Kirk, Marilyn M. and Ransick, Andrew and McRae, Stephen E. and Kirk, David L. (1993) The relationship between cell size and cell fate in Volvox carteri. Journal of Cell Biology, 123 (1). pp. 191-208. ISSN 0021-9525 http://resolver.caltech.edu/CaltechAUTHORS:KIRjcb93
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In Volvox carteri development, visibly asymmetric cleavage divisions set apart large embryonic cells that will become asexual reproductive cells (gonidia) from smaller cells that will produce terminally differentiated somatic cells. Three mechanisms have been proposed to explain how asymmetric division leads to cell specification in Volvox: (a) by a direct effect of cell size (or a property derived from it) on cell specification, (b) by segregation of a cytoplasmic factor resembling germ plasm into large cells, and (c) by a combined effect of differences in cytoplasmic quality and cytoplasmic quantity. In this study a variety of V. carteri embryos with genetically and experimentally altered patterns of development were examined in an attempt to distinguish among these hypotheses. No evidence was found for regionally specialized cytoplasm that is essential for gonidial specification. In all cases studied, cells with a diameter > approximately 8 microns at the end of cleavage--no matter where or how these cells had been produced in the embryo--developed as gonidia. Instructive observations in this regard were obtained by three different experimental interventions. (a) When heat shock was used to interrupt cleavage prematurely, so that presumptive somatic cells were left much larger than they normally would be at the end of cleavage, most cells differentiated as gonidia. This result was obtained both with wild-type embryos that had already divided asymmetrically (and should have segregated any cytoplasmic determinants involved in cell specification) and with embryos of a mutant that normally produces only somatic cells. (b) When individual wild-type blastomeres were isolated at the 16-cell stage, both the anterior blastomeres that normally produce two gonidia each and the posterior blastomeres that normally produce no gonidia underwent modified cleavage patterns and each produced an average of one large cell that developed as a gonidium. (c) When large cells were created microsurgically in a region of the embryo that normally makes only somatic cells, these large cells became gonidia. These data argue strongly for a central role of cell size in germ/soma specification in Volvox carteri, but leave open the question of how differences in cell size are actually transduced into differences in gene expression.
|Additional Information:||Copyright © 1993 by The Rockefeller University Press. Received for publication l0 May 1993 and in revised form 8 July 1993. We wish to thank the following colleagues who read and commented on an earlier version of this manuscript: Alexei Desnitski, Gary Kochert, Steven Miller, Rüdiger Schmitt, Richard Starr, and Manfred Sumper. This work was supported by grants from the National Institutes of Health (GM27215) and the National Science Foundation (DMB-005233) to D.L. Kirk and M.M. Kirk; S.E. McRae was supported by a special supplement to GM27215 that was provided under the program to support minority participation in biological and biomedical research.|
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|Deposited On:||22 Apr 2008|
|Last Modified:||26 Dec 2012 09:58|
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