Space-time decoupling in the branching process in the mutant étoile of the filamentous brown alga Ectocarpus siliculosus

Ectocarpus siliculosus is being developed as a model organism for brown algal genetics and genomics.1,2 Brown algae are phylogenetically distant from the other multicellular phyla (green lineage, red algae, fungi and metazoan)3 and therefore might offer the opportunity to study novel and alternative developmental processes that lead to the establishment of multicellularity. E. siliculosus develops as uniseriate filaments, thereby displaying one of the simplest architectures among multicellular organisms.4 The young sporophyte grows as a primary filament and then branching occurs, preferentially at the center of the filament. We recently described the first morphogenetic mutant etoile (etl) in a brown alga, produced by UVB mutagenesis in E. siliculosus.5 We showed that a single recessive mutation was responsible for a defect in both cell differentiation and the very early branching pattern (first and second branch emergences). Here, we supplement this study by reporting the branching defects observed subsequently, i.e. for the later stages corresponding to the emergence of up to the first six secondary filaments, and we show that the branching process is composed of at least two distinct components: time and position.   The developmental pattern of E. siliculosus is characterized by a very high level of morphological plasticity.6 Observations followed by statistical analyses allowed analyzing the morphometric features accompanying the establishment of the branching pattern in the mutant etoile, compared with the wild type (WT) organism (strain Ec32). The branching pattern can be deciphered in two main components: (1) the timing of branching and (2) the position of branching.