Life cycle Patterns

Alternation of generations
 

Introduction:

• In plants, both haploid and diploid cells can divide by mitosis. This ability leads to the formation of different plant bodies - haploid and diploid.
• The haploid plant body/gametophyte produces gametes by mitosis. This plant body represents a gametophyte.
• Following fertilization, the zygote also divides by mitosis to produce a diploid sporophytic plant body.
• Haploid spores are produced by this sporophytic plant body by meiosis.
• Spore in turn, divide by mitosis to form a haploid plant body once again.
• Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between gamete-producing haploid gametophyte and spore-producing diploid sporophyte.

Detailed Explanation:

The relationship between sporophyte and gametophyte is different in different groups of plants

• In Algae, the sporophyte and gametophyte are separate independent organisms that may or may not have the same appearance.
• In bryophytes, the sporophyte is less developed than gametophyte.However, moss and hornwort sporophytes can perform photosynthesis. They require extra photosynthate from the gametophyte for their growth and spore development.
• However, in modern vascular plants the gametophyte is less well developed as compared to the sporophyte.
• In ferns, the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte is dependent for its nutrition.
• In flowering plants, the reduction of the gametophyte is much more extreme; it consists of just a few cells that grow entirely inside the sporophyte.
• A typical life cycle has the following event series:
  - Two haploid gametes, each with n unpaired chromosomes, fuse together to produce a diploid zygote with n pairs of chromosomes, for a total of 2n chromosomes.
  - The zygote (diploid) grows and divides normally (mitosis), keeping the chromosome number at 2n to form a multicellular diploid organism which is known as the sporophyte (produces spores).
  - When the sporophyte matures, it forms many sporangia, which are the body parts that produce diploid sporocytes or spore mother cells. These divide through a unique process called meiosis, which reduces the chromosome number to half. It produces four haploid spores having n number of unpaired chromosomes.
  - The haploid spore germinates and divides normally (mitosis), keeping the chromosome number constant at n. The end result is a multicellular haploid organism known as a gametophyte which forms gametes at maturity.
  - When the gametophyte matures, it yields many gametangia, organs that generate haploid gametes. At least one type of gamete has a mechanism for reaching out to another gamete in order to get fused with it.
• Thus, the 'alternation of generations occurs between a diploid generation of sporophytes and a haploid generation of gametophytes in the life cycle of an organism.

Life Cycle Patterns

• However, different plant groups, as well as individuals representing them, differ in the following patterns:
  i.Haplontic life cycle
  ii.Diplontic life cycle
  iii.Haplodiplontic life cycle

Introduction:

• A life cycle is a sequence of changes that an organism goes through in form before returning to its starting state.
• Growth, asexual reproduction, or sexual reproduction are all examples of transitions or changes in form.
• During the life cycle of some organisms, various "generations" of the organisms succeed each other.
• Life cycles involving sexual reproduction also include alternating haploid (n) and diploid (2n) phases, implying a ploidy change.
• Meiosis is required for the transition from a diploid to a haploid stage.
• There are three types of cycles when it comes to ploidy changes: haplontic life cycle, diplontic life cycle and haplodiplontic life cycle.

Haplontic life cycle
 

Detailed Explanation:

• The main plant body is haploid gametophyte.
• The dominant, photosynthetic phase in such plants is the free-living gametophyte.
• Gametophyte divide mitotically to form haploid gametes.
• Fertilization of gamete leads to formation of zygote (2N)
• Sporophytic generation is represented only by the zygote.
• There are no free-living sporophytes.
• Meiosis in the zygote (zygotic meiosis) results in the formation of haploid spores.
• The haploid spores divide mitotically and form the gametophyte.
• This kind of life cycle is termed as haplontic.
• Examples: Ulothrix, Chlamydomonas, Spirogyra and Volvox.
  
