Biology subjects

Billoud, B.

Publications and source records attributed to Billoud, B..

8 recordsLinked to original sources

bioRxiv2026

Radical cell identity bifurcation in Saccharina embryos coincides with the expression of newly acquired genes.

Brown algae evolved independently from animals, land plants and other algae, and we know very little about the spatio-temporal dynamics of their embryogenesis. Here, we used time-lapse, bright-field microscopy to study cell division and lineage development during early embryogenesis in the kelp Saccharina latissima, a large brown alga. We discovered a radical change of cell identity as early as the 4-cell stage: after fertilization, the zygote underwent two or three unequal cell divisions before the basal cell - that closest to the maternal tissue - stopped dividing and radically differentiated into a hyperpolarized cell, the rhizoid, which anchors the embryo to the substrate. RNA-seq analysis showed that differentiation of rhizoid cells was preceded by expression of 130 basal cell-specific genes. Phylostratigraphic analysis further revealed that more than 40% of these basal cell-specific genes appeared after the emergence of the brown algae group, and their functions are largely unknown. By contrast, the apical cell predominantly expressed more ancestral, metabolism-related genes, and it continued to divide to produce the long, blade-shaped thallus of the alga. The early and radical nature of cell differentiation in Saccharina embryos, combined with differential gene expression from various evolutionary periods, highlights the unique mechanisms of embryogenesis of this alga.

developmental biology↗

bioRxiv2026

Cell position is more important than cell shape or age for the acquisition of cell identity in the brown alga Ectocarpus.

The brown alga Ectocarpus is a morphologically simple organism, with cells in the growing filament changing shape and position over time. In this study, we investigated the roles of cell shape, position, and age in establishing cell identity in Ectocarpus. Using laser capture microdissection, we isolated cells with different relative ages, shapes and positions from young sporophytes, and analysed their transcriptomes using differential RNA-seq to attribute a molecular signature to each cell. Five distinct cell transcriptomic signatures were found along the filament of WT Ectocarpus; the apical cell signature differed most from the others. To dissociate cell shape, age, and position, we analysed the transcriptomes of two mutants in which the relationships between these three parameters were disrupted. We found that the transcriptomic profile observed in the WT was significantly altered in these mutants. Overall, our data revealed that molecular cell signatures depend primarily on cell position along the filament and secondarily on cell shape.

developmental biology↗

bioRxiv2024

The longitudinal growth of the embryo of the kelp Saccharina depends on actin filaments that control the formation of an alginate corset in the cell wall

The initiation of embryogenesis in the kelp Saccharina latissima is accompanied by significant anisotropy in cell shape. Using monoclonal antibodies, we show that this anisotropy coincides with a spatio-temporal pattern of accumulation of alginates in the cell wall of the zygote and embryo. Alginates rich in guluronates as well as sulphated fucans show a homogeneous distribution in the embryo throughout Phase I of embryogenesis, but mannuronate alginates accumulate mainly on the sides of the zygote and embryo, disappearing as the embryo enlarges at the start of Phase II. This pattern depends on the presence of cortical actin filaments. In contrast, within the embryo lamina, the alginate composition of the walls newly formed by cytokinesis is not affected by the depolymerisation of actin filaments. Thus, in addition to revealing the existence of a mannuronate-rich alginate corset that may restrict the enlargement of the zygote and the embryo, thereby promoting the formation of the apico-basal growth axis, we demonstrate stage- and cytoskeleton-dependent differences in cell wall deposition in Saccharina embryos.

developmental biology↗

bioRxiv2024

MUM, a maternal unknown message, inhibits early establishment of the mediolateral axis in the embryo of the kelp Saccharina latissima

Brown algae are multicellular photosynthetic organisms that have evolved independently of plants and other algae. Apart from studies on the polarisation of the Fucus zygote in the 1990s, little is known about the mechanisms controlling the embryogenesis of these organisms. Here, we studied the determinism of embryogenesis in the kelp Saccharina latissima, focusing on the formation of its body axes. This alga initially develops an oblong embryo formed of a cell monolayer, which subsequently thickens; growth axes are then established in temporally distinct stages, starting with the formation of a dominant apico-basal axis. Our experiments focused on the role of the stalk, an empty cell that retains the embryo attached to the maternal tissue, in the development of the growth axes in mature embryos. In response to the removal of the stalk, the embryos developed as a monolayered disc rather than an elongated blade, demonstrating that attachment to the stalk inhibits the formation of the medio-lateral axis at the onset of embryogenesis. In addition, compared with embryos attached to the stalk, the cells of detached embryos were smaller and displayed an altered shape. The topology of the tissue was also disrupted, as cells had a higher number of cell neighbours. Observation of cell division patterns just after removal of the stalk showed that the stalk represses longitudinal cell divisions, thereby reinforcing the establishment of the main apico-basal axis. This unique quantitative study of brown algal embryogenesis revealed that, in kelps, a signal from maternal tissue (MUM for maternal unknown message) is necessary for the establishment of growth axes at the onset of embryogenesis and of the organisation of growing embryonic tissues. In addition, we discovered that, although the stalk persists for several weeks until the embryo reaches at least 500 cells, MUM is emitted in the first 4 days after fertilisation only, before the embryo reaches the 8-cell stage. Finally, transplantation experiments indicated that MUM does not diffuse in seawater, but requires contact between the embryo and the stalk. The potential chemical or mechanical nature of MUM is discussed.

