Acetabularia is an extraordinary organism. It is a single cell alga, yet its size (2 to 3 cm) and its complexity would suggest otherwise (Fig. 1). Moreover, cell differentiation is controlled by a single nucleus located at the basal end of the cell. The molecular and mechanical basis for the morphogenesis of this complex cell geometry remains for the most part a mystery.

We are interested in understanding how complex cellular shapes are achieved in Acetabularia and other dasyclads. Most of the morphological diversity in the dasyclads is observed in the complex whorled structures formed by the apex. Among these, the best known are the vegetative and reproductive whorls of Acetabularia acetabulum (Fig. 1). We are doing experiment to elucidate the origin of these structures.

Fig.1: Young reproductive whorl (cap) of Acetabularia acetabulum. Two vegetative whorls are also seen.

We identified five pattern forming events in the morphogenesis of the vegetative whorl of Acetabularia (Fig. 2) and similar events are observed for the reproductive whorl.

Fig. 2: Developmental stages in the growth of the vegetative whorl of Acetabularia.

Fig. 3: Stage 3 reproductive whorl in Acetabularia. Wall pits are the first sign of the location of lateral appendages.

The observed vegetative whorls can have as few as three hairs and as many as 35 hairs. Such variation gives the impression that whorl formation is poorly regulated but it is not so. Seminal work by Lionel Harrison at the University of British Columbia has shown that although the number of appendages in a whorl can vary greatly, the spacing between appendages remains nearly constant. Therefore, the large variation observed in the number of appendages reflects simply differences in the size of the tip when the whorl is formed. This observation suggests a "measuring" mechanism for the positioning of appendages. The exact nature of this mechanism remains unknown although several proposals have been made (see Dumais and Harrison, 2000).