Spirogyra ppt.pptx

This PPT gives brief idea about Spirogyra Life Cycle

1. Sem. I, Paper I Chlorophyta SPIROGYRA Associate Prof. Dr. S. A. Gaikwad Dept. of Botany, V. M. Sangola .

2. Unit 5 : Chlorophyta Division Chlorophyta- General Characters 1. Div. Chlorophyta - 1.Chlorophyceae 2. Charophyceae Includes both freshwater (90%) & marine Forms (10%). Chl.a & chl.b are important photosynthetic pigments. Reserved food material is starch . Motile reproductive cells are bi- or quadriflagellate . Flagella are equal in length & whiplash type. Commonly known as “Green algae” .

3. SPIROGYRA * Systematic position / classification Division : Chlorophyta Class : Chlorophyceae Order : Zygnematales Family : Zygnemataceae Genus : Spirogyra

4. Occurrence Spirogyra is a cosmopolitan, freshwater, filamentous green alga, represented by about 300 species. Most of them are green floating in stagnant ponds and pools, a few species are attached and some occur in running water ( S. adnata ). It was discovered by Link. Spirogyra is commonly known as pond silk, water silk, pond scum or mermaid’s trees because of their bright green silky appearance.

5. Structure of Thallus / Filament The plant body of Spirogyra is an un-branched multicelular filamentous thallus measuring about 1 mt. in length. The Spirogyra filament is very slimy due to the presence of mucilage sheath that lines the whole filament. The mucilage often holds the filaments together in a matted blanket. Each un-branched filament of Spirogyra consists of a number of elongated cylindrical cells of similar type joined end to end. The terminal cell is dome-shaped. In attached species, the lower non-green cell is called holdfast or hapteron (with irregular lobes, e.g., S. fluviatilis ). Presence of non-green rhizoids (branches) reported by Jao (1936) in two species, S . rhizobra and S. rhizopus .

6. Cell structure Each cell consists of a cell wall enclosing the protoplast. The cell wall consists of two concentric layers the inner is cellulose in nature. The outer is a pectose layer covered with a mucilage sheath. The protoplast is differentiated into a plasma membrane, cytoplasm, nucleus, one or more chloroplast with pyrenoids and a central vacuole. The single large nucleus surrounded by a tiny mass of cytoplasm remains suspended in the center of the vacuole by a number of cytoplasmic strand. The number of chloroplast varies from one to several. Each chloroplast is a green spiral band and is parietal in position. Each chloroplast bears a number of pyrenoids .

7. Reproduction In the life cycle, the thalloid vegetative filaments multiples by vegetative and sexual reproductions are most common while asexual reproduction occur only occasionally.

8. Vegetative Reproduction During the growing season, the vegetative filament of Spirogyra undergoes fragmentation, so that each fragment independently develops into a new filament by repeated cell division and elongation. In favorable conditions, fragmentation is a most common method of multiplication. The fragmentation is caused by ( i ) mechanical injury (ii) dissolution of middle lamella (Hi) development of H-shaped pieces (e.g. in S. colligates ).

9. Asexual Reproduction Asexual life cycle is less common and reported only in a few Spirogyra sp. Asexual cycles involve the formation of aplanospores , akinetes , azygospores Formation of aplanospores : Under un- favourable conditions, the protoplast of each vegetative cell shrinks and develops a wall around it to form an aplanospore . Each non-motile aplanospore germinates to form a new filament. For example, S. articulate, S. mirabilis etc. In S. aplanospora the life cycle involves the formation of aplanospores only.

10. Formation of akinetes : Under unfavorable conditions, the vegetative cells of S. farlowii develop thick-walled cells called akinetes . On the return of favorable conditions each akinete germinate into a new filament. Formation of azygospores or parthemspores : In the life cycle of S. varians and S. groenlandica , sometimes the gametes fail to fuse and each get enclosed by thick cell wall to become azygospore or parthenospore . Each azygospore germinates in to a new filament.

11. Sexual reproduction Sexual reproduction of Spirogyra involves conjugation, which may be defined as a primitive type of isogamy called aplanogametic isogamy. In this type of isogamy, the entire protoplasmic contents of vegetative cells (viz., Gametangia ) function as gametes. It always involves the gametangial contact. Conjugation is of two type— Scalariform and Lateral conjugation.

12. Scalariform Conjugation :- It is most common method in most heterothallic species of Spirogyra . It takes place when two filaments come close and lie opposite and parallel to each other. Now, opposite cells develop outgrowths or protuberances called conjugation processes. At the point of contact, the tips of these processes dissolve to form conjugation tube between opposite cells. This resembles ladder. Hence, the conjugation is called scalariform (Gr. scala = ladder; forma = shape) or H-shape conjugation.

13. Scalariform Conjugation: When the conjugation tube is forming, the protoplasts of conjugating cells ( gametangia ) recede, round up and function as gametes. The male gametes transfer through the conjugation tube into the opposite cell (female gametangia ) and fuse with female gamete to form zygote (2n). The zygote secretes a thick wall to become zygospore . Therefore, in the late stage of conjugation, male gametangia remain empty and the female gametangia contain zygospores (2n)

14. Lateral Conjugation: - It is of rare occurrence and involves the fusion of gametes from two adjacent cells of the same filament in monoecious or homothallic species. It is of two types: Indirect and Direct Lateral conjugation. Indirect lateral conjugation: Two outgrowths emerge on both side of a transverse septum of two adjunct cells and after some growth the two protuberances unite to form a conjugation tube. Of the two adjacent cells, one behaves as male gametangium whiles the other as female gametangium . The content (now behaving as gamete) from male gametangium passes through conjugation tube and enters into female gametangium .By the fusion of male and female gametes a diploid zygospore is formed. Thus, in each second cell of a filament, a zygospore is formed, e.g., S. tenuissima ,

15. Direct lateral conjugation: In this type of conjugation, the male gametangium after passing through an aperture in the transverse septum of adjoining gametangium enters the female gametangium and fuses with the female gamete, and a diploid zygospore is formed, e.g., S . jogensis

16. Germination of Zygospore : Zygospore is the only diploid phase in the sexual life cycle. The decay of female gametangia causes the liberation of zygospore which remains dormant in the pond bottom till the favorable conditions return. Zygospore wall is thick and differentiated in to 3-layers. Outer exosporium (cellulose), middle mesosporium (cellulose & chitin) and inner endosporium (cellulose). On return of favourable condition the diploid nucleus of zygospore undergo meiosis to produce 4 haploid nuclei out of which 3 degenerate. The zygospore with one haploid nucleus gradually enlarges and burst open to release a germ tube. The repeated transverse divisions of germ tube form a new filament.