Types of life-cycles in algae:
The growth and development of an algae passes through a number of distinct morphological and cytological stages. The sequence of these orderly changes is said to be life-cycle or life history. In other words, the sequence of all the different phases or events, through which an organism passes from the zygote of one generation up to the zygote of next generation, is known as its life cycle. The union of male and female gametes results in the formation of zygote. As there is doubling of chromosomal complement in zygote, it represents diploid state.
In some groups the original chromosome number is achieved by meiotic division of the zygote nucleus. Thus on germination the zygote produces haploid plants. In others, the zygotic nucleus divides mitotically and the resulting cells from a diploid phase, which later produces cells which are capable of meiotic division.
The various types of life-cycle met among algae can be broadly classified into the following five categories.
Haplontic type:
In this type, plant is haploid and bears haploid gamets in the gametangium. The gametic fusion results in the formation of a diploid zygote which is the only diploid phase in the life cycle. The zygote nucleus divides meiotically to produce four meospores, each of these develops into a new individual. Thus there is an alternation of a haploid plant with a diploid zygote. This type of life cycle occur in the motile unicellular forms and most multicellular green algae.
Diplontic type:
Here the plant is diploid and it bears sex organs which are also diploid. The reduction division takes place at the time of gamete formation and gametes become haploid. After gamete fusion the diploid stage is re-established in the form of zygote. The zygote does not undergo any reductional division and it give rise to a diploid plant body. Thus, there is an alternation of a diploid plant with haploid gametes. The gametes represents the only haploid stage in the life cycle of the plant.
Isomorphic type:
In this type of life cycle, there is an alternation of two generation which are externally similar, but one is haploid producing gametes and the other diploid producing zoospores. The zygote germinate directly into a diploid plant without undergoing reduction division. Sporangia develop on the diploid plant body and reduction division takes place prior to formation of zoospores. The haploid zoospores thus formed grow into haploid plants. Sex organ develop on the haploid plants and these give rise to haploid gametes. The haploid gamete fuse to form diploid zygote.
Heteromorphic type:
In this type of life cycle, both diploid and haploid plant bodies are morphologically distinct and they alternate with each other. Mostly the sporophyte is large and the gametophyte small. Heteromorphic life cycle seen mainly among brown algae.
In genera like Cutleria and Urospora, haploid gametophytic plants are larger where as diploid sporophytic plants are smaller in size.
Triphasic type:
In triphasic life cycle, there is a succession of three generations It may be of the following 2 types:
Gonimoblast filaments are found from the basal part of the carpogonium; the upper most cell of these filament function as carposporangium. The first division in the zygote nucleus is meiotic and as such the gonimoblast filaments are haploid. The haploid carposporangium bears a haploid carpospore. The gonimoblast filaments, carposporangia and carpospores are enveloped by numerous sterile filaments, and together represent carposporophyte generation which is haploid. On liberation, carpospores germinate into heterotrichous chantransia stage. The projecting filaments of the latter develop into new gametophytic plant body.
ii) Diplobiontic:
In diplobiontic triphasic life cycle there are two diploid phases alternating with haploid gametophytic phase. This type of life cycle is well exemplified by Polysiphonia. Here, the haploid phase is represented by male and female gametophytic plants, sex organs and gamets. The male and female gametes fuse to form a diploid zygote. The zygote produces gonimoblasts which represents an additional diploid phase. Diploid carpospores are formed in the carposporophyte. On liberation, the carpospore germinate to produce a diploid tetrasporophyte plant. The tetrasporophytic plant produces diploid tetrasporangia. Four haploid tetraspore are formed in s tetrasporangium by reduction division. Tetraspores give rise to haploid gametophytic plants.