Volvox carteri

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cellular organisms - Eukaryota - Viridiplantae - Chlorophyta - Chlorophyceae - Chlamydomonadales - Volvocaceae - Volvox - Volvox carteri

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Brief facts

• Volvox is a spherical multicellular green alga, which contains many small biflagellate somatic cells and a few large, non-motile reproductive cells called gonidia, and swims with a characteristic rolling motion with a distinct anterior and posterior.
• The name Volvox comes from the Latin volvere, to roll, and -ox, as in atrox, fierce.
• A medium in which the organism would thrive and reproduce in captivity was discovered only in the 1960s. Only after that it became possible to exploit Volvox as a laboratory model system.
• In nature Volovox is found in ponds and ditches.

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Life cycle

Volvox has 2 modes of reproduction: sexual and asexual.

• sexual in nature V. carteri reproduces sexually at least once each year when temporary ponds where the organism lives start to dry out in the heat of late summer; the stimulus for switching from the asexual to the sexual mode of reproduction is known to be a sex-inducing pheromone, a 32-kDa glycoprotein triggers sexual development of gonidia at concentrations as low as 10(-16) M.
  • sexual induction gonidia that have been exposed to the sex-inducing pheromone for at least 6–8 h before the initiation of embryonic cleavages modify their developmental program and produce sexual progeny containing immotile eggs or motile sperm, depending on the genetic sex of the individual; the sexual cycle is initiated by a heat shock that causes the somatic cells of the asexual Volvox spheroid to produce the sex-inducing pheromone; the level of pheromone is then further amplified by the ability of sperm cells to produce more sex-inducing pheromone
  • gametogenesis
    MeSH the 64–128 cell transition in sexual females, and the 128–256 cell transition in sexual males; in sexual males, somatic cells (smaller spheres) and androgonidia (larger spheres) arise in a 1:1 ratio; androgonidia undergo further cleavages to form sperm packets, each containing 64 or 128 sperm; when the gametes are mature, sperm packets are released into the surroundings
  • zygote MeSH on contact with females, the sperm packets break up into individual sperm, which can fertilize the eggs. The resulting diploid zygotes have tough cell walls that can resist drying, heat and cold. When favourable conditions cause the zygotes to germinate, they undergo meiosis to produce haploid males and females that reproduce asexually until the sex-inducing pheromone induces the sexual cycle again
• asexual males and females are indistinguishable in their asexual form; under standard conditions, the asexual life cycle takes 48 h and is synchronized by a 16 h light–8h dark cycle
  • embryogenesis MeSH
    • cleavage embryogenesis takes ~8 hours; mature gonidia undergo a rapid series of cleavage divisions (11–12 divisions), some of which are asymmetric: the larger cells resulting from these unequal divisions will become the gonidia of the next generation, whereas the smaller cells will become part of the somatic cell population; at the end of cleavage, the embryo is inside out with respect to the adult configuration: its gonidia are on the outside and the flagella of its somatic cells are pointing towards the interior of the hollow sphere
    • inversion the morphogenetic process of inversion taking place at the end of embryogenesis returns the embryo to its adult configuration through a series of cell movements that resemble the gastrulation of animal embryos
    • expansion the juveniles expand by deposition of extracellular matrix
  • release juveniles hatch from their parent spheroid
• juvenile organism with immature gonidia
• adult organism with mature gonidia
• senescent MeSH somatic cells of the parent, lacking reproductive cells and thus being incapable of further cleavage, undergo senescence and die

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References

Articles

• Hallmann A, Godl K, Wenzl S, Sumper M.The highly efficient sex-inducing pheromone system of Volvox. Trends Microbiol. 1998 May;6(5):185-9. PMID: 9614342
• PubMed free full text articles: major topic "Volvox"

Websites

• CELL BIOLOGY: ON VOLVOX
• Wikipedia: Volvox

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Appendix

Nematollahi G, Kianianmomeni A, Hallmann A. Quantitative analysis of cell-type specific gene expression in the green alga Volvox carteri. BMC Genomics. 2006 Dec 21;7:321. PMID: 17184518

Phenotype of Volvox carteri and appearance of separated cell types. A) Wild-type phenotype of an asexual female of Volvox carteri f. nagariensis containing ~2000 small, terminally differentiated somatic cells at the surface and ~16 large reproductive cells (gonidia) in the interior. More than 95% of the volume of such a spheroid consists of a complex but transparent extracellular matrix. B) Isolated somatic cell sheets of V. carteri. C) Isolated gonidia of V. carteri.

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