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Multicellular eukaryotes are characterized by an expanded extracellular matrix (ECM) with a diversified composition. The ECM is involved in determining tissue texture, screening cells from the outside medium, development, and innate immunity, all of which are essential features in the biology of multicellular eukaryotes. This review addresses the origin and evolution of the ECM, with a focus on multicellular marine algae. We show that in these lineages the expansion of extracellular matrix played a major role in the acquisition of complex multicellularity through its capacity to connect, position, shield, and defend the cells. Multiple innovations were necessary during these evolutionary processes, leading to striking convergences in the structures and functions of the ECMs of algae, animals, and plants.

• Brown algae are an important group of multicellular eukaryotes, phylogenetically distinct from both the animal and land plant lineages. Ectocarpus has emerged as a model organism to study diverse aspects of brown algal biology, but this system currently lacks an effective reverse genetics methodology to analyse the functions of selected target genes. • Here, we report that mutations at specific target sites are generated following the introduction of CRISPR-Cas9 ribonucleoproteins into Ectocarpus cells, using either biolistics or microinjection as the delivery method. • Individuals with mutations affecting the ADENINE PHOSPHORIBOSYL TRANSFERASE (APT) gene were isolated following treatment with 2-fluoroadenine, and this selection system was used to isolate individuals in which mutations had been introduced simultaneously at APT and at a second gene. This double mutation approach could potentially be used to isolate mutants affecting any Ectocarpus gene, providing an effective reverse genetics tool for this model organism. • The availability of this toolwill significantly enhance the utility of Ectocarpus as amodel organism for this ecologically and economically important group of marine organisms. Moreover, the methodology described here should be readily transferable to other brown algal species.

A reference genome from the coccolithophore Emiliania huxleyi is presented, along with sequences from 13 additional isolates, revealing a pan genome comprising core genes and genes variably distributed between strains: E. huxleyi is found to harbour extensive genetic variability under different metabolic repertoires, explaining its ability to thrive under a diverse range of environmental conditions. Emiliania genomes reveal great diversity This paper presents a reference genome from the coccolithophore Emiliania huxleyi strain CCMP1516. Coccolithophores are a major component of marine phytoplankton and can account for 20% of total carbon fixation in some systems, so have an important influence on global climate. Comparison of the reference genome to sequences from 13 other strains reveals a pan genome composed of core genes and genes variably distributed between strains. The findings indicate extensive genome variability reflected in different metabolic repertoires, explaining in part how E. huxleyi can thrive and form large-scale episodic blooms under a wide variety of environments. Coccolithophores have influenced the global climate for over 200 million years 1 . These marine phytoplankton can account for 20 per cent of total carbon fixation in some systems 2 . They form blooms that can occupy hundreds of thousands of square kilometres and are distinguished by their elegantly sculpted calcium carbonate exoskeletons (coccoliths), rendering them visible from space 3 . Although coccolithophores export carbon in the form of organic matter and calcite to the sea floor, they also release CO 2 in the calcification process. Hence, they have a complex influence on the carbon cycle, driving either CO 2 production or uptake, sequestration and export to the deep ocean 4 . Here we report the first haptophyte reference genome, from the coccolithophore Emiliania huxleyi strain CCMP1516, and sequences from 13 additional isolates. Our analyses reveal a pan genome (core genes plus genes distributed variably between strains) probably supported by an atypical complement of repetitive sequence in the genome. Comparisons across strains demonstrate that E. huxleyi , which has long been considered a single species, harbours extensive genome variability reflected in different metabolic repertoires. Genome variability within this species complex seems to underpin its capacity both to thrive in habitats ranging from the equator to the subarctic and to form large-scale episodic blooms under a wide variety of environmental conditions.

