Triphasic life cycle in algae and alternation of generation

The life cycle of a plant is the epoch of time it takes from germination of the plant to the production of seed or completion of reproduction of that plant.

Alternation of generations is a sort of life cycle found in plants and  algae in which successive generations of individuals alternate between haploid and diploid organisms. This can be compared to sexual reproduction in animals, in which both haploid and diploid cells are found in every generation.

There are four main patterns of life cycle in algae. These are:

• 1.Haplontic Life Cycle
• 2. Diplontic Life Cycle
• 3. Diplohaplontic Life Cycle
• 4. Triphasic Life Cycle.

Here we are going to study triphasic life cycle in detail. In this type, there is succession of three distinct generations.

It is of two types:

1. Haplobiontic Type:

In this cycle the gametophytic (haploid) phase is intricate, dominant and persists for long period than sporophytic (diploid) phase which is characterised only by zygote i.e., haplobiontic type and two successive haploid generations are interrupted only by diploid zygote stage specify its triphasic nature. This type of life cycle is found in the  members of Rhodophyceae like Batrachos permum and Nemalion.

In Batrachospermum the haploid plant body develops sex organs and produces male (spermatium) and female (egg) gametes. The gametes after fusion form zygote.

The zygote then undergoes meiosis and form another haploid gametophytic plant, the carposporophyte. The carposporophyte develops carposporangium which produces haploid carpospores. The carpospores germinate and develop new free-living gametophytic plant.

So in this cycle, three phases are present :

1. 1. 1. 1. Haploid carposporophyte,
         2. Haploid gametophyte, and
         3. Diploid zygote

1. Diplobiontic Type:

In this type there is one haploid phase and two diploid phases show its triphasic nature and the sporophytic phase is more elaborate and persists for long period than the gametophyte i.e., diplobiontic type.such type of life cycle is found in Polysiphonia, a member of Rhodophyceae.

In Polysiphonia, the gametophytic phase is represented by two types of gametophytic  individuals  i.e., male and female plant, those bear spermatangium and carpogonium respectively. The spermatangium and carpogonium develop sperms and egg. These male and female gamete undergo fusion and form zygote. The zygote (2n) develops into a diploid carpospophytic phase. The diploid carpospores are formed in the carposporophyte.

The carpospores after germination develop the diploid tetrasporophytic plants. The tetrasporophytic plant develops diploid tetrasporangia. each of the tetrasporangia produce four tetraspores (n) by meiotic division. They are liberated by splitting of sporangial wall. Out of four tetraspores two produce male gametophyte and the remaining two into female gametophyte.

So in this cycle, three phases are found:

1. 1. Haploid gametophyte,
   2. Diploid carposporophyte, and
   3. Diploid tetrasporophyte.

This way the triphasic life cycle is very significant and advance type of life cycle.

Blog by- Ms. Kanchan Sharma
Asst. Prof, Department of Science