Reproduction and Life Cycle in Algae:
It is to be noted that life cycle of alga is handled by environmental factors such as light, temperature, seasons, and availability of nutrients, and also salinity, wave action and periodicity of tides in case of marine forms. Observations prepared by people in different times from different geographical locations and at times experimentally studied under controlled surroundings, provide comprehensive if not complete picture of life cycle of alga.
Sexual reproduction in the alga illustrates all three different kinds depending on species. Isogamy is discovered in C.moewusii, C.gynogama and C.media
Isogamy is of 2 kinds:
In clonal populations (cells attained by repeated divisions of single parent cell) fusion may occur between gametes that are homothallic or in self compatible strains. For instance, fusion happens between any 2 cells of C.gynogama and C.media.
In C.moewusii and C.reinhardii fusion of gametes can occur only when they come from 2 different unconnected (heterothallic, self incompatible) strains.
In several isogamous scies parent cell may split to produce 16 to 64 biflagellate gametes whereas in some adult cell themselves may directly act as gametes and fuse. Anisgamous form of gametic fusion is discovered in C.braunii. The female cell separates and produces 4 large cells. Each of these cells has two flagella but is less active. Male cells are approx 8 in number but smaller in size.
Oogamy is advanced kind of sexual reproduction found in C. coccifera. The parent cell discards its flagella and directly turns into a non-motile egg or ovum. Whereas male parent cell by repeated divisions generates sixteen male gametes. These are biflagellate and extremely motile. Process of gametic attraction, fusion and connected phenomena have been examined is some detail in laboratory. In proper light condition and carbon dioxide concentration, production of gametes can be started by nitrogen starvation. Formation of male or female gametes (even in case of isogamy) is attributed to differing concentration of gamones made by them. Attraction between gametes was discovered because of presence of glycosidic mannose at tips of flagella of one strain that in complementary way binds with substance present in flagella of gamete of opposite strain. Once this sticking of flagella of plus and minus gametes occurs, flagella twist about each other bringing anterior ends of gametes close. This is followed by cellular and nuclear fusion.
Zygote secretes the thick wall and collects large amount of food materials such as lipids, starch, and orange-red pigments. It is now called as zygospore that remains dormant till environmental conditions are favorable for the germination. It has been illustrated that in germination of zygospore meiosis occurs followed by mitosis resulting in haploid Chlamydomonas cells.
Life Cycle of Chlamydomonas:
Chlamydomonas is unicellular, haploid and reproduces asexually several times by forming zoospores. Under unfavorable environmental conditions it generates gametes that fuse to form diploid zygospores. In germination reduction division occurs and haploid cells are generated. Chlamydomonas is of great concern to biologists. There are many interesting features of biological significance of Chlamydomonas, which are given below:
i) Existence of DNA in chloroplasts of alga.
ii) Existence of cytoplasmic genes.
iii) Construction of genetic mutations-affecting nutrition, photosynthesis and production of mutants lacking flagella or cell wall.
iv) Discovery of gamones and role in sexual reproduction.
v) Existence of anisogamy, isogamy and oogamy in the single genus
vi) Control of reproduction through environmental conditions.
Sexual reproduction occurs by isogamous, biflagellate gametes. Fusion occurs only between plus and minus mating types. Gametes are from different filaments (heterothallic). Zygote grows thick wall and stays dormant till conditions are favorable for germination. When conditions turn into favorable meiosis occur and 4-16 haploid zoospores are made that settle down and give rise to vegetative filaments. It has been discovered that Ulothrix generates gametes when developed under long day conditions while short day conditions begin formation of zoospores.
Life Cycle of Ulothrix:
The thallus of Ulothrix is haploid and diploid stage is signified by zygote only. In few species (U.specuiosa, U.flcca and in U.implexa) zygote grows into the independent, univellular thallus that is diploid in nature. It generates zoospores asexually by meiosis. Zoospores grow in haploid filaments.
Therefore in Ulothrix two kinds of life cycles can be differentiated:
Thallus is haploid and only zygote is diploid like U.zonata.
