Life Cycle

Many life-cycle patterns are found in algae. However, there is no regular and fixed alternation of generation, as found in higher palnts. In blue-green algae and certain chlorophyceae which reproduced asexually, there is n alternation generation.

Haplontic type : This is a simplest and most primitive type of life-cycle. The other pattern of life-cycle have originated from this type. This type is found in all chlorophyceae.

In such cases the somatic phasse (plant) is haploid (Gametophyte) while the diploid phase (sporophte) is represented by zygote. During germination the zygote (2n) divides meiotically producing haploid (n) zoospores, which develop into individual plant. Here the unicellular or filamentous gametophyte (n) alternates with one-celled zygote or sporophyte (2n). The haploid filamentous plants are known as haplonts which reproduce asexually by zoospores or aplanospores producing the individals like parents.

Diplontic type: This pattern is reverse of haplontic type. In this case somatic phase (plant) is diploid (sporophyte 2n) while the haploid phase (gametophyte) is restricted to gmetes which are produced by meiotic division. After gametic union a diploid zygote is formed, which develops into a diploid (sporophyte 2n) plant by mitotic divisions.

Isomorphic type: In this type there are two exactly similar somatic phases (morphologically identical plants) showing alernaion of generations. Here the one phase is diploid (sporophyte 2n) while the other haploid (gametophyte n). This pattern of life cycle is found in ulvaceae, chaetophoraceae, cladophoraceae in chlorophyceae and ectocarpales, dictyotaes of phaeophyceae.

In such cases, the zygote develops into a diploid multicellular plant (sporophyte) by postponement of meiosis. Prior to zoospore (meiospore) formation thee is meiosis. These zoospores (n) develop into haploid plants (gametophyte n). The haploid plants prouce gametes (n) which after fusion develop int zygote (2n)

Hateromorphic type : This pattern of life cycle is exactly like that of the preceding one (isomorphic type) only with the difference that the alternating haploid (n) and diploid (2n) somatic phases (plant) and morphologically different. In such cases the diploid multicellular sporophytic plants produces haploid zoospores (meiospores) by meiosis. these zoospores develop into gameophytes. eah gametophytic plant (n) produce gametes which after their union form a zygote and the latter develops into a diploid sporophytic plant by mitotic division. this pattern is found in laminariales of phaeophyceae.

Haplobiontic type : In this patern there are three phases in the life cycle. Out of three phases two phases are haploid (n) and one diploid (2n). A haploid gametophytic phase produces gametess which on fusion form a zygote (2n). The latte is the only diploid phase which divides meiotically forming a haploid asexual phase known as carposporophyte. The latter reproduce asexually by haploid carpospores which again develop into gametophytes (haploid plants n). Thus two morphologically different haploid phases (gametophyte and carposporophyte) alternate with the zygote (2n)

 

Diplobiontic type : This type of life cycle is found in almost all Rhodophyceae except Nemalionales. Here the life cycle is triphasic and involves an alternation of two diploid (2x) or sporophytic generations. i.e. carposporophyte and tetrasporophyte with one haploid (x) or gametophytic generation. Thus there are two diploid phases and one haploid phase. the gametophyte produce gametes which unite and orm a zygote (2x). Now the zygote divides mitotically forming a carposporophyte (2x) bearing diploid (2x) carpospores. On germination these diploid carpspores form another diploid plant, the tetrasporophyte. The latter produce tetraspores by meiosis. The haploid tetraspores erminate and give rise to haploid gametophytic plants.

Thus there are several pattern of life cycle in algae and there is no regular and fixed alternation of generations as found in higher plants

Life cycle of Ulva

Life cycle of Sargassum

Life cycle of Porphyra