The Florideophyceae is the most abundant and taxonomically diverse class of red algae (Rhodophyta). However, many aspects of the systematics and divergence times of the group remain unresolved. Using a seven-gene concatenated dataset (nuclear EF2, LSU and SSU rRNAs, mitochondrial cox1, and plastid rbcL, psaA and psbA genes), we generated a robust phylogeny of red algae to provide an evolutionary timeline for florideophyte diversification. Our relaxed molecular clock analysis suggests that the Florideophyceae diverged approximately 943 (817–1,049) million years ago (Ma). The major divergences in this class involved the emergence of Hildenbrandiophycidae [ca. 781 (681–879) Ma], Nemaliophycidae [ca. 661 (597–736) Ma], Corallinophycidae [ca. 579 (543–617) Ma], and the split of Ahnfeltiophycidae and Rhodymeniophycidae [ca. 508 (442–580) Ma]. Within these clades, extant diversity reflects largely Phanerozoic diversification. Divergences within Florideophyceae were accompanied by evolutionary changes in the carposporophyte stage, leading to a successful strategy for maximizing spore production from each fertilization event. Our research provides robust estimates for the divergence times of major lineages within the Florideophyceae. This timeline was used to interpret the emergence of key morphological innovations that characterize these multicellular red algae.

Red algae (Rhodophyta) underwent two phases of large-scale genome reduction during their early evolution. The red seaweeds did not attain genome sizes or gene inventories typical of other multicellular eukaryotes. We generated a high-quality 92.1 Mb draft genome assembly from the red seaweed Gracilariopsis chorda, including methylation and small (s)RNA data. We analyzed these and other Archaeplastida genomes to address three questions: 1) What is the role of repeats and transposable elements (TEs) in explaining Rhodophyta genome size variation, 2) what is the history of genome duplication and gene family expansion/reduction in these taxa, and 3) is there evidence for TE suppression in red algae? We find that the number of predicted genes in red algae is relatively small (4,803-13,125 genes), particularly when compared with land plants, with no evidence of polyploidization. Genome size variation is primarily explained by TE expansion with the red seaweeds having the largest genomes. Long terminal repeat elements and DNA repeats are the major contributors to genome size growth. About 8.3% of the G. chorda genome undergoes cytosine methylation among gene bodies, promoters, and TEs, and 71.5% of TEs contain methylated-DNA with 57% of these regions associated with sRNAs. These latter results suggest a role for TE-associated sRNAs in RNA-dependent DNA methylation to facilitate silencing. We postulate that the evolution of genome size in red algae is the result of the combined action of TE spread and the concomitant emergence of its epigenetic suppression, together with other important factors such as changes in population size.

No abstract

Dictyochophyceae (silicoflagellates) are unicellular freshwater and marine algae (Heterokontophyta, stramenopiles). Despite their abundance in global oceans and potential ecological significance, discovered in recent years, neither nuclear nor organellar genomes of representatives of this group were sequenced until now. Here, we present the first complete plastid genome sequences of Dictyochophyceae, obtained from four species: Dictyocha speculum, Rhizochromulina marina, Florenciella parvula and Pseudopedinella elastica. Despite their comparable size and genetic content, these four plastid genomes exhibit variability in their organization: plastid genomes of F. parvula and P. elastica possess conventional quadripartite structure with a pair of inverted repeats, R. marina instead possesses two direct repeats with the same orientation and D. speculum possesses no repeats at all. We also observed a number of unusual traits in the plastid genome of D. speculum, including expansion of the intergenic regions, presence of an intron in the otherwise non‐intron‐bearing psaA gene, and an additional copy of the large subunit of RuBisCO gene (rbcL), the last of which has never been observed in any plastid genome. We conclude that despite noticeable gene content similarities between the plastid genomes of Dictyochophyceae and their relatives (pelagophytes, diatoms), the number of distinctive features observed in this lineage strongly suggests that additional taxa require further investigation.

