Calcium localization during Acetabularia whorl formation: evidence supporting a two-stage hierarchical mechanism, Development 104

The giant single-celled marine alga Acetabularia metU-terranea (or A. acetabulum) repeatedly generates whorls of hairs at its growing tip. Theoretical con-siderations, and physicochemical analysis of the effect of extracellular calcium concentration on hair spacing, have suggested a two-stage mechanism, hi which whorl pattern is preceded by a simple annular pattern. This analysis requires that, at the time of whorl determination, morphogenetically related cal-cium should still be distributed in the first-stage annular pattern. This paper reports studies of calcium distribution at closely spaced developmental stages from unpatterned growing tip to whorl expression. The sequence observed is that expected from the theoretical analysis. The fluorescence chelate used, chlorotetracycline, is believed to be specific for mem-brane-bound calcium. The results therefore suggest, but do not prove, that morphogenetically related calcium is attached to membrane-bound structures rather than intracellular ones