Useful notes on the alternation of generations in algae!
In algae, there is no regular and fixed alternation of generations, as found in higher plants. In all blue green algae, and some Chlorophyceae, such as Protococcus, Scenedesmus, etc., the reproduction is sexual type, and there is no alternation of generations.
In the case of simple unicellular Chlorophyceae, such as Chlamydomonas, Sphaerella, etc., which reproduce sexually, there is no distinct sporophytic generation.
In such cases, the zygote is regarded as a sporophyte, because it is diploid (2n) and during germination undergoes reduction division; producing gametophyte. The life-cycle of such primitive algae consists of an alternation of generations of one celled haploid phase with a one-celled diploid phase.
The above-mentioned primitive type of life-cycle is succeeded by such type of cycle in which the haploid phase, i.e., gametophyte is filamentous showing vegetative cell division and alternates with a one-celled diploid phase, i.e., sporophyte, e.g., in Oedogonium, Ulothrix, Spirogyra, Zygnema, etc. The haploid filamentous plants, are known as haplonts reproduce asexually by means of zoospores or aplanospores.
In the case of Bryopsis and Codium of Siphonales, there is an alternation of generations of one-celled haploid phase with diploid coenocyte or siphonous filament, which is supposed to be morphologically equivalent multicellular diploid generation or diplont.
In the life-cycle of certain Ulvaceae and Cladophoraceae, there is an alternative of many- celled haploid generation with a many-celled diploid generation. In such cases, the two alternating generations are morphologically identical, i.e., isomorphic, and cannot be distinguished from each other until the time of reproduction.
In Ulvaceae and Cladophoraceae the meiosis in the zygote is delayed and the nuclei in the cell of the diploid plants usually do not divide reductionally until the plant is fully mature. Meiosis may take place in the nuclei of .all the cells of diploid plant, such as, in Ulva or may be restricted to the terminal actively growing cell, at the branch apices as in Cladophora.
In one species of Cladophora, i.e., Cladophora glomerata, reduction division does not occur in the development of the zoospores. The zoospores remain diploid like the plant that produces them. They are true asexual spores. In C. glomerata, meiosis occurs in the development of isogametes, which are consequently haploid. The zygote formed with the result of conjugation is diploid so that the haploid generation is represented only by the isogametes.
In this way, two types of life-cycles occur in different species of Cladophora.
In one of the brown algae, i.e., Dictyota the two generations are morphologically identical but physiologically different. In this case, a flat ribbon-like sexual plant is identical with an asexual plant. The sexual plants which are haploid and bear antheridia or oogonia, whereas, the asexual plant which is produced from the oospore is diploid and shows reduction division (meiosis) of the spore mother cell during tetraspore formation.
The tetraspores are haploid and produce the haploid sexual plants which bear antheridia or oogonia on their germination. Such type of life-cycle bears only two phases, i.e., diploid and haploid and is known as biphasic life-cycle. This type of alternation of generations is known as isomorphic or homologous. The life-cycle of Ectocarpus is also of isomorphic type, where the gametophyte and sporophyte are alike.
In other brown algae, the two alternating generations are quite dissimilar. For example, in Laminaria, the sporophyte is a large complex plant and male and female gametophytes are simple and small. The type of life-cycle is also biphasic but the type of alternation of generations is heteromorphic, as the two phases are morphologically dissimilar.
In Fucus, reduction division (meiosis) occurs during the gamete formation in the antheridia and oogonia and therefore, the eggs and sperms become haploid. Such type of life-cycle is found in animals as well as in higher seed plants where the gametophytes are reduced to a few nuclei.
In the red algae, there is a great variation in the life-histories. In these algae, the sexual reproduction and the female sex organs are more complicated. The alternation of generations in the lower genera of red algae such as Batrachospermum and Nemalion is more or less like that of green algae where the plant body is haploid and, therefore, gametophyte.
The zygote formed by the fusion of egg and sperm is only diploid structure. The cystocarp or the fruit body of these algae produced from the oospore, though apparently sporophytic, is haploid because during its formation the zygote nucleus undergoes reduction division.
In these lower red algae, there is a definite alternation of haploid to diploid number of chromosomes and back to haploid. In these cases, there is no alternation of gametophytic and sporophytic generations with different functions. Such life-cycle is triphasic, possessing three phases, e.g., Batrachospermum plant, cystocarp and zygote. The first two phases are haploid and the last phase is diploid one. This way, there is alternation of two haploid generations (gametophyte) with one diploid (sporophyte) generation. Such life-cycle is known as haplobiontic type of life-cycle.
There is distinct alternation of generations in the higher red algae, e.g., Polysiphonia. The plants are different in their chromosome numbers as well as in functions. The sexual plants bear male and female sex organs on them, and the asexual plants bear tetraspores. The tetraspores produce male and female plants (gametophytes) on germination with the result of fertilization the number of chromosomes being doubled, i.e., diploid.
The diploid zygote produces the asexual plant, i.e., tetraspores, bearing tetrasporangia on germination. The tetrasporangia produce tetraspores. The reduction division takes place during tetraspore formation and therefore, the spores become haploid.
Such life-cycle is also triphasic involves an alternation of two diploid or sporophytic generations with one haploid or gametophytic generation. Here, the diploid phases are represented by tetrasporophyte and carposporophyte, and the haploid phase by sexual plants. Such life-cycle is known as diplobiontic type of life-cycle.