In diplobiontic cycle, alga comprises of haploid thallus which generates gametes and diploid unicellular stalked thallus that produces zoospores after meiotic division. Two generations - haploid and diploid, alternate with each other. (Alternation of generations). As two thalli are extremely different in size and morphology it is called as heteromorphic, diplobiontic life cycle.
The thalli of sporophyte and gametophyte. Both are morphologically alike. Though, gametophyte is hploid(n) while sporophyte is diploid(2n). Haploid gametophyte generates gametes and diploid sporophytes produces after meiosis zoospore which germinate to form haploid gametophytes. Gametophytes of Ulva generate gametes that are isogamous or anisogamous. After fusion zygote is formed that develops into diploid sporophyte. Life cycle of Ulva is explained as isomorphic, diplobiontic type.
Sexual reproduction in Laminaria is oogamous type. Mature diploid thalli of sporophytes generate sori or unicular sporangia on surface of lamina. Every sporangium splits by meiosis to give rise to 32 biflagellate zoospores that germinate to create male and female gametophytes.
Gametophytes of both sexes are microscopic with the few branches and fertility is handled by environmental conditions. Any cell of female gametophyte can grow into oogonium, contents of which form single egg. Egg protrudes out when mature but remains joined to mouth of empty oogonial cell Antheridia are made singly as lateral outgrowths of male gametophyte. Just 1 sperm is made from each antheridium, that is pear shaped and includes two flagell of unequal length. After fertilization zygote directly splits mitotically without any resting period and grows into sporophyte.
Life Cycle in Laminaria:
In Laminaria there is distinct modification of haploid gametophyte and dominant diploid sporophyte. Reduction division occurs in sporagia of sporophyte before formation of zoospore that germinate to form male and female gametophytes.
Two dissimilar generations - one simple filamentous gametophyte and other highly differentiated, complex multicelluar thallus - alternate with each other therefore life cycle is known as heteromorphic alternation of generations.
Focus has advanced kind of reproductive structures, known as receptacles, that are swollen at tips of branches. Distributed over surface of every receptacle are small pores, called as ostioles that lead in cavities, known as conceptacles. Every conceptacle may generate only eggs, only sperms or as in some cases both. The thallus may be unisexual - either including male receptacle or just female ones. At base, inside conceptacle is fertile layer of cells that grows into oogonia. Every oogonium has basal stalk cell and upper cell that experience reduction division and produces 8 haploid eggs. These are liberated in conceptacle. Some of cells inside conceptacle generate unbranched multicellular hairs known as paraphyses that emerge out of ostoile as tufts. Antheridia are made on branched paraphyses inside concepacle. Every antheridium is like unilocular sporangium that splits meiotically and then by further divisions generates 64 haploid sperm. Biflagellate sperm has longer flagellum pointing backwards and shorter one projecting toward front. It includes single chloroplast and prominent orange eye spot. Release of gametes is connected with sea tides. At low tide, Fucus fronds shrink because of loss of water released in surrounding sea water. Eggs of Fucus are known to attract sperms by secreting gamone. Immediately after fertilization the wall is secreted around zygote. It has been illustrated that unfertilized eggs can grow in germlings parthenogenetically if treated with dilute acetic acid. Diploid zygote germinates by producing the rhizoidal outgrowth on one side. It is afterward cut by further divisions gives rise to focus fronds.
Life Cycle of Fucus:
Fucus plants are diploid and haploid stage is signified by gametes only. Life cycle of focus is explained as diplontic life cycle. Four essential kinds of life cycles described are summarized. When dominant stage is haploid gametophyte, life cycle is known as haplontic life cycle. In cycle diploid state or sporophyte is signified by zygote that produces spore by meiosis which develops in gametophytes. In diplontic cycle Main or dominant stage is diploid sporophyte. Zygote directly germinates into sporophyte. Afterward meiosis occurs producing haploid gametes which fuse to form zygote. In diplontic algae it is to be noted that no free living haploid thalli are discovered.
When both gametophyte and sporophyte are evenly developed and look morpholoigically related, we have isomorphic alternation of generations. Though, if gametophyte is underdeveloped compared to sporophyte the life cycle is called as heteromophic alternation.
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