Calcium localization duringAcetabulariawhorl formation: evidence supporting a two-stage hierarchical mechanism

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Abstract
The giant single-celled marine alga Acetabularia mediterranea (or A. acetabulum) repeatedly generates whorls of hairs at its growing tip. Theoretical considerations, and physicochemical analysis of the effect of extracellular calcium concentration on hair spacing, have suggested a two-stage mechanism, in which whorl pattern is preceded by a simple annular pattern. This analysis requires that, at the time of whorl determination, morphogenetically related calcium 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 membrane-bound calcium. The results therefore suggest, but do not prove, that morphogenetically related calcium is attached to membrane-bound structures rather than intracellular ones.