Volvox

Volvox is a polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Antonie van Leeuwenhoek in 1700. Volvox diverged from unicellular ancestors approximately 200 million years ago.[1]

Description

Volvox colony: 1) Chlamydomonas-like cell, 2) Daughter colony, 3) Cytoplasmic bridges, 4) Intercellular gel, 5) Reproductive cell, 6) Somatic cell.

Volvox is the most developed in a series of genera that form spherical colonies.[2] Each mature Volvox colony is composed of numerous flagellate cells similar to Chlamydomonas, up to 50,000 in total,[1] and embedded in the surface of a hollow sphere or coenobium containing an extracellular matrix[1] made of a gelatinous glycoprotein.[3] Except during the formation of daughter colonies, vegetative cells comprise a single layer with the flagella facing outward. The cells swim in a coordinated fashion, with distinct anterior and posterior poles. The cells have eyespots, more developed near the anterior, which enable the colony to swim towards light. The individual algae in some species are interconnected by thin strands of cytoplasm, called protoplasmates.[4] They are known to demonstrate some individuality and working for the good of their colony, acting like one multicellular organism.

Reproduction

An asexual colony includes both somatic (vegetative) cells, which do not reproduce, and large, non-motile gonidia in the interior, which produce new colonies through repeated division. The daughter colonies are initially held within the parent coenobium and have their flagella directed inwards. Later, the parent disintegrates and the daughters invert. In sexual reproduction two types of gametes are produced. Volvox species can be monoecious or dioecious. Male colonies release numerous microgametes, or sperm, while in female colonies single cells enlarge to become oogametes, or eggs.[2][5]

Volvox ordinarily grows asexually when the environment provides optimum conditions. V. carteri occupies temporary pools of water that tend to dry out in the heat of late summer. As their environment dries out, haploid asexual volvox quickly die. Shortly before drying is complete, Volvox switch to the sexual phase that leads to production of dormant desiccation resistant zygotes.

Kirk and Kirk[6] showed that sex-inducing pheromone production can be triggered in somatic cells by a short heat shock given to asexually growing organisms. The induction of sex by heat shock is mediated by oxidative stress that likely also causes oxidative DNA damage.[7][8] It has been suggested that switching to the sexual pathway is the key to surviving environmental stresses that include heat and drought.[9] Consistent with this idea, the induction of sex involves a signal transduction pathway that is also induced in Volvox by wounding.[9]

Habitats

Volvox is a genus of freshwater algae found in ponds and ditches, even in shallow puddles.[5] According to Charles Joseph Chamberlain,[10]

"The most favorable place to look for it is in the deeper ponds, lagoons, and ditches which receive an abundance of rain water. It has been said that where you find Lemna, you are likely to find Volvox; and it is true that such water is favorable, but the shading is unfavorable. Look where you find Sphagnum, Vaucheria, Alisma, Equisetum fluviatile, Utricularia, Typha, and Chara. Dr. Nieuwland reports that Pandorina, Eudorina and Gonium are commonly found in summer as constituents of the green scum on wallows in fields where pigs are kept. The flagellate, Euglena, is often associated with these forms."

History

Antonie van Leeuwenhoek first reported observations of Volvox in 1700.[11][12]

After some drawings of Henry Baker (1753),[13] Linnaeus (1758)[14] would describe the genus Volvox, with two species: V. globator and V. chaos. Volvox chaos is an amoeba now known as Chaos sp.[15][16]

Evolution

Ancestors of Volvox transitioned from single cells to form multicellular colonies at least 200 million years ago, during the Triassic period.[1][17] An estimate using DNA sequences from about 45 different species of Volvox and related species suggests that the transition from single cells to undifferentiated multicellular colonies took about 35 million years.[1][17]

References

  1. 1 2 3 4 5 University of Arizona (February 22, 2009). "Single-celled algae took the leap to multicellularity 200 million years ago". Science Daily.
  2. 1 2 Kirk, David L. (1998). Volvox: A Search for the Molecular and Genetic Origins of Multicellularity and Cellular Differentiation. Cambridge University Press. ISBN 978-0-521-45207-6.
  3. ↑ Hallmann, A. (2003). "Extracellular matrix and sex-inducing pheromone in Volvox". International Review of Cytology. International Review of Cytology 227: 131–182. doi:10.1016/S0074-7696(03)01009-X. ISBN 978-0-12-364631-6.
  4. ↑ Ikushima, N.; Maruyama, S. (1968). "The protoplasmic connection in Volvox". Journal of Eukaryotic Microbiology 15 (1): 136–140. doi:10.1111/j.1550-7408.1968.tb02098.x.
  5. 1 2 Powers, J. H. (1908). "Further studies in Volvox, with descriptions of three new species". Transactions of the American Microscopical Society 28: 141–175. doi:10.2307/3220908. JSTOR 3220908.
  6. ↑ DL, Kirk; Kirk, MM (1986). "Heat shock elicits production of sexual inducer in Volvox". Science 231 (4733): 51–4. doi:10.1126/science.3941891. PMID 3941891.
  7. ↑ Nedelcu, AM; Michod, RE (2003). "Sex as a response to oxidative stress: the effect of antioxidants on sexual induction in a facultatively sexual lineage". Proc. Biol. Sci. 270 Suppl 2: S136–9. doi:10.1098/rsbl.2003.0062. PMC 1809951. PMID 14667362.
  8. ↑ Nedelcu, AM; Marcu, O; Michod, RE (2004). "Sex as a response to oxidative stress: a twofold increase in cellular reactive oxygen species activates sex genes". Proc. Biol. Sci. 271 (1548): 1591–6. doi:10.1098/rspb.2004.2747. PMC 1691771. PMID 15306305.
  9. 1 2 Amon, P; Haas, E; Sumper, M (1998). "The sex-inducing pheromone and wounding trigger the same set of genes in the multicellular green alga Volvox". Plant Cell 10 (5): 781–9. doi:10.2307/3870664. PMC 144025. PMID 9596636.
  10. ↑ Chamberlain, Charles Joseph (2007) [1932]. "Chlorophyceae". Methods in Plant Histology. Read Books. pp. 162–180. ISBN 978-1-4086-2795-2.
  11. ↑ van Leeuwenhoek, Antonie (1700). "Part of a Letter from Mr Antony van Leeuwenhoek, concerning the Worms in Sheeps Livers, Gnats, and Animalcula in the Excrements of Frogs" (PDF). Philosophical Transactions of the Royal Society 22 (260–276): 509–518. doi:10.1098/rstl.1700.0013.
  12. ↑ Herron, M. (2015). “…of the bignefs of a great corn of fand…”. Fierce Roller Blog, .
  13. ↑ Baker, H. (1753). Employment for the microscope. R. Dodsley: London, pl. XII, f. 27, .
  14. ↑ 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.
  15. ↑ Herron, M. (2016). Moving without limbs! Linnaeus on Volvox. Fierce Roller Blog, .
  16. ↑ Spencer, M.A., Irvine, L.M. & Jarvis, C.E. (2009). Typification of Linnaean names relevant to algal nomenclature. Taxon 58: 237-260, .
  17. 1 2 Herron, MD; Hackett, JD; Aylward, FO; Michod, RE (2009). "Triassic origin and early radiation of multicellular volvocine algae". Proceedings of the National Academy of Sciences, USA 106 (9): 3254–3258. doi:10.1073/pnas.0811205106.

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