Orientation and rate of locomotion of individual amebas in the life cycle of the cellular slime mold Dictyostelium mucoroides

Abstract

1. 1. Rate of locomotion of individual amebas of the cellular slime mold Dictyostelium mucoroides were measured throughout the life cycle and considerable variation was found from one stage to the next. However, when individual amebas were isolated from various stages of the life cycle their mean rates of locomotion fell into one of two categories; prefeeding amebas were consistently slow, whereas amebas from all other stages showed a similar high rate of locomotion. 2. 2. There was no significant difference between the mean rates of locomotion of individual amebas isolated from different positions along the main axis of migrating pseudoplasmodia. 3. 3. Rate of locomotion appeared to be directly related to the frequency of pseudopod formation. 4. 4. Rate of locomotion was found to be independent of the size of the ameba. 5. 5. Of all the physical factors in the environment normally encountered in laboratory procedures, wetness of the substrate was found to be the most significant for it greatly increased the rate of locomotion of the amebas. The increase in speed was effected by producing more elongated pseudopods rather than by altering the frequency of pseudopod formation. 6. 6. Examination of the role of the chemical constitution of the medium revealed that peptone and amino acids maintained, whereas dextrose inhibited, the rate of locomotion of isolated amebas over a period of time. 7. 7. Isolated amebas showed clear-cut orientation toward E. coli although the presence of the bacteria did not affect their mean rate of locomotion. 8. 8. Isolated amebas wander away from one another; they exhibit a definite negative chemotaxis. 9. 9. The presence of large quantities of postfeeding (but not yet aggregating) amebas does not affect the rate of movement of isolated amebas of the same stage. However, this is not true of recently germinated amebas, whose rate is increased by the presence of other amebas. 10. 10. The mean rate of locomotion of isolated amebas from nonaggregating phases of the life cycle were not influenced by acrasin when actively secreting centers of aggregation were placed in their midst. 11. 11. In preaggregation and aggregation phases there was a correlation between orientation and the reduction of the rate of movement. These coupled phenomena, in the case of the preaggregation stages of D. purpureum, occurred as a wave originating and moving away from a normal-appearing cell before there is any sign of local crowding. In D. mucoroides, this slowing also preceded the period of conspicuous contact. 12. 12. These results show that over and above the well-known orientation mechanism of acrasin there are a number of other important systems of communication between these social amebas. These systems involve relations between amebas that affect rate of locomotion and orientation as well as relations between the amebas and their environment.