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.
Many aspects of Drosophila segmentation can be discussed in one-dimensional terms as a linear pattern of repeated elements or cell states. But the initial metameric pattern seen in the expression of pair-rule genes is fully two-dimensional, i.e. a pattern of stripes. Several lines of evidence suggest a kinetic mechanism acting globally during the syncytial blastoderm stage may be responsible for generating this pattern. The requirement that the mechanism should produce stripes, not spots or some other periodic pattern, imposes preconditions on this act, namely (1) sharp anterior and posterior boundaries that delimit the pattern-forming region, and (2) an axial asymmetrizing influence in the form of an anteroposterior gradient. Models for Drosophila segmentation generally rely on the gradient to provide positional information in the form of concentration thresholds that cue downstream elements of a hierarchical control system. This imposes restrictions on how such models cope with experimental disturbances to the gradient. A shallower gradient, for example, means fewer pattern elements. This need not be the case if the gradient acts through a kinetic mechanism like reaction-diffusion that involves the whole system. It is then the overall direction of the gradient that is important rather than specific concentration values.(ABSTRACT TRUNCATED AT 250 WORDS)