Editing Volvox (section)

== Description ==
[[File:Volvox.svg|thumb|''Volvox'' colony: 1) ''[[Chlamydomonas]]''-like cell, 2) Daughter colony, 3) Cytoplasmic bridges, 4) Intercellular gel, 5) Reproductive cell, 6) Somatic cell.]]
Mature colonies of ''Volvox'' [[colony (biology)|colony]] are composed of hundreds, up to tens thousands of cells from two differentiated cell types: numerous [[flagellate]] [[Somatic (biology)|somatic cells]] and a smaller number of [[germ cell]]s lacking in soma that are embedded in the surface of a hollow sphere or [[Coenobium (morphology)|coenobium]] containing an [[extracellular matrix]]<ref name="daily" /> made of [[glycoprotein]]s.<ref name="Hallmann">{{cite book |last=Hallmann |first=A. |date=2003 |title=Extracellular matrix and sex-inducing pheromone in ''Volvox'' |series=International Review of Cytology |doi=10.1016/S0074-7696(03)01009-X |pmid=14518551 |isbn=978-0-12-364631-6 |volume=227 |pages=131–182 |chapter=Extracellular Matrix and Sex-Inducing Pheromone in Volvox}}</ref>

Adult [[somatic cell]]s comprise a single layer with the flagella facing outward, forming a hollow spheroid. The cells swim in a coordinated fashion, with distinct anterior and posterior poles. Each cell is enclosed in a gelatinous sheath, which is either distinct or confluent depending on the species. Cells are ovoid, spherical, or star-shaped, each with two equal [[flagella]]. The cells have a cup-shaped [[chloroplast]] with a single [[pyrenoid]] and an anterior [[eyespot apparatus|eyespot]]<ref name=Nakada2014>{{cite book |editor-first1=John D.|editor-last1=Wehr|editor-first2=Robert G.|editor-last2=Sheath|editor-first3=J. Patrick|editor-last3=Kociolek |date= 2014 |edition=2 |title= Freshwater Algae of North America: Ecology and Classification |last1= Nakada|first1=Takashi| last2=Nozaki |first2=Hisayoshi |chapter= Chapter 6. Flagellate Green Algae |url= |location= |publisher= Elsevier Inc. |pages=265–313|isbn=978-0-12-385876-4 }}</ref> that enables the colony to swim toward light. The cells of colonies in the more basal ''[[Euvolvox]]'' clade are interconnected by thin strands of [[cytoplasm]], called protoplasmates.<ref>{{cite journal |last1=Ikushima |first1=N. |last2=Maruyama |first2=S. |date=1968 |title=The protoplasmic connection in ''Volvox'' |journal=[[Journal of Eukaryotic Microbiology]] |doi=10.1111/j.1550-7408.1968.tb02098.x |volume=15 |issue=1 |pages=136–140}}</ref> Cell number is specified during development and is dependent on the number of rounds of division.<ref name="Kirk">{{cite book |last=Kirk |first=David L. |date=1998 |title=''Volvox'': A Search for the Molecular and Genetic Origins of Multicellularity and Cellular Differentiation |publisher=[[Cambridge University Press]] |isbn=978-0-521-45207-6}}</ref>

=== Reproduction ===

''Volvox'' is facultatively sexual and can reproduce both sexually and asexually. In the lab, asexual reproduction is most commonly observed; the relative frequencies of sexual and asexual reproduction in the wild is unknown. The switch from asexual to sexual reproduction can be triggered by environmental conditions<ref name="pmid14667362" /> and by the production of a sex-inducing pheromone.<ref>{{Cite journal |last=Hallmann |first=Armin |year=2003 |title=Extracellular Matrix and Sex-Inducing Pheromone in Volvox |journal=International Review of Cytology |volume=227 |pages=131–182 |doi=10.1016/S0074-7696(03)01009-X |pmid=14518551 |isbn=978-0-12-364631-6}}</ref> Desiccation-resistant diploid [[zygote]]s are produced following successful fertilization.

An [[asexual reproduction|asexual]] colony includes both somatic (vegetative) cells, which do not reproduce, and large, non-motile ''[[Gonidium|gonidia]]'' in the interior, which produce new colonies asexually through repeated division. In [[sexual reproduction]] two types of [[gamete]]s are produced. ''Volvox'' species can be [[monoecious]] or [[dioecious]]. [[Male]] colonies release numerous sperm packets, while in female colonies single cells enlarge to become oogametes, or eggs.<ref name="Kirk" /><ref name="Powers">{{cite journal |last=Powers |first=J. H. |date=1908 |title=Further studies in ''Volvox'', with descriptions of three new species |journal=[[Transactions of the American Microscopical Society]] |doi=10.2307/3220908 |jstor=3220908 |volume=28 |pages=141–175}}</ref>

