Figure 3.—

Prestalk and stalk cell heterogeneity. (Top) The slug displayed was transformed with two different ecmA promoter-to-GFP fusions that are expressed in pstA cells (red), pstO cells, or anterior-like cells (ALCs) (green) (D. Dormann, N. Zhukovskaya, J. G. Williams and C. J. Weijer, unpublished results). (Bottom) A representation of the different prestalk cell populations present in the slug. The anterior prestalk region contains four partially overlapping populations (Gaudet et al. 2008; http://dictybase.org/Dicty_Info/dicty_anatomy_ontology.html). The pstA and pstO cells, respectively, occupy the front and rear halves and are identified by their ability to use different parts of the promoter of the ecmA gene. The pstAB cells are pstA cells that prematurely and sporadically express the ecmB gene. This is the same transitional event that occurs continuously during culmination. The pstAB cells sporadically drop backward and fall out the back of the migrating slug. At culmination, they form the inner part of the basal disc. The tip organizer cells are characterized by their ability to use a specific region of the promoter of the cudA gene. They control the migration and maintain the integrity of the slug. The pstB cells express ecmB at a higher level than ecmA and move backward and forward as a group along the anterior–posterior axis of the ventral surface of the prespore region. At culmination, they form the outer part of the basal disc. The ALCs are a mixed group of cells that are the direct precursors of the upper and lower cups of the culminant.