DISTAG/TBCCd1 Is Required for Basal Cell Fate Determination in Ectocarpus

Abstract

Brown algae are one of the most developmentally complex groups within the eukaryotes. As in many land plants and animals, their main body axis is established early in development, when the initial cell gives rise to two daughter cells that have apical and basal identities, equivalent to shoot and root identities in land plants, respectively. We show here that mutations in the Ectocarpus DISTAG (DIS) gene lead to loss of basal structures during both the gametophyte and the sporophyte generations. Several abnormalities were observed in the germinating initial cell in dis mutants, including increased cell size, disorganization of the Golgi apparatus, disruption of the microtubule network, and aberrant positioning of the nucleus. DIS encodes a TBCCd1 protein, which has a role in internal cell organization in animals, Chlamydomonas reinhardtii, and trypanosomes. Our study highlights the key role of subcellular events within the germinating initial cell in the determination of apical/basal cell identities in a brown alga and emphasizes the remarkable functional conservation of TBCCd1 in regulating internal cell organization across extremely distant eukaryotic groups.

Ectocarpus: an evo-devo model for the brown algae

“…2; [10,15]). Both generations consist of uniseriate, branched filaments, but there are some morphological differences between the two generations [15,16]. Gametophyte germlings are made up of a rhizoid and an upright filament, the latter consisting of cylindrical cells (Fig.…”

“…Male and female gametes are released into the surrounding seawater, where they fuse to produce zygotes that initiate the sporophyte generation. Gametophytes and sporophytes are subtly different in terms of cell types, cell size and angle of branching [16]. The Ectocarpus life cycle also includes several alternative pathways (dashed arrows).…”

“…Classical genetic analysis of mutants is possible, with protocols having been developed both for carrying out genetic crosses and for the isolation of meiotic progeny [9,10,13,56]. Using these approaches large segregating populations can be generated for mapping experiments [30] and mutant alleles have also recently been identified by a cloning-by-sequencing approach [16].…”

“…Immunostaining protocols have been employed successfully to describe the cellular modifications associated with mutations in Ectocarpus genes involved in pattern formation (e.g. [16]). Immunostaining using specific monoclonal antibodies directed against sulphated cell wall polysaccharides has also been employed to investigate cell wall dynamics during early development of Ectocarpus [58] and to describe cellular responses to osmotic stress [59].…”

Axis of Algae: Disruption of Basal Cell Fates in the Brown Alga Ectocarpus

“…The basal cell forms filamentous rhizoids that anchor the algae to the substratum, analogous to roots in land plants. Godfroy et al (2017) identified an Ectocarpus mutant that does not produce rhizoids (see figure) and named this mutant distag (dis), which means "detached" in Breton. Ectocarpus has alternating gametophytic and sporophytic generations, which start from a single cell and develop similar structures.…”