while it is still attached. 
• Fragmentation: The parent organism breaks into small fragments. Each fragment can develop into a new organism. 
• Vegetative propagation: Vegetative propagation is another type of asexual reproduction in plants where an offspring grows from vegetative parts, such as specialized stems, leaves, and roots. 
• Parthenogenesis: Parthenogenesis is referred to as a type of asexual reproduction which occurs in complex, multicellular animals. Parthenogenesis is when an embryo forms and develops without fertilization by a male gamete. 

Sexual reproduction 

Sexual reproduction involves the fusion of a male and a female gamete. The female gamete (egg cell) is haploid, and it fuses with the male gamete (sperm cell), which is also haploid. The fusion of these gametes occurs at the time of fertilization and results in the formation of a diploid zygote. The zygote further develops into an individual organism that is genetically different from the parent organisms.  

It is a more complex and slow process compared to asexual reproduction. In asexual reproduction, an organism is a clone of the parent. Whereas in sexual reproduction formation of gametes increases genetic variation and expands the gene pool. Thus, we can say that sexual reproduction gives rise to a greater level of genetic variation than asexual reproduction. It also ensures that the chromosome number of organisms in a particular species will remain the same across generations. It is generally seen in multicellular organisms and in plants and animals, which have more complex organization.  

Events occurring during sexual reproduction may be categorized as pre-fertilization, fertilization and post-fertilization events. In pre-fertilization events, gametogenesis and gamete transfer take place, while in post-fertilization events, the formation of zygote and embryogenesis takes place. 

Gametes are haploid in nature and are produced by meiotic division. In haploid organisms, gametes are formed by mitotic division. The transfer of male gametes is an important event in sexual reproduction. In unisexual animals, it occurs through copulation or simultaneous release, whereas it’s relatively easy in bisexual organisms. In angiosperms, pollination ensures the transfer of pollen grains from pollen tube to stigma.  

Syngamy is a type of fertilization that occurs between the male and female gametes. It may occur either externally or internally.  Syngamy results in the formation of a zygote. The process of development of an embryo from the zygote is called embryogenesis. In animals, the zygote starts developing soon after its formation. 

Alternation of generation: 

Many organisms use more than one method of reproduction and have both asexual and sexual reproductive capabilities. Such organisms alter their life cycle between two generations, i.e., sexual and asexual. The alternation of the sexual and asexual phases in the life cycle of an organism is called the alternation of generation. The sporophyte and gametophyte generations are morphologically distinct most of the time. Sometimes they can also be distinct chromosomally. When these two generations are similar morphologically, then it is said to be isomorphic. 

Alternation of generations is common in algae, fungi, and plants. The two generations are often not easy to observe due to the microscopic size of one or the other generations.  

Sporophyte generation:  

The sporophyte phase is the asexual phase in the alternation of generations. The sporophyte is diploid and has two sets of chromosomes.  

When the sporophyte matures, it develops a reproductive organ called sporangia which produces spores. The diploid sporophyte produces haploid spores, which are produced by meiotic cell division.  

Spores are released in the environment when the conditions are favourable and are transported by air, water, etc. 

Gametophyte generation: 

The gametophyte phase is the sexual phase in the alternation of generations. The gametophyte is haploid and has only one set of chromosomes.  

When the gametophyte matures, it develops gametangia which produce gametes. The haploid gametophyte produces gametes that fuse to form a diploid zygote. It involves mitotic cell division. 

Gametes are released into the environment, and gametes of the opposite sex fuse together to form a diploid zygote that gives rise to the sporophyte. 

Stages of Alternation of Generations 

Alternation of generation takes place in two stages: 

Sporophyte Generation: 

In this generation, two haploid gametes of the opposite sex fuse to form a diploid zygote. This diploid zygote then divides multiple times by mitotic cell division to produce the sporophyte, which is also diploid.  

The sporophyte is diploid and a multicellular organism. After attaining maturity, the sporophyte bears sporangia which are its reproductive organs. This is a significant step in the alternation of generations.  