developmental biology↗

bioRxiv2023

Analysis of a shark reveals ancient habenular asymmetries in gnathostomes and points to Wnt regulation as a driving force in their diversification

The origin of left-right asymmetries in the vertebrate habenula remains largely unknown. Using a transcriptomic approach, we show that in a cartilaginous fish, the catshark Scyliorhinus canicula, habenulae exhibit marked asymmetries both in their medial and their lateral component. Comparisons across gnathostomes suggest that asymmetries in the catshark lateral habenulae reflect an ancestral gnathostome trait, independently lost in tetrapods and neopterygians. Analysis of the mechanisms underlying their formation highlights an essential role of Wnt signaling. Wnt activity is submitted to a dynamic, asymmetric regulation during habenula development, with a Nodal dependent left repression at a stage when precursors for lateral habenulae have exited cell cycles. Pharmacological treatments during this time window reveal that Wnt signaling promotes lateral right neuronal identities in the right lateral habenula, while its repression by Nodal in the left one promotes lateral left neuronal identities. Based on comparisons with the zebrafish and the mouse, we propose that habenular asymmetry formation and diversification in gnathostomes involve the same developmental logic, relying on a conserved temporal regulation of neurogenesis, shaping neuronal identities on both sides, and its modification by a dynamic Wnt activity, right-restricted in the ancestral state and prone to variations in time and space during evolution.

developmental biology↗

bioRxiv2022

Chromosomal assembly of the flat oyster (Ostrea edulis L.) genome as a new genetic ressource for aquaculture

The European flat oyster (Ostrea edulis L.) is the endemic species of the European coasts. Its exploitation has been reduced during the last decades, because of the appearance of two parasites that have led to the collapse of the stocks and the strong decline of the natural oyster beds. O. edulis has been the subject of numerous studies and programs in population genetics and on the presence of the parasites Bonamia ostreae and Marteilia refringens. These studies investigated the effects of these parasites mainly on immunity at the molecular and cellular levels. Several genetic selection programs especially related to resistance to the parasite have been initiated. Within the framework of a European project (PERLE 2) which aims to produce genetic lines of O. edulis with hardiness traits (growth, survival, resistance) for the purpose of repopulating natural oyster beds in Brittany and reviving the culture of this species on the foreshore, obtaining a reference genome has proved to be essential as done recently in many bivalve species of aquaculture interest. Here, we present a chromosome-level genome assembly and annotation for the European flat oyster, generated by combining PacBio technology, Illumina, 10X linked and Hi-C sequencing. The finished assembly is 887.2 Mb with a scaffold-N50 of 97.1 Mb scaffolded on the expected 10 pseudo-chromosomes. Annotation of the genome revealed the presence of 35962 protein-coding genes. We analyzed in details the transposable elements (TE) diversity in the flat oyster genome, highlight some specificities in tRNA and miRNA composition and provide first insights in the molecular response of O. edulis to M. refringens. This genome will serve as a reference for genomic studies on O. edulis to better understand its basic physiology or developing genetic markers in breeding projects for aquaculture or natural reef restoration.

genomics↗

bioRxiv2021

Tip growth in the brown alga Ectocarpus is controlled by a RHO-GAP-BAR domain protein independently from F-actin organisation

The brown alga Ectocarpus is a filamentous seaweed that grows by tip growth and branching. In the morphometric mutant etoile, tip growth is slower than in the WT and eventually stops. In this paper, we show that the causal etoile mutation is a null mutation in a bi-domain BAR-RhoGAP gene. By quantitative RT-PCR, we showed that ETOILE is ubiquitously expressed in prostrate filaments of the Ectocarpus sporophyte, and is downregulated in the etoile mutant. We immunolocalised both domains of the protein in WT and etoile, as well as RAC1, the known target of Rho-GAP enzymes. Thus, ETOILE would be localised at the apical cell dome where it would control the localisation of EsRAC1 to the plasma membrane. Actin staining showed that the mutant is not affected in F-actin structures. Overall, these results suggest that in Ectocarpus, BAR-RhoGAP controls tip growth by controlling RAC1 localization and through an actin-independent mechanism.

developmental biology↗

bioRxiv2021

Acquisition of cell identity in the brown alga Ectocarpus: which of time, cell shape or position matters most?

During development, cells undergo simultaneous changes of different types that together depict cell "identity". In the multicellular brown alga Ectocarpus sp., while ageing, cells change shape and relative position within the filament. Understanding how these factors act and interact to specify cell identity requires markers of cell identity and the ability to genetically separate age, shape and position. Here we used laser capture microdissection (LCM) to isolate specific cell types from young sporophytes of Ectocarpus, and performed differential RNA-seq analysis. Transcriptome profiles of cell types in the wild-type strain provided signatures of the five cell types that can be identified by shape and position. In two mutants, where the relationship between cell shape, position and age are altered, transcriptome signatures revealed that little differential expression could be identified when only shape was perturbed. More generally, although the two mutants are characterised by opposite morphological phenotypes, their transcriptomes were remarkably similar. We concluded that despite the robustness of cell differentiation during WT development, neither the shape nor the position of the cell could serve as a faithful gauge for tracking differentiation.

developmental biology↗