Pollen wall and tapetal development in Cymbalaria muralis: the role of physical processes, evidenced by in vitro modelling

Our aim was to unravel the underlying mechanisms of pollen wall development in Cymbalaria muralis. By determining the sequence of developing substructures with TEM, we intended to compare it with that of other taxa and clarify whether physical processes of self-assembly and phase separation were involved. In parallel, we tried to simulate in vitro the substructures observed in Cymbalaria muralis exine development, using colloidal mixtures, to determine whether purely physical self-assembly processes could replicate them. Exine ontogeny followed the main stages observed in many other species and was initiated by phase separation, resulting in heterogeneity of the homogeneous contents of the periplasmic space around the microspore which is filled with genome-determined substances. At every stage, phase separation and self-assembly come into force, gradually driving the substances through the sequence of mesophases: spherical micelles, columns of spherical micelles, cylindrical micelles arranged in a layer, laminate micelles. The final two of these mesophases define the structure of the columellate ectexine and lamellate endexine respectively. Structures obtained in vitro from colloidal mixtures simulated the developing exine structures. Striking columella-like surface of some abnormal tapetal cells and lamella-like structures in the anther medium confirm the conclusion that pattern generation is a feature of colloidal materials, after genomic control on material contents. Simulation experiments show the high pattern-generating capacity of colloidal interactions.