DEVELOPMENTAL BIOLOGY 52, 158-160 (1976) BRIEF NOTES The Absence of a Growth Inhibitor during the Log Phase of Growth of Dictyostelium discoideum RICHARD FERGUSON AND DAVID R. SOLL' Department of Zoology, The University of Iowa, Iowa City, Iowa 52242 Accepted January 30,1976 When amebae of the cellular slime mold Dictyostelium discoideum Ax-3 reach stationary phase in liquid nutrient medium, an extracellular factor appears which will inhibit cell multiplication when added to a logarithmically multiplying culture (Yarger et al., 1974). Recently, Hanish (1975) presented evidence that an inhibitor also accumulated during log phase and concluded that the cessation of cell multiplication and the final cell concentration were controlled by the accumulation of this inhibitor to an effective level. We have reexamined this extremely interesting observation, but have found no evidence to support Hanish’s conclusions. INTRODUCTION Amebae of the axenic strain of Dictyos- telium discoideum Ax-3 multiply in the axenic medium HL-5 with a doubling time of 12 hr (Cocucci and Sussman, 1971; Yar- ger et al., 1974). When the cell concentra- tion reaches approximately 6 x lo6 per ml, the rate of cell multiplication begins de- creasing, and after 20 to 30 hr it reaches zero, at a stationary phase cell concentra- tion of 2 to 2.5 X 10’ per ml. Yarger et al. (1974) presented evidence which demon- strated that (1) regardless of the origin or concentration of the initial cell inoculum, a final net increase of 2 to 2.5 X 10’ cells per ml occurs by stationary phase, (2) the final cell concentration is directly propor- tional to the concentration of nutrients in the medium, and (3) a growth inhibitor accumulates in the depleted medium at stationary phase. They proposed that the cessation of cell multiplication was due to the depletion of growth-limiting nutrients, and they hypothesized that the growth in- hibitor was a cell leakage product which appeared early in stationary phase and played an intracellular, autoregulatory role in the expression of the stationary phase phenotype. Evidence was also pre- ’ Author to whom all correspondence should be directed. sented that stationary phase cell popula- tions are blocked at a point late in the cell cycle (Sol1 et al., 1976) and accumulate 3 hr of aggregation information (Sol1 and Wad- dell, 1975; Shea et al., 1976). These obser- vations add new dimensions to the possible role of the observed growth inhibitor. Recently, Hanish (1975) reported results indicating that a growth inhibitor also ac- cumulates on or in log phase amebae and concluded that the cessation of cell multi- plication at stationary phase is due to the accumulation of this inhibitor to an effec- tive level. Hanish’s conclusion is ex- tremely interesting, but it is in conflict with the observations and conclusion of Yarger et al. (1974). To resolve this issue, we have reexamined Hanish’s conclusion by repeating several of her experiments as well as by performing new experiments. We have found no evidence to support her conclusion that a growth inhibitor accu- mulates during log phase and controls cell concentration at stationary phase. RESULTS Washing Log Phase and Stationary Phase Amebae Hanish presented evidence that if log phase cells of the axenic strain Ax-3 were washed twice with buffered salts solution 158. Copyright Q 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.