Brown algae are important primary constituents of marine coastal ecosystems, characterised by complex life cycles and various levels of complex multicellular development. However, the molecular processes that underlie development and life cycle progression in the brown algae remain poorly understood. In this study, pairwise comparisons of gametophyte and sporophyte transcriptomes across 10 diverse brown algal species showed that the total number of genes exhibiting generation-biased or generation-specific expression in each species was correlated with the degree of dimorphism between life cycle generations. However, analysis of gene ontology terms assigned to the generation-biased/generation-specific genes indicated that each generation (i.e. the sporophyte and the gametophyte) also has characteristic broad life-cycle-related features that have been conserved during evolution. A more detailed analysis of Ectocarpus species 7 identified progressive transcriptome changes over its entire life cycle, with a particularly marked change in transcriptome composition during the first day of sporophyte development, characterised by downregulation of flagellar and transcription factor genes and upregulation of a subset of translation genes. Comparison with a similar transcriptomic time series for the evolutionarily distant (about 250 My) brown alga Dictyota dichotoma indicated considerable conservation of co-expressed gene modules between the two species, particularly for modules that were enriched in genes assigned to evolutionarily conserved functional categories. This study therefore identified broad life-cycle- and development-related patterns of gene expression that are conserved across the brown algae. Coastal marine environments are often populated by large underwater forests of brown algae, such as seaweeds and kelps. The brown algae are key components of these coastal ecosystems, providing food and habitat for numerous organisms. From a developmental perspective, brown algae are particularly interesting because they evolved multicellularity independently of land plants and animals. They therefore represent an alternative system to investigate the evolution and molecular bases of developmental processes. Most brown algae have life cycles that alternate between a spore-producing generation (the sporophyte) and a gamete-producing generation (the gametophyte). In some cases, these distinct life stages may enhance survival under variable conditions. However, the genetic basis of development in these complex multicellular organisms – which could shed more light on their ecology – remains poorly understood. Ratchinski et al. investigated the activity of genes across multiple life-cycle stages in a broad range of brown algal species to identify groups of genes that potentially play key roles at specific stages of development. The results showed that brown algal species with strongly differentiated life-cycle generations tended to have more genes that are differentially expressed during each generation, correlating with the differences in morphological traits. Furthermore, several groups of genes that exhibited coordinated patterns of expression over the course of the life cycle (so-called co-expression modules) tended to share related functions. Ratchinski et al. also noted extensive changes in gene expression during the early stages of sporophyte development, and similar patterns were observed in two evolutionarily distant brown algae species. Seaweed cultivation is attracting growing interest for producing biomass for human consumption and other applications. At the same time, wild populations of brown algae are increasingly threatened by global warming. A deeper understanding of brown algae biology is therefore vital for developing sustainable cultivation methods and effective conservation strategies to protect natural brown algal ecosystems.

Background Brown algae evolved complex multicellularity independently of the animal and land plant lineages and are the third most developmentally complex phylogenetic group on the planet. An understanding of developmental processes in this group is expected to provide important insights into the evolutionary events necessary for the emergence of complex multicellularity. Here, we focus on mechanisms of epigenetic regulation involving post-translational modifications of histone proteins. Results A total of 47 histone post-translational modifications are identified, including a novel mark H2AZR38me1, but Ectocarpus lacks both H3K27me3 and the major polycomb complexes. ChIP-seq identifies modifications associated with transcription start sites and gene bodies of active genes and with transposons. H3K79me2 exhibits an unusual pattern, often marking large genomic regions spanning several genes. Transcription start sites of closely spaced, divergently transcribed gene pairs share a common nucleosome-depleted region and exhibit shared histone modification peaks. Overall, patterns of histone modifications are stable through the life cycle. Analysis of histone modifications at generation-biased genes identifies a correlation between the presence of specific chromatin marks and the level of gene expression. Conclusions The overview of histone post-translational modifications in the brown alga presented here will provide a foundation for future studies aimed at understanding the role of chromatin modifications in the regulation of brown algal genomes.

Although evolutionary transitions from sexual to asexual reproduction are frequent in eukaryotes, the genetic bases of these shifts remain largely elusive. Here, we used classic quantitative trait analysis, combined with genomic and transcriptomic information to dissect the genetic basis of asexual, parthenogenetic reproduction in the brown alga Ectocarpus. We found that parthenogenesis is controlled by the sex locus, together with two additional autosomal loci, highlighting the key role of the sex chromosome as a major regulator of asexual reproduction. We identify several negative effects of parthenogenesis on male fitness, and different fitness effects of parthenogenetic capacity depending on the life cycle generation. Although allele frequencies in natural populations are currently unknown, we discuss the possibility that parthenogenesis may be under both sex-specific selection and generation/ploidally-antagonistic selection, and/or that the action of fluctuating selection on this trait may contribute to the maintenance of polymorphisms in populations. Importantly, our data provide the first empirical illustration, to our knowledge, of a trade-off between the haploid and diploid stages of the life cycle, where distinct parthenogenesis alleles have opposing effects on sexual and asexual reproduction and may help maintain genetic variation. These types of fitness trade-offs have profound evolutionary implications in natural populations and may structure life history evolution in organisms with haploid-diploid life cycles.

