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Life History of Volvox Systematic Position Phylum

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Life History of Volvox Systematic Position Phylum Life history of Volvox Systematic position Phylum - Chlorophyta Class - Chlorophyceae Order - Chlamydomonadales Family - Volvocaceae Genus - Volvox Habitat - fresh water, Ponds, ditches, lagoons, shallow puddles. Thallus structure – Volvox is a prophyletic genus in the volvocine green algae. It consists of around 20 species of freshwater algae. Volvox is unicellular but remains in hollow spherical colonial forms. Thousands of Volvox individual forms a colony covered under mucilaginous sheath composed of glycoproteins. Colony of volvox is coenobium. Colony is spherical in shape and number of individual ranges from 500 to 60000 volvox individual in a single colony. A single volvox colony measures about 0.5mm in diameter. Volvox species is classified on the basis of number of individuals in a colony. Volvox was firsts of all reported by Antony van Leeuwenhoek in 1700. Common species of Volvox is Volvox aureus, V. carteri, V. barberi etc. Colony consists of two types of cells viz. germ cells and flagellated somatic cells. The individual cells of colony are spherical in shape with cup-shaped chloroplast. Chloroplast contains pyrenoids. The cells have nucleus, eye spots, vacuoles and two equal-sized flagella at their anterior end. Flagella facilitate movement of colony in the water body. Although, volvox appear in colony, but all the members of colony perform their metabolic activity independently. Each individual of colony is interconnected with cytoplasmic strands. Eye spot present near the anterior end helps in light reception. Volvox significantly support native environment by adding oxygen to it. It has been reported that rapid multiplication of V. aureus may result into harmful algal bloom development in the pond. Life cycle Life cycle of Volvox is haplontic. It means that life cycle of Volvox is dominated with haploid (gametophyte) generation and diploid phase (sporophyte) is represented only by zygote. Volvox reproduces both sexually and asexually. Sexual reproduction occurs during unfavourable condition while asexual reproduction occur during favourable condition. Asexual Reproduction Asexual reproduction is followed with development of some cells of posterior part of colony become reproductive. These cells lose flagella and become enlarge and rounded. Cells thus developed are called as gonidia. Gonidia also lose eye spot but number of pyrenoids increased. Now gonidia are pushed towards interior of the colony and first division of gonidia occur longitudinally to plane of the coenobium resulting into formation of two cells. Second division is also ;ongitudinal but right angle to first developing four cells. Third longitudinal division produces 8 cells i.e. four central and 4 peripheral arranged in curved-plate-like structure. 8-celled stage is called plakea stage. Now, 8 cell divides longitudinally to form 16 cells and are arranged in the form of hollow sphere. Hollow sphere is open at exterior end called phialopore. Under this condition, cells divide and redivide till the the no of cells become equal to the characteristics number of that particular species. At this stage, cells are naked and pointed end of cells are towards inside. Now inversion of colony takes place in which pointed end is directed towards anterior end due to inversion. Inversion of colony It is the most important steps in both sexual and asexual reproduction. In this case, pointed anterior end directed towards interior of the colony is reversed towards periphery. Inversion of new colony starts with development of constriction of opposite end of phialopore. The cells of posterior end along with constriction are pushed inside the hollow sphere of colony. This processes of pushing continues till the whole structures come out of phialopore. Such process of inversion results into the development of anterior end of cells/individual of colony towards periphery. Now, phialopore closes and forms the periphery of the colony. After inversion, cells develop cell wall, flagella and eye spot. Each cell becomes separated due to development of gelatinous sheath around each cell. In this way, new colony i.e. daughter colony is formed. At this stage, daughter colony remains attached to gelatinous wall of the parent colony. Daughter colony is released in water after disintegration of parent colony. In this way, Volvox reproduces asexually. Sexual Reproduction: The sexual reproduction in Volvox is oogamous type. Volvox species are either monoecious or homothallic e.g., V. globator or dioecious or heterothallic e.g. V. rousseletii. In monoecious species both sex organs i.e. antheridia and oogonia develop on same plants whereas in dioecious, both sex organs develop on different plants. Monoecious species are mostly protandrous i.e. antheridia develop before oogonia. But, some species are prototogynous too. Both sex organs develop in posterior part of the colony. Male reproductive cells are called antheridia or androgonia and female sex organs are called oogonia or gynogonidia. Initially, reproductive cells enlarge and lose flagella. Such cells are called gametangia. Development of Antheridium The development of antheridium starts with formation of antheridial initial or androgonidial cell in posterior side of the colony. Antheridial initial lose flagella, become enlarge and dense in protoplasm. At this stage, antheridial initial reamin connected to other cells with cytoplasmic strands. Protoplast of antheridial initial divides longitudinally and form around 16-512 elongated cells. The cell remain arranged in palte-like structure or in a hollow sphere. Here, inversion of cells takes place as discussed in asexual reproduction. Each cell differentiate into spindle-shaped, elongated, biflagellated antherozoids with two contractile vacuole, nucleus, cup-shaped chloroplast, pyrenoid and eye spot. Antherozoids are generally pale yellow or green in colour. On maturity, anthrozoids are released individually or in group for fertilization with female gametes. Development of Oogonium: Likewise antheridia, oogonia also develop in the posterior region of the colony. Oogonial initial enlarges their cell size as well as nucleus and loses flagella and eye spot. Protoplast becomes dense and pyrenoid appears. The mature ovum is either ovoid or flask-shaped. The egg is uninucleateand and beak of flask shaped oogonium behaves like receptive spot. Fertilization in Volvox: After liberation of antherozoids and maturity of egg, antherozoids swim and reach to oogonia due to chemotactic response. Out of many antherozoids, only one enters inside oogonum to fertilize female nuclei. First of all, fusion of protoplast i.e. plasmogamy occurs which is followed by fusion of nucleus i.e. karyogamy resulting into formation of zygote. Zygote secretes a three layered thick wall i.e. exospores (thick), mesospore (thin and smooth) and endospore (thin and smooth) and become dormant. Wall contains haematochrome pigment which provides red colour to zygote. Germination of Zygote: Onset of favourable condition, dormant zygote divides through meiotic division and forms four haploid cells. At this stage, exospores and mesospore burst and endospore comes out as vesicle. The four haploid cells migrate along with the vesicle. Zygote divides to form cells reaching to the characteristics number of that particular species. In some cases, zygote produces biflagellate zoospore which divides and increase the number of cells up to characteristics of that species. Further, inversion of daughter colony as discussed in asexual reproduction and mature daughter colony of volvox is formed.In this way, Volvox reproduces and maintains its species. Fig.- Germination of Zygote Graphical Representation of Life Cycle of Volvox .
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