Types of Reproduction:
The reproduction of algae can be explained under three types, namely: vegetative, asexual and sexual reproduction.
The most common kind of reproduction in algae is through binary fission. In unicellular prokaryotic algae such as Anacystis it is the only process of reproduction found in nature. In filamentous and multicellular forms, the algae might get broken by accident into small pieces, 1each building up into a new one.
It takes place by the formation of different kinds of sporangia. The spore escape from the parent plant, experience germination and give mount to new plants. All types of spores that the exception of zoospores are non-motile. The most common kinds of asexual spores, found in algae, comprise zoospores, autospores, aplanospores, monospores, hypnospores, exospores, carpospores, endospores, tetraspores and paraspores.
a) Zoospores: flagellated, asexual reproductive bodies frequently by means of an eyespot.
b) Synzoospore: multinucleate, multi-flagellate zoospore or compound zoospore.
c) Aplanospore: non-motile thin walled zoospore made by cleavage of protoplast in a cell.
d) Hypnospore: thick walled Aplanospore.
e) Autospores: thin walled spores made by parent cell division.
f) Tetraspore: 1n, thin walled, non-motile spores made after reduction division in 2n tetrasprangia.
Sexual reproduction in algae such as in other organisms comprises the fusion of two cells from opposite sex termed as gametes, resulting in the formation of a zygote. Some fundamental features of this process of reproduction are as follows:
Gametes are for all time haploid and might or might not be different in morphology. When both the sex cells look similar, they could be male termed plus (+) or female termed minus (-) mating types of strains. Gametes can fuse only if one is plus and the other is minus. Both of them + and - might be produced through a single parent. This is termed as monoecious or homothallic condition. If they come from different plus or minus thallus types it is termed as dioecious or heterothallic condition.
There are three kinds of gametic fusion:
a) Isogamy: If both the gametes are of similar size and morphology.
b) Anisogamy: The two gametes are definitely different in size or shape, the larger of the two is minus (female) kind.
c) Oogamy: The female gamete, ovum or egg is big in size and consists of no flagella therefore it is non-motile. Male gametes are flagellated and extremely motile. They are as well termed as antherozoids, spermatozoids and sperms.
The male gametes are attracted through the female cells because of special hormones termed as gamones (a volatile hydrocarbon) formed by them.
Fusion of the gametes leads to the development of a zygote. When the conditions are inappropriate for growth, the zygote might build up a thick wall and become a resting zygospore. Gametes being haploid are generated by mitosis in a haploid thallus. When the thallus is diploid as in Fucus, then the reproductive cells experience meiosis or reduction division to form haploid gametes.
In haploid thallus, after the fusion of gametes, the diploids zygote experiences meiosis throughout germination. Though, in diploid algae a zygote might divide or split mitotically and give mount to a diploid thallus (that is, Fucus).
Both haploid and diploid thallus is found in the Ulva. They look much identical in size and shape.
Life cycle in algae:
1) Haplontic type:
This is the simplest and most primitive kind of life-cycle. The other prototypes of life-cycle encompass originated from this kind. This kind is found in all Chlorophyceae apart from a few. At times this is termed as Ulothrix or Chlamydomonad kind.
In such situations, the somatic phase (plant) is haploid (that is, gametophyte) whereas the diploid phase (that is, sporophyte) is showed by zygote. All through germination, the zygote (2n) divides or splits meiotically producing haploid (n) zoospores that build up into individual plants.
Here the unicellular (example: Chlamydomonas) or filamentous (example: Ulothrix, Spirogyra, Oedogonium, Chara and so on) gametophyte (n) alternates by a one-celled zygote or sporophyte (2n). The haploid filamentous plants are termed as haplonts which reproduce asexually through zoospores or aplanospores producing the individuals similar to parents.
2) Diplontic type:
This prototype is reverse of Haplontic type. In this situation, somatic phase (that is, plant) is diploid (sporophyte 2n) whereas the haploid phase (gametophyte n) is limited to gametes that are produced through meiotic division. After gametic union, a diploid zygote is made, which builds up into a diploid (sporophyte 2n) plant through mitotic division. The well known illustrations of this pattern are: Fucus, Codium, Sargassum, Bryopsis and so on.
3) Isomorphic type:
In this kind, there are two precisely similar (morphologically similar) somatic stages (plants) exhibiting alternation of generations. Here the one stage is diploid (sporophyte 2n) whereas the other haploid (gametophyte n). Among Chlorophyceae, this is found in the Ulvaceae, Chaetophoraceae and Cladophoraceae. The orders Ectocarpales, Cutleriales, Tilopteridales, Sphacelariales and Dictyotales of Phaeophyceae as well represent this pattern of life-cycle.
4) Heteromorphic type:
This prototype of life-cycle is precisely similar to that of preceding one (isomorphic type) only by the difference that the alternating haploid (n) and diploid (2n) somatic phases (plant) are, morphologically dissimilar. In such situations, the diploid multicellular sporophytic plant generates haploid zoospores (meiospores) through meiosis. Such zoospores build up into gametophytes. Each gametophytic plant (n) generates gametes that after their union form a zygote and the latter builds up into a diploid sporophytic plant through mitotic divisions. This pattern is found in Sporochnales, Laminariales, Desmarestiales and so on of Phaeophyceae and Urospora of the Chlorophyceae.
5) Haplobiontic type:
In this prototype, there are three stages in the life-cycle. Out of three, two stages are haploid (n) and one diploid (2n). The illustrations are found among Nemalionales of Rhodophyceae, and Coleochaete of Chlorophyceae.
6. Diplobiontic type:
This kind of life-cycle is mainly found in nearly all Rhodophyceae apart from Nemalionales. The most general illustration is Polysiphonia of order Ceramiales. In this, the life-cycle is triphasic and comprises an alternation of the two diploid (2n) or sporophytic generations, that is, carposporophyte and tetrasporophyte having one haploid (n) or the gametophytic generation.
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