Asymmetric cell division and differentiation; fern spore germination as a model. I. Physiological aspects

Abstract

SynopsisDuring germination of Onoclea sensibilis spores, the spore nucleus moves from a central location to one end. Cell division partitions the spore into a small cell which differentiates into a rhizoid, and a larger cell which gives rise, by continued division, to the prothallus. Spore germination is a valuable system in which to study the relationship between asymmetric cell division and the initiation of cell differentiation. It appears that cytoplasmic microtubules and some lipophilic site in the spore are both involved in premitotic nuclear migration. Asymmetric cell division is an obligate step in the initiation of rhizoid differentiation. One hypothesis is that the nucleus must be confined to a small, localised region of the spore for a sufficient time before differentiation can occur, and the confinement is accomplished by the asymmetric cell division. Metal-binding sites are present in the spore coat, specifically on the proximal face. Several types of evidence suggest that the metal-binding region is involved in spore polarity. The sequence of events during germination appears to involve both polarity which is inherent in the spore and polarity which may be imposed by external stimuli. Experiments in which spores are treated with colchicine and polarised red light reveal both types of polarity.