Whorl morphogenesis in the dasycladalean algae: the pattern formation viewpoint

The dasycladalean algae produce diverse whorled structures, among which the best known are the vegetative and reproductive whorls of Acetabularia acet abulum. In this paper, we review the literature pertaining to the origin of these structures. The question is addressed in terms of the necessary patt ern-forming events and the possible mechanisms involved, an outlook we call the pattern formation viewpoint. The pattern-forming events involved in th e morphogenesis of the vegetative and reproductive whorls of Acetabularia h ave been used to define five and six morphogenetic stages, respectively. We discuss three published mechanisms which account, at least in part? for th e pattern-forming events. The mechanisms are mechanical buckling of the cel l wall, reaction-diffusion of morphogen molecules along the cell membrane, and mechanochemical interactions between Ca2+ ions and the cytoskeleton in the cytosol. The numerous differences between these mechanisms provide expe rimental grounds to test their validity. To date, the results of these expe riments point towards reaction-diffusion as the most likely patterning mech anism. Finally, we consider the evolutionary origin of the vegetative and r eproductive whorls and provide mechanistic explanations for some of the maj or evolutionary advances.