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Name derivation:
‘Fierce Roller’ -- To roll Volvo
Classification:
Volvox Linnaeus 1758; 24 of 135 species descriptions are currently accepted taxonomically (Guiry and Guiry 2013). Order Chlamydomonadales; Family Volvocaceae
Morphology:
Spherical colony of up to 2^15 (32,768) individual cells stuck in the periphery of a gelatinous matrix. The colony sphere is hollow. Cells range in shape from spherical to star shaped with two equal flagella and a stigma; they are all enclosed by a species dependent gelatinous sheath and connected by cytoplasmic strands. The flagellated cells propel the entire colony causing it to both rotate and translate in motion. A small number of cells lose their flagella enlarge to many times their orignal volume without dividing. Subsequently they divide into the species number of cells, separating growth from cell division (‘palintomy’), consistent with other volvicine genera starting with Chlamydomonas (Leliaert et al. 2012).
Reproduction:
Volvox can reproduce either asexually or sexually, and in either case reproductive cells develop synchronously. Asexually no gametes, no nuclear fusion and no meiosis occur. Two cell types are specialized with division of labor (function) – flagellated cells that never reproduce (the vast majority of colonial cells), and non-flagellated ‘gonidia’ relegated only to division. Sexually the few reproductive cells enlarge and produce either packets of 64 or 128 elongate sperm (microgametes) shaped like bundles of miniature matchsticks, or one large egg per cell. Sperm are released from their 'packet' and attracted to an egg, presumably guided by a pheromone release. Fusion occurs followed by meiosis, and the zygote begins repreated mitotic divisions until the species specific number of cells is produced in a miniature colony. At this point parallels can be drawn to vertebrate embryo development from morula through blastula to gastrula stages. In the ‘blastula’ stage flagella are formed, facing inward toward the hollow interior of the miniature colony. A rapid series of 11 or 12 cell divisions occurs (cleavage stage lasts less than 7 h), 35 minutes per division cycle (Kirk and Nishii 2001). Afterward cells in the embryo change from pear-shape to spindle shape causing contraction of the embry and opening of the ‘phialopore’ through which cells micgrate (Viamontes and Kirk 1977). At this point a kind of gastrulation occurs with inversion by cell migration so that the flagella are now facing outward ready to propel the colony. Asexual development is similar minus the sexual stage. The few asexual reproductive cells called gonidia that act like undifferentiated stem cells are located toward the posterior pole of the spherical colony, generally in quartets. As they enlarge they go through the stages described above, producing the next generation of both cell types, flagellated and non-flagellated reproductive gonidia. (Kirk and Nishii 2001). The flagellated cells propel the entire colony causing it to both rotate and translate in motion. A small number of cells lose their flagella enlarge to many times their orignal volume without dividing (the gonidia). Subsequently they divide into the species number of cells, separating growth from cell division (‘palintomy’), consistent with other volvicine genera starting with Chlamydomonas (Leliaert et al. 2012). For far greater detail on 1) morphogenesis and the role of a complex cytoskeleton of microtubules in Volvox and related flagellated colonial genera, as well as in Chlamydomonas and related flagellated unicellular genera; 2) a mechanism for colony inversion (‘gastrulation’); and 3) genetic analysis of mutants, is found in Kirk and Nishii (2001).
Similar genera:
Eudorina and Pleodorina have fewer cells and a higher proportion of gonidia.
Habitat:
Freshwater, widely distributed, more common in richer waters.
References:
Guiry, M.D. and G.M. Guiry 2013. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 10 September 2013. Kirk, D.L. and I. Nishii 2001. Volvox carteri as a model for studying the genetic and cytological control of morphogenesis. Developmental Growth Differentiation 43:621-631. Leliaert, F., D.R. Smith, Herve Moreau et al. 2015. Phylogeny and molecular evolution of the green algae Critical Reviews in Plant Sciences 31:1-46. online 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. Viamontes, G.K., and D.L. Kirk 1977. Cell shape changes and the mechanism of inversion in Volvox. The Journal of Cell Biology 75:719-730. online
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