Background Histones are among the most conserved proteins in eukaryotes. They not only ensure DNA compaction in the nucleus but also participate in epigenetic regulation of gene expression. These key epigenetic players are divided into replication-coupled histones, expressed during the S-phase, and replication-independent variants, expressed throughout the cell cycle. Compared with other core histones, H2A proteins exhibit a high level of variability but the characterization of algal H2A variants remains very limited. Results In this study, we exploit genome and transcriptome data from 22 species to identify H2A variants in brown seaweeds. Combined analyses of phylogenetic data, synteny and protein motifs enable us to reveal the presence of new H2A variants as well as their evolutionary history. We identify three new H2A variants: H2A.N, H2A.O and H2A.E. In brown seaweeds, the H2A.E and H2A.O variants arose from the same monophyletic clade while the H2A.N variant emerged independently. Moreover, the H2A.E variant seems to have a shared ancestry with RC H2A while the H2A.O variant has an H2A.X-characteristic signature without being orthologous to this variant. Based on mass spectrometry, we identify distinct epigenetic marks on these H2A variants. Finally, the H2A.Z, H2A.N and H2A.O from brown seaweeds are ubiquitously expressed while expression of H2A.E has tissue-specific patterns, especially in reproductive tissues. Conclusions We thus hypothesize that H2A.O and H2A.X might have convergent functions while H2A.E might fulfil some functions of replication-coupled H2As and/or compensate for the absence of repressive histone marks along with H2A.N.

Males and females often have marked phenotypic differences, and the expression of these dissimilarities invariably involves sex differences in gene expression. Sex-biased gene expression has been well characterized in animal species, where a high proportion of the genome may be differentially regulated in males and females during development. Male-biased genes tend to evolve more rapidly than female-biased genes, implying differences in the strength of the selective forces acting on the two sexes. Analyses of sex-biased gene expression have focused on organisms that exhibit separate sexes during the diploid phase of the life cycle (diploid sexual systems), but the genetic nature of the sexual system is expected to influence the evolutionary trajectories of sex-biased genes. We analyze here the patterns of sex-biased gene expression in Ectocarpus, a brown alga with haploid sex determination (dioicy) and a low level of phenotypic sexual dimorphism. In Ectocarpus, female-biased genes were found to be evolving as rapidly as male-biased genes. Moreover, genes expressed at fertility showed faster rates of evolution than genes expressed in immature gametophytes. Both male- and female-biased genes had a greater proportion of sites experiencing positive selection, suggesting that their accelerated evolution is at least partly driven by adaptive evolution. Gene duplication appears to have played a significant role in the generation of sex-biased genes in Ectocarpus, expanding previous models that propose this mechanism for the resolution of sexual antagonism in diploid systems. The patterns of sex-biased gene expression in Ectocarpus are consistent both with predicted characteristics of UV (haploid) sexual systems and with the distinctive aspects of this organism's reproductive biology.

Hi-C exploits contact frequencies between pairs of loci to bridge and order contigs during genome assembly, resulting in chromosome-level assemblies. Because few robust programs are available for this type of data, we developed instaGRAAL, a complete overhaul of the GRAAL program, which has adapted the latter to allow efficient assembly of large genomes. instaGRAAL features a number of improvements over GRAAL, including a modular correction approach that optionally integrates independent data. We validate the program using data for two brown algae, and human, to generate near-complete assemblies with minimal human intervention.

Sex discriminating genetic markers are commonly used to facilitate breeding programs in economically and ecologically important animal and plant species. However, despite their considerable economic and ecological value, the development of sex markers for kelp species has been very limited. In this study, we used the recently described sequence of the sex determining region (SDR) of the brown algal model Ectocarpus to develop novel DNA-based sex-markers for three commercially relevant kelps: Laminaria digitata, Undaria pinnatifida and Macrocystis pyrifera. Markers were designed within nine protein coding genes of Ectocarpus male and female (U/V) sex chromosomes and tested on gametophytes of the three kelp species. Seven primer pairs corresponding to three loci in the Ectocarpus SDR amplified sex-specific bands in the three kelp species, yielding at least one male and one female marker for each species. Our work has generated the first male sex-specific markers for L. digitata and U. pinnatifida, as well as the first sex markers developed for the genus Macrocystis. The markers and methodology presented here will not only facilitate seaweed breeding programs but also represent useful tools for population and demography studies and provide a means to investigate the evolution of sex determination across this largely understudied eukaryotic group.