Life Cycle in Algae Notes

Algae Life Cycle Patterns

Algae exhibit diverse life cycle patterns, characterized by alternating haploid (n) and diploid (2n) generations. The sequence of events from the zygote to the zygote of the next generation defines the life cycle pattern.

1. Haplontic Life Cycle

   In haplontic life cycles, the haploid phase is dominant. The gametophytic generation, primarily responsible for gamete production, takes precedence in the life cycle. Diatoms, for instance, showcase a unique diploid-dominant life cycle.

2. Diplontic Life Cycle

   Contrasting with haplontic life cycles, diplontic life cycles feature a dominant diploid phase (2n). The sporophytic generation, characterized by spore production, is the focal point of this life cycle type.

3. Alternation of Generations

   Many algae exhibit an alternation of generations, where both haploid and diploid phases are integral to the life cycle. The zygote gives rise to a multicellular organism that undergoes meiosis, producing haploid spores that develop into gametophytes, completing the cycle.

4. Absence of Sexual Reproduction

   Certain algae, such as blue-green algae and specific chlorophyceae like Protococcus, deviate from traditional life cycle patterns. In these cases, sexual reproduction is entirely absent, and these organisms reproduce exclusively through asexual means. Consequently, there is no alternation of generations in these algae.

5. Variability in Higher Plants

   Unlike algae, higher plants do not adhere to a regular and fixed alternation of generations. Their life cycles are more complex, with variations in the dominance of haploid and diploid phases.

Haplontic Life Cycle in Algae

The haplontic type of life cycle is considered the simplest and most primitive among various life cycle patterns. It serves as the evolutionary basis for other life cycle types. This pattern is predominantly found in members of the Chlorophyceae, where the main plant body is thalloid and can exist as unicellular, multicellular, or colonial structures. The gametes are produced in specialized structures called gametangia, and the primary plant body is referred to as the gametophyte (n).

In the haplontic life cycle, the majority of the organism's life is spent in the haploid phase, with the diploid (2n) phase represented solely by the zygote resulting from the fusion of gametes. The zygote undergoes immediate meiotic or reduction division, giving rise to four haploid zoospores, or meiospores. These meiospores then develop into individual plants, continuing the life cycle.

An illustrative example of the haplontic type of life cycle is observed in Chlamydomonas, a unicellular alga, as well as in various multicellular algae such as Oedogonium, Spirogyra, and Chara. The simplicity and ancestral nature of the haplontic life cycle make it a fundamental model for understanding the reproductive strategies and life histories of algae.

 

Diplontic Type Life Cycle in Algae

Diplontic Life Cycle in Fucus (Brown Alga):

1. Diploid Sporophyte (2n) Phase:

   • The main and visible phase of the life cycle is the diploid sporophyte (2n).
   • The sporophyte produces specialized structures called conceptacles, which contain diploid sporangia.

2. Meiosis and Haploid Spore Formation:

   • Within the sporangia, meiosis occurs, resulting in the formation of haploid spores (n).

3. Release and Germination of Haploid Spores:

   • The haploid spores are released into the water and, under favorable conditions, germinate to develop into multicellular structures known as the haploid gametophyte (n).

4. Haploid Gametophyte (n) Phase:

   • The haploid gametophyte is less conspicuous and usually microscopic compared to the diploid sporophyte.
   • The gametophyte produces gametangia, specialized structures where gametes are formed.

5. Gamete Formation on Gametophyte:

   • The male gametangia produce haploid sperm (n), and the female gametangia produce haploid eggs (n).

6. Fertilization and Zygote Formation:

   • Fertilization occurs when a sperm fertilizes an egg, resulting in the formation of a diploid zygote (2n).

7. Development into a New Sporophyte:

   • The zygote undergoes mitotic divisions and develops into a new diploid sporophyte, completing the life cycle.

Example:

• Fucus vesiculosus (Bladderwrack): This brown alga exhibits a diplontic life cycle. The conspicuous thallus seen in the intertidal zone represents the diploid sporophyte, while the microscopic, filamentous structures in conceptacles are the haploid gametophytes.

The diplontic life cycle in algae, such as Fucus, emphasizes the dominance of the diploid phase, contributing to genetic diversity and adaptation to changing environmental conditions.

