THE RELATIONSHIP BETWEEN CELL-SIZE AND CELL FATE IN VOLVOX-CARTERI

In Volvox carteri development, visibly asymmetric cleavage divisions s et apart large embryonic cells that will become asexual reproductive c ells (gonidia) from smaller cells that will produce terminally differe ntiated 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 s pecification, (b) by segregation of a cytoplasmic factor resembling ge rm plasm into large cells, and (c) by a combined effect of differences in cytoplasmic quality and cytoplasmic quantity. In this study a vari ety of V carteri embryos with genetically and experimentally altered p atterns of development were examined in an attempt to distinguish amon g these hypotheses. No evidence was found for regionally specialized c ytoplasm that is essential for gonidial specification. In all cases st udied, cells with a diameter > approximately 8 mum at the end of cleav age-no matter where or how these cells had been produced in the embryo -developed as gonidia. Instructive observations in this regard were ob tained by three different experimental interventions. (a) When heat sh ock was used to interrupt cleavage prematurely, so that presumptive so matic cells were left much larger than they normally would be at the e nd of cleavage, most cells differentiated as gonidia. This result was obtained both with wild-type embryos that had already divided asymmetr ically (and should have segregated any cytoplasmic determinants involv ed in cell specification) and with embryos of a mutant that normally p roduces 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 e ach 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 beca me gonidia. These data argue strongly for a central role of cell size in germ/soma specification in Volvox carteri, but leave open the quest ion of how differences in cell size are actually transduced into diffe rences in gene expression.