Inheritance of Pattern: Analysis from Phenotype to Gene

The form and pattern of multicellular organisms are developmentalphenotypes. They are long term processes rather than staticstructures. They involve myriad events at multiple locations.The efficient encoding of such phenotypes is analyzed here intwo stages. First, the complex developmental behavior is brokendown so it can be accounted for by cell or tissue rules. Themost effective rules have the instantaneous character foundin time-based differential equations. When integrated over timeand space, the rules produce the behavior. Second, the cytologicaland nuclear basis of the rules is sought. One thus studies acomplex phenotype in terms of its successive antecedent causes,refining understanding as one gets closer to the genome. The approach is applied here to phyllotactic (leaf placement)patterns. Leaves may be alternating in a plane, whorled, orin a helical arrangement. In all three cases a new leaf formsas an arc-like bulge at a site apical to a small number of neighboringleaves. The leaf-forming sites are irregularities in the patternof cellulose reinforcement in the surface of the apical dome.Two organ-level rules combine to produce new leaf sites. First,each established leaf develops a single reinforcement field,with gently curved reinforcement lines, on the region of thedome just above the leaf. Second, where parts of two or threesuch fields abut on the dome they combine to make the irregularityfor the next leaf. Hence a given reinforcement pattern on thedome produces a leaf; the action of the leaves in turn reestablishesthe reinforcement pattern. The cellular basis of generatinga reinforcement field appears to be a cytoskeletal responseto excessive stretch, brought on by rapid growth of adjacentleaf bases. The large scale patterns are thus traceable to cytoskeletalphenomena and from there to genes involving microtubular behavior.