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Abstract

In the model brown alga Ectocarpus sp.7, the independent gametophytic (GA) and sporophytic (SP) generations deploy a distinct developmental program autonomously from an initial cell. Both generations possess a simple thallus organization composed of few cell-types that can be grouped into 1) a basal body part, allowing attachment to the substratum (rhizoids in GA and rhizoids and prostrate filaments in SP) and 2) an erected body part that bears the reproductive structures (filaments in GA and upright filaments in SP). We identified mutants affected in two different genes, DISTAG and BASELESS, which lost basal structures in both generations. Several cellular abnormalities are observed in the initial cell including disruption of the germination pattern, modification of cytoskeleton organisation, nucleus mis- positioning and Golgi apparatus fragmentation. Altogether, those phenotypes emphasize the crucial role of the first cell division in the establishment of major developmental axes and cell fates and the importance of these genes for pattern development in both the gametophyte and sporophyte. DISTAG and BASELESS encode, respectively, a Tubulin-binding co-factor C d1 (TBCCd1) protein and a Ca 2+ -sensitive type B" regulatory subunit of protein phosphatase 2A (PP2A), highlighting the role of the cytoskeleton and Ca 2+ signalling in the initial cell division. Orthologues of both proteins regulate internal cell organization in plants and animals demonstrating the deep evolutionary conservation of those intra-cellular processes.