Mound-cell movement and morphogenesis in Dictyostelium

To examine the mechanisms of cell locomotion within a three-dimensional (3- D) cell mass, we have undertaken a systematic 3-D analysis of individual ce ll movements in the Dictyostelium mound, the first 3-D structure to form du ring development of the fruiting body. We used time-lapse deconvolution mic roscopy to examine two strains whose motion represents endpoints on the spe ctrum of motile behaviors that we have observed in mounds. In AX-2 mounds, cell motion is slow and trajectories are a combination of random and radial , compared to KAX-3, in which motion is fivefold faster and most trajectori es are rotational. Although radial or rotational motion was correlated with the optical-density wave patterns present in each strain, we also found sm all but significant subpopulations of cells that moved differently from the majority, demonstrating that optical-density waves are at best insufficien t to explain all motile behavior in mounds. In examining morphogenesis in t hese strains, we noted that AX-2 mounds tended to culminate directly to a f ruiting body, whereas KAX-3 mounds first formed a migratory slug. By alteri ng buffering conditions we could interchange these behaviors and then found that mound-cell motions also changed accordingly. This demonstrates a corr elation between mound-cell motion and subsequent development, but it is not obligatory. Chimeric mounds composed of only 10% KAX-3 cells and 90% AX-2 cells exhibited rotational motion, suggesting that a diffusible molecule in duces rotation, but many of these mounds still culminated directly, demonst rating that rotational motion does not always lead to slug migration. Our o bservations provide a detailed analysis of cell motion for two distinct mod es of mound and slug formation in Dictyostelium. (C) 1999 Academic Press.