Human activity is an important driver of ecological and evolutionary change on our planet. In particular, domestication and biological introductions have important and long-lasting effects on species’ genomic architecture and diversity. However, genome-wide analysis of independent domestication and introduction events within a single species has not previously been performed. The Pacific kelp Undaria pinnatifida provides such an opportunity because it has been cultivated in its native range in Northeast Asia but also introduced to four other continents in the past 50 years. Here we present the results of a genome-wide analysis of natural, cultivated and introduced populations of U. pinnatifida to elucidate human-driven evolutionary change. We demonstrate that these three categories of origin can be distinguished at the genome level, reflecting the combined influence of neutral (demography and migration) and non-neutral (selection) processes.

No abstract

Among the brown algal lineages, ectocarpales species have isogamous fertilization in which male and female gametes are morphologically similar. in contrast, female gametes are much larger than male gametes in the oogamous species found in many other brown algal lineages. it has been reported that the plastids of isogamous species are biparentally inherited whereas the plastids of oogamous species are maternally inherited. in contrast, in both isogamous and oogamous species, the mitochondria are usually inherited maternally. to investigate whether there is any relationship between the modes of inheritance and organellar genome architecture, we sequenced six plastid genomes (ptDnA) and two mitochondrial genomes (mtDnA) of isogamous species from the ectocarpales and compared them with previously sequenced organellar genomes. We found that the biparentally inherited ptDnAs of isogamous species presented distinctive structural rearrangements whereas maternally inherited ptDnAs of oogamous species showed no rearrangements. our analysis permits the hypothesis that structural rearrangements in ptDnAs may be a consequence of the mode of inheritance. The brown algae (Phaeophyceae) are a group of photosynthetic heterokonts (=stramenopiles), a secondary endosymbiotic lineage containing red algal-derived plastids 1. The four brown algal orders analyzed in this study, Ectocarpales, Fucales, Laminariales, and Dictyotales, have macroscopic thalli and are distributed worldwide in low-to-mid latitudes where they play important roles in marine ecosystems. Also, they have great potential for commercial uses such as food or in other seaweed industries. From a taxonomic point of view, the orders Ectocarpales, Fucales and Laminariales are grouped into the subclass Fucophycidae, which is defined as a lineage derived from the brown algal crown radiation (BACR), whereas Dictyotales is classified into the Dictyotophycidae 2 (Fig. S1). The life cycles of most brown algae, including species of the Laminariales and Ectocarpales, are characterized by an alternation between a diploid sporophyte and a haploid gametophyte (i.e., a diploid-haploid life cycle). In contrast, in the Fucales meiosis occurs in the parental diploid plants and they directly produce gametes (i.e., a diplontic life cycle) 3,4. Three fertilization types can be defined based on the morphologies and flagella of gametes, independent of whether they are produced via a gametophyte stage or not: the isogamous type in which flagellated gametes are morphologically similar; the anisogamous type where one flagellated gamete is larger than the other; and the oogamous type in which one gamete is a non-motile cell called the egg cell (or oocyte) and the other is a smaller, flagellated sperm cell 5. Most brown algae, including the Laminariales and most species of the Fucales, are oogamous. Anisogamy is only observed in a few lineages such as the Onslowiales, Asterocladales, Nemodermatales, and in some Fucales, Sphacelariales and Ectocarpales species. With the exception of the orders Ecto...

Background The Cyanidiophyceae is an early-diverged red algal class that thrives in extreme conditions around acidic hot springs. Although this lineage has been highlighted as a model for understanding the biology of extremophilic eukaryotes, little is known about the molecular evolution of their mitochondrial genomes (mitogenomes). Results To fill this knowledge gap, we sequenced five mitogenomes from representative clades of Cyanidiophyceae and identified two major groups, here referred to as Galdieria-type (G-type) and Cyanidium-type (C-type). G-type mitogenomes exhibit the following three features: (i) reduction in genome size and gene inventory, (ii) evolution of unique protein properties including charge, hydropathy, stability, amino acid composition, and protein size, and (iii) distinctive GC-content and skewness of nucleotides. Based on GC-skew-associated characteristics, we postulate that unidirectional DNA replication may have resulted in the rapid evolution of G-type mitogenomes. Conclusions The high divergence of G-type mitogenomes was likely driven by natural selection in the multiple extreme environments that Galdieria species inhabit combined with their highly flexible heterotrophic metabolism. We speculate that the interplay between mitogenome divergence and adaptation may help explain the dominance of Galdieria species in diverse extreme habitats.