Nature is a machine. The family is a machine. The life cycle is like a machine.
Ray Dalio
What is life cycle?
The growth and development consists of a number of distinct morphological and cytological stages. The sequence of these orderly changes is called as LIFE CYCLE. It is the sequence of all different phases or events through which an organism passes from a diploid zygote of one generation to the zygote of the next generation through haploid gametes.
Life Cycle of Algae
There are five different types of life cycles in algae based on the number of haploid and diploid generations.
1. Haplontic life cycle
2. Diplontic life cycle
3. Haplodiplontic life cycle
4. Haplobiontic life cycle
5. Haplo-diplobiontic life cycle
(1). Haplontic life cycle
Ø Most common type of life cycle in algae
Ø Life cycle is diphasic (two phases)
Ø The prominent phase is haploid gametophytic phase
Ø The diploid (sporophytic) phase in the life cycle is represented by the ZYGOTE
Ø Zygote is formed by the fusion of haploid male and female gametes
Ø Zygote immediately undergo meiosis to produce haploid zoospores
Ø Zoospores germinate and grow by mitosis to produce the haploid gametophytic generation
Ø Gametophytic plant produce male and female gametes by mitosis
Ø Haplontic life cycle is the most primitive and simplest type of life cycle
Ø Other types of life cycles in algae are developed from this type
Ø Example: Most of the green algae such as Chlamydomonas and Ulothrix
Diplontic Life Cycle
(2). Diplontic life cycle
Ø This type is just a reversal of the haplontic type of life cycle
Ø Life cycle is diphasic, but the prominent phase is diploid sporophytic phase
Ø Haploid gametophytic phase in the life cycle is represented only by gametes
Ø Here gametes are produced in the gametangia by meiosis
Ø Moreover zygote do not undergo meiosis, rather it develop into a diploid sporophytic phase by mitosis
Ø Example: Sargassum, Codium, Bryopsis, Fucus
(3). Haplodiplontic life cycle
Ø Life cycle is diphasic
Ø One phase is haploid gametophyte and the other is diploid sporophyte
Ø Diploid zygote develop into diploid sporophytic generation
Haplo-diplontic Life Cycle
Ø Sporophytic plant produce sporangia which produce haploid zoospores by meiosis
Ø Zoospores develops into haploid gametophytic generation
Ø Gametophyte produce gametes
Ø Male and female gametes fuse to form the diploid zygote
Ø There are two types of haplodiplontic life cycle
(a). Isomorphic: gametophytic and sporophytic plants are morphologically similar (example: Ulva, Chaetophora)
(b). Heteromorphic: gametophytic and sporophytic plants are morphologically dissimilar (Laminaria, Urospora)
Haplobiontic Life Cycle (Eg. Batrachospermum)
4. Haplobiontic life cycle
Ø Here the life cycle is triphasic (three phases)
Ø In this type, there will be three phases in the life cycle, one diploid and two haploid phases
Ø The three phases are:
(a). Gametophyte phase (n): haploid phase 1
(b). Zygote (2n): diploid phase
(c). Carposporophyte phase (n): haploid phase 2
Ø Haplobiontic type of life cycle is shown by Nemalionales of Rhodophyceae (Eg. Batrachospermum)
Ø Gametophytic phase produce haploid gametes
Ø Male and female games fuse to form the zygote, the only diploid phase in the life cycle
Ø Zygote undergo reduction division (meiosis) to produce haploid spores which germinate into an intermediate haploid phase called carposporophyte
Ø Carposporophyte reproduce asexually by carpospores (n).
Ø Carpospores germinate and develop in to haploid gametophytic generation
5. Haplo-diplobiontic life cycle
Ø Most complex and advanced type of life cycle in algae
Ø Life cycle is triphasic with one haploid phase and two diploid phases
Haplo-diplobiontic Life Cycle (Eg. Polysiphonia)
Ø Among the three phases, two will be diploid and one haploid phase Diplobiontic life cycle is found in all Rhodophycean members except those in the order Nemalionales
Ø Polysiphonia is the most common example showing haplo-diplobiontic life cycle
Ø The life cycle of Polysiphonia includes three phases
(a). Carposporophyte – diploid (2n)
(b). Gametophyte – haploid (n)
(c). Tetrasporophyte – diploid (2n)
Ø Diploid zygote (2n) develop mitotically to diploid carposporophytic phase
Ø Carposporophyte produce diploid carpospores (2n)
Ø Carpospore germinate into diploid tetrasporophytic phase
Ø Tetrasporophyte produce haploid tetraspores by meiosis
Ø Tetraspore germinate into the haploid gametophytic generation
Ø Gametophytic generation produce male and female gametes
Ø Games fuse to form diploid zygote
Ø Thus in this haplo-diplobiontic life cycle, two diploid phases (carposporophyte and tetrasporophyte) alternate with a haploid gametophytic phase.
Learn more: Polysiphonia: Thallus Structure, Reproduction, Post Fertilization Changes and Life Cycle
You may also like…