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The family Volvocaceae contains several genera of green flagellated algae that are intermediate in size and complexity between unicellular Chlamydomonas and Volvox. Molecular phylogenetic analysis indicates that the family is monophyletic, and shares common ancestor with Chlamydomonas that existed about 50-200 million years ago. Thus, these algae provide great opportunity to analyze evolutionary pathway leading from unicellularity to multicellularity with complete division of labor.
Genomes of Chlamydomonas and Volvox are remarkably similar: both genomes contain ~14,500 protein-coding genes, and the Volvox genome is slightly larger, 138-118 megabases, mostly because of its greater transposon/repetitive DNA content.
Over a relatively short period of time Volvox evolved:
- assymetric cell division, which generates large gonidial precursors;
- multicellularity with germ-soma division of labor;
- embryonic morphogenesis - a gastrulation-like process tha tflips the organism's polarity to position flagellar ends osomatic cells externally;
- complex extracellulaar matrix (ECM) ralated to Chlamydomonas cell wall;
- oogametic (egg/sperm) sexual program that is very different from the isogametic one (same-sized gametes).
- 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.
Volvox has 2 modes of reproduction: sexual and asexual.
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 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 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.
Males and females are indistinguishable in their asexual form;
under standard conditions, the asexual life cycle takes precisely 48 h and is
synchronized by a 16 h light–8 h dark cycle.
- 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. The cell-sheet bending occurs at a specific site known as the phialopore, a swastika-shaped opening found at the anterior pole of the embryo. To initiate inversion, cells at the edges of the phialopore adopt an asymetric flask-like shape.
- 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 The parent sphere devoid of gonidia and consisting only of somatic cells undergoes senescence and die. Somatic cells are specialized for motility and are destined to die when they are only about four days old.
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.
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.
- 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.
- PubMed free full text articles: major topic "Volvox"
- Nishii I, Miller SM. Volvox: simple steps to developmental complexity? Curr Opin Plant Biol. 2010 Dec;13(6):646-53.
- Kirk DL. Volvox. Curr Biol. 2004 Aug 10;14(15):R599-600.
- Cole DG, Reedy MV. Algal morphogenesis: how volvox turns itself inside-out. Curr Biol. 2003 Sep 30;13(19):R770-2.
- PubMed free full text articles: major topic "Volvox"