The role of self-assembly in biological systems: evidence from iridescent colloidal sporopollenin in <i>Selaginella</i> megaspore walls

Abstract Iridescent exines have previously been reported in both fossil and modern megaspores of selaginellalean affinity. This striking feature has been shown to result from a colloidal crystal configuration of the exine units in at least part of the wall. This paper discusses the mechanics of formation of the colloidal crystal and of the adjacent wall layers (by depletion attraction), and provides a detailed explanation of the cause of the iridescence. Selaginella leaves and virus aggregates provide comparisons with the iridescent arrangement present in the spore wall. The self-assembly aspect of colloidal structures in biological systems is explored. Periods of selfassembly in organismal development may represent a saving in terms of stored information retrieval. Self-assembly, and its often chaotic behaviour, offer the prospect of large changes in organismal construction associated with relatively little change in the underlying genetic configuration.