The morphogenetic sequence followed by migrating slugs of Dictyostelium discoideum during reentry into the fruiting mode

Abstract

Given the appropriate environmental cues, a migrating slug of Dictyostelium discoideum can, at any time, stop migrating, reenter the fruiting mode, and complete fruiting body construction over approximately an 8 h period. The reentry sequence exhibited by single slugs of known size and developmental condition has been documented in still photomicrographs and time-lapse videotape recordings. Reentry, culminating in the construction of a single fruiting body, has been found to involve a complex series of concerted morphogenetic movements by the slug tip and posterior regions which are remarkably invariant both in pattern and timing. At a frequency positively correlated with slug length, a variant scenario is impressed upon this basic sequence that leads to the formation of multiple fruits. The latter involves one or more transverse splits within the posterior region that can be preceded, attended, or succeeded by the appearance of additional apical tips in (relatively) anterior, medial, or posterior locations. The paradigmal sequences described here can serve as topographic groundwork upon which to construct or with which to test formal models of morphogenetic regulation. Repeated examinations of the videotapes have given us the impression that slug reentry resembles a reprise of aggregation involving the movement of the posterior region as a single outlying stream toward and under the tip, accompanied over the 2–2.5 h period by the accelerated progression of the tip and underlying region to the Mexican Hat stage. An earlier study [3,7] had shown that slug reentry triggers the quantal resynthesis of aggregation-associated proteins such as UDPG-pyrophosphorylase and UDP galactosyl transferase plus a delayed first round of UDP Gal-4-epimerase, a post-aggregation protein uniquely associated with the fruiting mode. It is noteworthy that the same pattern of protein synthesis is exhibited by cells mechanically dispersed from aggregates after 16–18 h development while they (rapidly) reaggregate and recapitulate previously accomplished morphogenesis [10].