EARLY DEVELOPMENT PATTERN OF THE BROWN ALGA ECTOCARPUS SILICULOSUS (ECTOCARPALES, PHAEOPHYCEAE) SPOROPHYTE(1).

The distant phylogenetic position of brown macroalgae from the other multicellular phyla offers the opportunity to study novel and alternative developmental processes involved in the establishment of multicellularity. At present, however, very little information is available about developmental patterning in this group. Ectocarpus siliculosus (Dillwyn) Lyngb. has uniseriate filaments and displays one of the simplest architectures in the Phaeophyceae. The aim of this study was to decipher the morphogenetic steps that lead to the development of the Ectocarpus sporophyte. We carried out a detailed morphometric study of the events that occurred between gamete germination and the 100-cell stage. This analysis was performed on two ecologically distant isolates to assess plasticity in developmental patterning within this species. Cell sizes were measured in both isolates, allowing the definition of two main cell types based on their shape (round and elongated). On average, the filament is composed of about 40% round cells, which are present in the central region of the filament, but different combinations of the two cell types within filaments were observed and quantified. Young sporophytes grew apically, with elongated cells progressively differentiating into round cells. Secondary filaments emerged preferentially on round cells, primarily from the older central cells. Statistical analyses showed that the pattern of branching was regulated to ensure a stereotyped architecture. This description of the developmental patterning during the growth of the E. siliculosus sporophyte will serve as a base for more detailed studies of development, in this species and in brown algae in general.