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• W4252048175 abstract "What is Volvox? The name comes from the Latin volvere, to roll, and -ox, as in atrox, fierce. 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. Ever since van Leeuwenhoek first viewed these algal ‘fierce rollers’ with utter fascination in 1700, one biologist after another has pointed to Volvox as a model organism that could be used to support or refute some important biological concept of the day, such as spontaneous generation, preformation, epigenesis, the continuity of the germ plasm, and so on. All attempts to exploit Volvox as a laboratory model system failed, however, until the 1960s, when Richard Starr's group finally discovered a medium in which the organism would thrive and reproduce in captivity. Starr then circled the globe, bringing into culture all 18 known (and several previously unknown) Volvox species. By 1970, he concluded that a mating pair of isolates of V. carteri from Japan had the best combination of properties to serve as a genetic model system. Most studies of Volvox reported in the last 30 years have used those strains of V. carteri or their descendants, and so this guide will be similarly restricted in scope. How does Volvox reproduce? Although V. carteri has a sexual cycle that can be induced and exploited for Mendelian analysis, the sexual cycle is not used for reproduction in nature; it is used to produce dormant, diploid zygotes that are able to survive adverse conditions. During all active phases, Volvox (like other green algae) is haploid and reproduces asexually. In V. carteri, an asexual cycle begins when each mature gonidium initiates a rapid series of cleavage divisions, certain of which are visibly asymmetric and produce large gonidial initials and small somatic initials. The fully cleaved embryo contains all of the cells of both types that will be present in an adult, but it is inside out, and to achieve the adult configuration it must turn right-side-out in a gastrulation-like process called inversion. Cleavage and inversion together take about 8 hours, and the complete asexual cycle takes precisely two days when it is synchronized by a suitable light–dark cycle. Following inversion, both the adult spheroid and the juvenile spheroids within it increase in size (without further cell division) by depositing large quantities of a glycoprotein-based extracellular matrix. Part way through the expansion phase, the juveniles digest their way out of the parental matrix and become free-swimming. By that time, the somatic cells of the parental spheroids, having fulfilled their function, are already moribund, and will soon be history. Thus, whereas the gonidia are non-motile and potentially immortal, the somatic cells are specialized for motility, but destined to die when they are only about four days old. This difference raises with particular clarity a central question of developmental biology: how are cells with entirely different phenotypes produced from the descendants of a single cell? What is the genetic program for germ–soma differentiation in Volvox? In most close relatives of Volvox, all cells first execute motility and other vegetative functions, and then they redifferentiate and engage in asexual reproduction. Mutational analysis has defined three types of gene that play the major roles in converting this ancestral program for biphasic development into a germ–soma dichotomy in V. carteri: first, the gls (onidiaes) genes act during cleavage to permit asymmetric division and formation of large–small sister-cell pairs; then the regA (enerator ) gene acts in the small cells to prevent all aspects of reproductive development, while the lag (te onidia) genes act in the large cells to prevent formation of somatic features such as flagella and eyespots. Genetic and experimental analysis indicates that it is the difference in cell size at the end of cleavage that determines whether regA or the lag genes will be activated. Are tools available for molecular analysis of this developmental program? Although there have never been more than a handful of labs investigating V. carteri at any one time, an array of important modern analytical tools have been developed for studying it. These include a transposon-based gene-tagging system, nuclear transformation, inducible-promoter and reporter-gene constructs, GFP-tagging and so on. Such tools have been used to examine in some detail the functions of the glsA gene product, which is required for asymmetric division, and the regA gene product, which is required for terminal differentiation of somatic cells. More recently, they have also been used to show that the invA gene encodes a novel kinesin that drives inversion. What can Volvox tell us about the evolutionary origins of multicellularity and cellular differentiation? The family Volvocaceae, in which Volvox is placed, contains several genera of g reen flagellates that are intermediate in size and complexity between unicellular Chlamydomonas and Volvox. Molecular phylogenetic analysis indicates that the family is monophyletic, and shares a common ancestor with Chlamydomonas that existed about 50 million years ago. Thus, these algae provide an unrivalled opportunity to analyze a genetic pathway leading from unicellularity to multicellularity with a complete division of labor. Is there a Volvox genome project? Yes. One bacterial artificial chromosome (BAC) library has been produced, a second is in preparation; sequencing of the complete 108 base pair V. carteri genome is underway. Where can I find out more?" @default.
• W4252048175 created "2022-05-12" @default.
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• W4252048175 date "2004-08-01" @default.
• W4252048175 modified "2023-10-16" @default.
• W4252048175 title "Volvox" @default.
• W4252048175 cites W1549918097 @default.
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• W4252048175 doi "https://doi.org/10.1016/j.cub.2004.07.034" @default.
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