Kirk and Kirk<ref name="pmid3941891">{{cite journal |last1=DL |first1=Kirk |last2=Kirk |first2=MM |date=1986 |title=Heat shock elicits production of sexual inducer in Volvox |journal=Science |pmid=3941891 |doi=10.1126/science.3941891 |volume=231 |issue=4733 |pages=51–4 |bibcode=1986Sci...231...51K}}</ref> showed that sex-inducing pheromone production can be triggered in [[somatic cell]]s 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.<ref name="pmid14667362">{{cite journal |last1=Nedelcu |first1=AM |last2=Michod |first2=RE |date=2003 |title=Sex as a response to oxidative stress: the effect of antioxidants on sexual induction in a facultatively sexual lineage |journal=Proc. Biol. Sci. |pmid=14667362 |pmc=1809951 |doi=10.1098/rsbl.2003.0062 |volume=270 |issue=Suppl 2 |pages=S136–9}}</ref><ref name="pmid15306305">{{cite journal |last1=Nedelcu |first1=AM |last2=Marcu |first2=O |last3=Michod |first3=RE |date=2004 |title=Sex as a response to oxidative stress: a twofold increase in cellular reactive oxygen species activates sex genes |journal=Proc. Biol. Sci. |pmid=15306305 |pmc=1691771 |doi=10.1098/rspb.2004.2747 |volume=271 |issue=1548 |pages=1591–6}}</ref> It has been suggested that switching to the sexual pathway is the key to surviving environmental stresses that include heat and [[drought]].<ref name="Amon">{{cite journal |last1=Amon |first1=P |last2=Haas |first2=E |last3=Sumper |first3=M |date=1998 |title=The sex-inducing pheromone and wounding trigger the same set of genes in the multicellular green alga Volvox |journal=Plant Cell |pmid=9596636 |pmc=144025 |doi=10.2307/3870664 |volume=10 |issue=5 |pages=781–9 |jstor=3870664}}</ref> Consistent with this idea, the induction of sex involves a [[signal transduction]] pathway that is also induced in ''Volvox'' by wounding.<ref name="Amon" />

=== Colony inversion ===
Colony inversion is a special characteristic during development in the order Volvocaceae that results in new colonies having their [[flagella]] facing outwards. During this process the asexual reproductive cells (gonidia) first undergo successive cell divisions to form a concave-to-cup-shaped embryo or plakea composed of a single cell layer. Immediately after, the cell layer is inside out compared with the adult configuration—the apical ends of the embryo protoplasts from which flagella are formed, are oriented toward the interior of the plakea. Then the embryo undergoes inversion, during which the cell layer inverts to form a spheroidal daughter colony with the apical ends and flagella of daughter protoplasts positioned outside. This process enables appropriate locomotion of spheroidal colonies of the Volvocaceae. The mechanism of inversion has been investigated extensively at the cellular and molecular levels using the model species, ''[[Volvox carteri]]''.<ref name="Yamashita">{{cite journal |last1=Yamashita |first1=S |last2=Arakaki |first2=Y |last3=Kawai-Toyooka |first3=H |last4=Noga |first4=A |last5=Hirono |first5=M |last6=Nozaki |first6=H |date=Nov 2016 |title=Alternative evolution of a spheroidal colony in volvocine algae: developmentalanalysis of embryogenesis in Astrephomene (Volvocales, Chlorophyta) |journal=BMC Evol. Biol. |volume=16 |issue=1 |page=243 |pmid=27829356 |doi=10.1186/s12862-016-0794-x |pmc=5103382 |doi-access=free }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> Another species ''[[Volvox globator]]'' has a similar mode of colony inversion, but begins at the posterior instead of the anterior.<ref name=Herron/>

[[File:Inversion Process for Volvox.png|899x899px|thumb|center|Embryonic inversion in ''Volvox''. (a) Adult ''V. globator'' spheroid containing multiple embryos. (b) Embryo undergoing type-A inversion (e.g., ''V. carteri''). (c) Embryo undergoing type-B inversion (e.g., ''V. globator'', ''V. aureus''). (d) Light micrograph shows semi-thin section of V. globator embryo exhibiting different cell shapes. (e) Schematic representation of cells in region marked in (d). PC: paddle-shaped cells, two different views illustrate anisotropic shape; SC: spindle-shaped cells; red line:  position of cytoplasmic bridges (CB). (f) 3D renderings of a single ''V. globator'' embryo in three successive stages of inversion. (g) Optical midsagittal cross sections of embryo in (f). (h) Traced cell sheet contours overlaid on sections in (g), with color-coded curvature κ. (i) Surfaces of revolution computed from averaged contours.<ref>Stephanie Höhn, Aurelia R. Honerkamp-Smith, Pierre A. Haas, Philipp Khuc Trong, and Raymond E. Goldstein
Phys. Rev. Lett. 114, 178101 – Published 27 April 2015. [[File:CC-BY icon.svg|50px]] This article contains quotations from this source, which is available under the [https://creativecommons.org/licenses/by/3.0/ Creative Commons Attribution 3.0 (CC BY 3.0)] license.</ref>]]