  
  

Diplontic life cycle
 

Detailed Explanation:

• Diploid sporophyte is the dominant, photosynthetic, independent phase of the plant.
• Meiosis occurs in the diploid spore mother cells .
• The gametophytic phase is represented by a short lived, single to few-celled haploid gametophyte that is dependent on the photosynthetic sporophyte.
• This kind of life cycle is termed as diplontic.
• Example: All seed bearing plants(Gymnosperm and angiosperm), some green algae (Caulerpa) and some brown algae (Fucus, Sargassum).
  
  
  

Haplo-diplontic life cycle
 

Detailed Explanation:

• Bryophytes and pteridophytes exhibit an intermediate type of life cycle called haplo-diplontic in which both phases are multicellular and equally dominant.
• In bryophytes, a haploid gametophyte represents a dominant, photosynthetic, independent, thalloid, or erect phase that switches with the short lived multicellular sporophyte that is totally or partially dependent on the gametophyte for anchorage and nutrition. This pattern is shared by all bryophytes.
• In pteridophytes, a dominant, photosynthetic, independent, vascular plant body characterizes the diploid sporophyte. It alternates with a multicellular, independent and short-lived haploid gametophyte.
• Generally algae exhibit a haplontic life cycle but Ectocarpus, Polysiphonia, and kelps are haplo-diplontic.
  
  
  

Frequently asked questions (FAQ)

Q1. Name the algae that represent the haplo-diplontic life cycle.
Ans.

• Generally, algae exhibit a haplontic life cycle but algae like Ectocarpus, Polysiphonia, kelps exhibit haplo-diplontic life cycle patterns.
• In the haplo-diplontic life cycle, both phases (gametophyte and sporophyte) are multicellular and are equally dominant.
• Haplo-diplontic life cycle pattern is commonly seen in Bryophytes and Pteridophytes.

Q2. Explain the life cycle of pteridophytes.
Ans.

• In pteridophytes, a dominant, photosynthetic, independent, vascular plant body characterizes the diploid sporophyte. It alternates with a multicellular, independent and short-lived haploid gametophyte.

Q3. What is a diplontic life cycle pattern?
Ans:

• Diploid sporophyte is the dominant, photosynthetic, independent phase of the plant.
• Meiosis occurs in the diploid spore mother cells .
• The gametophytic phase is represented by a short lived, single to few-celled haploid gametophyte that is dependent on the photosynthetic sporophyte.
• This kind of life cycle is termed as diplontic.
• Example: All seed bearing plants(Gymnosperm and angiosperm), some green algae (Caulerpa) and some brown algae (Fucus, Sargassum).

Q4. Name the organisms exhibiting haplontic life cycle.Do they show the presence of embryo?
Ans.

• Algae, including Volvox, Ulothrix, Spirogyra,Chlamydomonas exhibit haplontic life cycle.
• In haplontic life cycle, the main plant body is haploid gametophyte.The dominant, photosynthetic phase in such plants is the free-living gametophyte. Gametophyte divide mitotically to form haploid gametes. Fertilization of gamete leads to formation of zygote (2n) which is the only diploid stage. There are no free-living sporophytes. Meiosis in the zygote (zygotic meiosis) results in the formation of haploid spores hence no embryo is formed. The haploid spores divide mitotically and form the gametophyte.

Q5. Define alternation of generations.
Ans.

• In plants, both haploid and diploid cells can divide by mitosis. This ability leads to the formation of different plant bodies - haploid and diploid.
• The haploid plant body/gametophyte produces gametes by mitosis. This plant body represents a gametophyte.
• Following fertilization, the zygote also divides by mitosis to produce a diploid sporophytic plant body.
• Haploid spores are produced by this sporophytic plant body by meiosis.
• Spore in turn, divide by mitosis to form a haploid plant body once again.
• Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between gamete-producing haploid gametophyte and spore-producing diploid sporophyte.

Q6. What is the dominant phase of bryophytes and angiosperms life cycle?
Ans. 
During the evolution from bryophytes to angiosperms there is gradual development of sporophyte and the gametophyte gets reduced. bryophytes, the dominant phase of the life cycle is the gametophytic phase while the sporophyte is completely or partially dependent on the gametophyte. Whereas in the angiosperms the main plant body is represented by a well developed sporophyte while the gametophyte is reduced and few celled.