 

 Introduction to the Diplohaplontic Life Cycle

As observed in previous life cycles such as haplontic and diplontic, a single vegetative phase, either gametophytic or sporophytic, is present. However, the diplohaplontic life cycle introduces an alternation of two distinct vegetative individuals with different chromosome numbers and functions. One individual is haploid (gametophyte), dedicated to sexual reproduction, while the other is diploid (sporophyte), responsible for producing meiospores through meiosis.

 Types of Diplohaplontic Life Cycle

(a) Isomorphic Diplohaplontic Life Cycle

In the isomorphic type, two generations alternate, externally appearing similar but with one being haploid (gametophyte) producing gametes and the other diploid (sporophyte) producing zoospores. The zygote germinates directly into a 2N plant without undergoing reduction division or meiosis, forming a sporophytic plant morphologically similar to the gametophytic plant. Meiosis occurs in the sporangia present on the 2N plant, with examples such as Cladophorales, Ulvalves, and Ectocarpus. Meiosis may occur in zoosporangia or unilocular sporangia, termed sporogenic meiosis. Haploid zoospores grow into a new haploid plant, initiating the cycle anew.

Examples: Ulva, Cladophora, Ectocarpus, and Dictyota

 

(b) Heteromorphic Diplohaplontic Life Cycle

In the heteromorphic type, both sporophyte (2n) and gametophyte (n) plants are morphologically distinct, alternating in the life cycle. The sporophytic plant is typically larger than the gametophytic plant. For instance, in Laminaria, the sporophyte can reach several meters in length, bearing diploid sporangia. This macroscopic diploid plant produces zoosporangia, which undergoes sporogenic meiosis to produce haploid meiospores. These spores germinate into minute gametophytes, or haploid plants, responsible for producing gametes. The fusion of gametes results in the development of a zygote, which directly develops into a diploid sporophytic plant, completing the life cycle.

Example: Laminaria

In summary, the diplohaplontic life cycle involves the alternation of two vegetative individuals, the gametophyte and the sporophyte, with sporogenic meiosis occurring in the sporangia. The two main types are isomorphic and heteromorphic, each exhibiting unique characteristics in the alternation of generations.

 

 Triphasic Life Cycle

(a) Haplobiontic Life Cycle:

The best examples of this life cycle are the members of Rhodophyceae (Batarachospermum and Nemalion) where two well-developed haploid phases are present in the life cycle. It is called haplobiontic triphasic. The diploid phase is represented by the zygote only.

• Gametophytic Phase (n): The plant body of Batarachospermum is gametophyte, bearing sex organs (spermatangium and carpogonia).
• Sexual Reproduction: Male gamete (spermatium) and female gamete (egg) are formed in these sex organs. Fusion results in the formation of a diploid zygote.
• Carposporophyte Generation (n): The basal portion of carpogonium gives rise to a haploid filament (gonimoblast filament). The uppermost cell functions as a carposporangium (n) which bears haploid carpospores.
• Alternation: There is alternation of three successive dissimilar, somatic haploid generations (carposporophyte, chantransia stage, and parent gametophyte) with a short-lived diploid phase.
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(b) Diplobiontic or Diplobiontic Triphasic Life:

During the diplobiontic life cycle, there are two distinct diploid phases - carposporophyte and tetrasporophyte - alternating with a haploid gametophytic phase. An example is Polysiphonia of Rhodophyceae.

• Gametophytic Phase (n): Male and female gametes are produced on their respective sex organs, developing on male and female gametophytic plants.
• Zygote Formation: Fusion of male and female gametes results in the formation of a zygote.
• Diploid Carposporophyte: The zygote divides mitotically, forming a small diploid carposporophyte attached to the gametophytic plant. Carposporophyte produces carposporangia with single diploid carpospores.
• Tetrasporophyte (2n): Diploid carpospores, upon liberation, germinate into a free-living diploid tetrasporophyte.
• Meiosis: Meiosis occurs in tetrasporangia, producing four haploid tetraspores.
• Alternation: Tetraspores, on germination, give rise to a free-living gametophytic plant.

In general, sexually producing algae complete their life cycle by passing through two distinct phases: gametophytic phase (n) concerned with the production of gametes and sporophytic phase (2n) concerned with the production of spores. These two phases alternate in a regular sequence, known as alternation of generations.

 

• Pigmentation In Algae 
• Reproduction In Algae