Phycology جلبک شناسی

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Volvox

Volvox Linnaeus, 1758: 820

Classification:
Empire Eukaryota
Kingdom Plantae
Subkingdom Viridaeplantae
Phylum Chlorophyta
Class Chlorophyceae
Order Volvocales
Family Volvocaceae

Lectotype species: Volvox aureus Ehrenberg

Original publication: Linnaeus, C. (1758). Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Editio decima revisa. Vol. 1 pp. [i-iv], [1]-823. Holmiae [Stockholm]: impensis direct. Laurentii Salvii.

 

Type designated in Ehrenberg, C.G. (1838). Infusionsthierchen als vollkommene Organismen. I-II. pp. 1-547, 64 pls.. Leipzig: Leopold Voss.

Taxonomic status: currently recognized as a distinct genus.

Description: Colonies spherical, subspherical, ellipsoidal or ovoid, containing 500-50,000 cells arranged at the periphery of a gelatinous matrix, forming a hollow sphere. Two to 50 large reproductive cells (gonidia) situated in posterior 1/2 to 2/3 of colony. Each cell enclosed by gelatinous sheath which is distinct or confluent (species dependent). Somatic cells spherical, ovoid or star-shaped, each with two equal flagella, a stigma, two contractile vacuoles at base of flagella, and a cup-shaped chloroplast with single pyrenoid. Cytoplasmic strands between cells are thick, thin or absent and this is species dependent. Asexual reproduction by autocolony formation, each reproductive cell dividing successively to form a hollow, spheroidal plakea, which then inverts to form a daughter colony. Sexual reproduction oogamous; in monoecious species the sexual colony with both sperm packets (bundles of sperm) and eggs. In dioecious species, male colonies with androgonidia which divide successively into sperm packets; such males may be markedly reduced in size (dwarf male) or nearly as large as asexual colonies. The female colony has eggs, whose number is nearly the same as that of gonidia in asexual colonies (facultative female) or much larger (special female). After fertilization zygotes develop a heavy cell wall that may be ornamented with reticulation or spines. Upon germination, zygotes produce a single biflagellate gone cell. Volvox is cosmopolitan in freshwater; V. aureus is common. Volvox was established with V. globator and more than 12 additional species have been described. The genera Besseyosphaera, Campbellosphaera, Merrillosphaera, Copelandosphaera and Janetosphaera were described by Shaw but are now included in Volvox. Each of these genera was later considered to be a section of Volvox, with Euvolvox [=Volvox] including species referable to Volvox by Shaw. Later, the sections Campbellosphaera and Besseyosphaera were rejected and 18 species were recognized in four sections based on differences in the gelatinous matrix, shape of somatic cells, and cytoplasmic strands connecting cells in the colony. Embryogenesis of V. carteri has been studied in detail, and chemical induction of sexual colonies were described for V. aureus, V. carteri, V. rousseletii, V. gigas, V. observus and V. capensis. Inducers are species-specific glycoproteins of ca. 30,000 MW produced by sperm, except in V. capensis where L-glutamic acid is reported. Ultrastructure of flagellar apparatuses in V. rousseletii and V. carteri implied a remote relationship. The extracellular matrix of the four sections of Volvox has also been studied ultrastructurally, and a system of nomenclature was proposed in which the matrix was divided into four main zones

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نویسنده : رضا رمضان نژاد قادی ; ساعت ٤:۱۱ ‎ق.ظ روز یکشنبه ٥ دی ۱۳۸٩