THE INITIATION OF BASAL DISC FORMATION IN DICTYOSTELIUM-DISCOIDEUM ISAN EARLY EVENT IN CULMINATION

We have analysed expression of the ecmA and ecmB genes of Dictyosteliu m by enzymatic double staining using beta-galactosidase and beta-glucu ronidase reporter gene constructs. Cells expressing the ecmA gene firs t appear as scattered cells at the mound stage of development and we s how that this is also true for cells expressing the ecmB gene, During tip formation the ecmA-expressing cells move to the apex of the mound, while the ecmB-expressing cells accumulate in the base, The ecmB-expr essing cells constitute part of the basal disc if the culminant is for med in situ but are discarded if a migratory slug is formed, During sl ug migration they are replaced by a band of ecmB-expressing cells, sit uated in the front half of the prespore zone and tightly apposed to th e substratum, When culmination is triggered these cells rapidly move t o the back half of the prestalk zone, possibly acting as a point of at tachment to the subtratum, Ultimately, they are joined by cells at the back of the slug, the rearguard cells, to form the basal disc. Thus, contrary to previous belief, basal disc formation is initiated very ea rly during culmination and occurs by the forward movement of cells loc ated in the anterior of the prespore zone.