Algae: Life cycles

Following types of life histories are found in algae:

(a)      Haplontic Monophasic: 

1. It is a primitive life cycle.
2. In this case, vegetative plant body remains haploid.

• Gametes are produced by mitosis and fuse to form diploid zygote.

1. First division in zygote is meiosis. It forms haploid vegetative body.
2. Thus zygote is the only diploid stage in such cycles. e.g. Chlamydomonas, Ulothrix, Oedogonium, Spirogyra, Zygnema

Fig: haplointic life cycle

(b) Diplontic Monophasic:

1. Itis more advanced life cycle.
2. In this case, vegetative stage is diploid.

• Haploid gametes are formed by meiosis.

1. 1. These gametes fuse to form diploid zygote.
   2. Zygote divides by mitosis to form vegetative body.
   3. Thus all life form is diploid except gametes, e.g., diatoms; Sargassum and

Fig: Diplontic life cycle of algae

(c) Diplohaplontic life cycle: 

There are present two distinct vegetative individuals, of which one is haploid or gametophytic and other is diploid or sporophytic in nature and both alternate with each other.

• Isomorphic Diplohaplontic life cycle:

If both generations (diploid sporophyte and haploid gametophyte) have

similar vegetative structure, then they are called isomorphic, e.g., Ulva, Ectocarpus

• Heteromorphic Diplohaplontic:

1. If plant body (sporophyte and gametophyte) has different structures,

then they are called heteromorphic.

1. Gametophyte is haploid generation.

• It produces gametes by mitosis. Gametes fuse to form diploid zygote.

1. Zygote form diploid sporophyte.
2. Sporophyte is diploid generation. It produces spores by meiosis.
3. Spores germinate to form gametophyte plant, e.g.,

Fig: Algae, Diplohaplontic life cycle

(d) Haplobiontic Life cycle:

There are present two well-developed haploid phases in the life cycle and hence are called haplobiontic. The diploid phase is represented only by zygote.

1. Plant body is gametophyte, which produces gametes
2. Zygote undergoes meiosis, produces haploid nuclei and develop into haploid plant (Carposprophyte)

• Carposprophyte produces haploid carpospores.

1. Carpospores germinate into heterotrichous chantransia stage.
2. Erect filaments develop into gametophyte.
   

   Fig: Algae haplobiontic life cycle

(e) Haplodiplobiontic or triphasic life cycle:

In this case there are present one haploid phase and two diploid phases. Because of presence of one haploid and two diploid phases, it may also name as triphasic.

1. Plant body is gametophyte that produces gametes
2. Zygote germinate to produce diploid carposporophyte.

• Carpospores (2n) are produced by mitosis by carposporophyte.

1. Carpospores on germination produces tetrasporophyte (2n)
2. Tetrasporophyte produces haploid tetraspores by meiosis.
3. Tetraspores germinate to produce gametophyte.

Fig: Haplodiplobiontic life cycle of algae