The sporangia produce haploid spores. These spores are released into the environment and carried away by air or water. When the conditions are favorable, these spores develop into a gametophyte. 

Gametophyte Generation: 

This is the second generation in the alternation of generations. The newly formed spore is haploid and has only one set of chromosomes of the parent organism. This spore divides several times mitotically to produce a gametophyte.  

The gametophyte generation leads to the production of gametes. These gametes are formed by gametangia, the reproductive organs of the gametophyte.  

On maturity, the gametes are transferred between plants or spread into the environment. When a male gamete encounters a female gamete, it fuses to form a zygote.  

This zygote divides and eventually becomes a sporophyte. 

Three types of alternation of generation: 

There are three patterns of life cycle: 

Haplontic:  

• The haploid gametophyte is multicellular, dominant and independent. 
• The sporophyte is not free-living and is dependent on the gametophyte. 
• The diploid sporophyte is represented only by a zygote which divides by meiosis to form haploid cells called spores. 
• These spores give rise to the multicellular haploid organisms. 
• E.g. Volvox, Spirogyra, Chlamydomonas, etc. 

Diplontic: 

• The diploid sporophyte is dominant, photosynthetic and independent generation. 
• Haploid gametes represent the gametophyte. 
•  E.g. some green algae and all seed bearing plants like gymnosperms and angiosperms. 

Haplodiplontic:  

• The gametophyte and sporophyte both are multi-cellular and free- living. 
• In some the gametophyte is dominant and the free-living sporophyte is small and has a short life –span. E.g., bryophytes. 
• Whereas in pteridophytes, the sporophyte is the independent and free-living dominant stage which alternates with the short-lived gametophyte. E.g.,Ulva, Polysiphonia, Ectocarpus. 

Summary

• Reproduction is a biological process in which an organism gives rise to its young ones,
  which are similar to itself. It is a continuous cycle of birth, growth and death.
• Genetic variation is created and inherited during reproduction.
• In asexual reproduction single parent produces offspring. As a result, the offspring are
  usually genetically identical to one another and their parent. It is a rapid process and does not give rise to the same degree of genetic variation as sexual reproduction.
• Asexual reproduction takes place by binary fission, spore formation, budding, fragmentation, vegetative propagation and parthenogenesis.
• In sexual reproduction male and female gametes are formed by the same individual or by different individuals of the opposite sex. Male and female gametes fuse to form zygote which later grows and develops into a new individual.
• It is a complex and slow process and gives rise to a great level of genetic variation.
• Many organisms use more than one method of reproduction and have both asexual and sexual reproductive capabilities. The alternation of the sexual and asexual phases in the
  life cycle of an organism is called the alternation of generation.
• The sporophyte phase is the asexual phase and is diploid. Sporophyte matures to form
  sporangia which produces spores. The diploid sporophyte produces haploid spores.
• Spores are produced by meiotic cell division and are released when the conditions are
  favorable.
• Mitotic spores and meiotic spores are the two types of spores. Asexual spores are mitotic
  spores, while sexual spores are meiotic spores.
• Sporophytes can produce both sexual and asexual spores. Chlamydomonas and algae
  both produce mitotic spores. .
• Bryophytes produce meiotic or sexual spores.
• Pteridophytes are either homosporous or heterosporous.
  • Homosporous: produce only one type of spore which is of the same size as the
    spore mother cell.
  • Heterosporous: produce two types of spores which are different in size, namely
    microspore and megaspore.
• The gametophyte phase is the sexual phase and is haploid.
• Gametophyte matures and develops gametangia which produces gametes. The haploid
  gametophyte produces gametes that fuse to form a diploid zygote.
• Gametes are released into the environment and gametes of opposite sex fuse together to
  form diploid zygote that gives rise to the sporophyte.
• The diploid sporophyte undergoes meiotic division to produce haploid spores which divides mitotically to form the haploid gametophyte.
• The haploid gametophyte then produces haploid gametes which fuse together to again form the diploid sporophyte. This results in alternation of generation in plants.