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.
In response to environmental constraints, living organisms organise their body according to axes, rotation and translation plans, or asymmetries. Cellular and molecular processes are involved in the establishment of this architecture. Hence, this review aims at presenting the molecular mechanisms controlling the main symmetries and axes in plants. Several genes, coding for transcription factors, have been identified in land plants (mainly Arabidopsis thaliana), as controlling the establishment of apico-basal and adaxial-abaxial axes mainly. The establishment of these axes allows the development in other spatial directions of radial or bilateral symmetries. These processes seem in most cases to be under the control of the phytohormone auxin. In brown algae, which are all multicellular marine plants, polarity plans are less obvious than in land plants. The development of the model brown alga Ectocarpus siliculosus is currently being studied. E. siliculosus develops a filamentous architecture, and primary observations show that branching along the main axis occurs in a non-stereotyped and regular way, even though it is mainly centred. However, more detailed morphometrical studies, accompanied by probabilistic analyses, have shown that, among the overall population of individuals, organisms obey yet unidentified biological constraints, that aim at refining the radial symmetry as the organism grows. The role of this symmetry in the adaptation of E. siliculosus to its environment, as well as the molecular actors involved in this process, are currently under study in our laboratory.
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