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Stigeoclonium is a genus of green algae that is widely distributed in freshwater habitats around the world. The genus comprises species with variously developed prostrates and erect systems of uniseriate branched filaments and grows attached to a wide range of different surfaces. It holds significant promise for applications in water quality indicators, sewage treatment, and the development of high-value-added products. Nevertheless, our comprehension of Stigeoclonium remains unclear and perplexing, particularly regarding its fundamental systematic taxonomy. Recent molecular analyses have revealed that the morphologically well-defined genus Stigeoclonium is polyphyletic and requires taxonomic revision. Phylogenetic analysis based on a single molecular marker and limited samples is insufficient to address the polyphyletic nature of Stigeoclonium. In the present study, 34 out of 45 strains of Stigeoclonium were newly acquired from China. Alongside the morphological data, a concatenated dataset of three markers (18S rDNA + ITS2 + tufA) was utilized to determine their molecular phylogeny. The phylogenetic analysis successfully resolved the broadly defined Stigeoclonium into three robustly supported clades (Stigeoclonim tenue clade, S. farctum clade, and S. helveticum clade). The morphological characteristics assessment results showed that the cell type of the main axis-producing branch, considered a crucial morphological characteristic of the Stigeoclonium taxonomy, did not accurately reflect the real phylogeny of the genus. A new taxonomical classification of the genus Stigeoclonium was proposed based on zoospores’ germination types, which aligned well with the phylogenetic topologies. Species where zoospores showed erect germination (S. helveticum clade) formed a distinct monophyletic clade, clearly separated from the other two clades, with zoospores showing prostrate germination or pseudo-erect germination. Consequently, a new genus, Pseudostigeoclonium gen. nov., is suggested to include all species in the broadly defined Stigeoclonium with zoospores with erect germination. The taxonomic diversity is supported by distinctive morphological differences and phylogenetic divergence within the broadly defined Stigeoclonium identified in this study. Further evaluation of the genus Stigeoclonium is necessary, especially via examining additional specimens and re-evaluating morphological characters under precisely defined laboratory conditions. Full article

Filamentous green chaetophoralean algae are distributed mainly in aquatic ecosystems, less known from terrestrial habitats. Many of them have a complicated thalli and complex life cycles that make it difficult to determine these organisms by morphology. Description of new representatives of the Chaetophorales continues. In this study, we have explored the filamentous green alga IRK–A 341 from soil of the Baikal Region by light and electron microscopy along with molecular phylogeny. Based on the results of morphological, ecological, and molecular phylogenetic analyses (18S–28S rDNA, tufA), we described the studied alga as the new species, Barranca variabilis sp. nov. The study complements the data on the diversity of soils green filamentous algae, and their biogeography. For the first time, the data on the structure of the cell walls and the cell ultrastructure of Barranca were established. The information on the morphology of the reproductive and resting cells is updated. Full article

Considering the phylogenetic differences in the taxonomic framework of the Chaetophorales as determined by the use of nuclear molecular markers or chloroplast genes, the current study was the first to use phylotranscriptomic analyses comparing the transcriptomes of 12 Chaetophorales algal species. The results showed that a total of 240,133 gene families and 143 single-copy orthogroups were identified. Based on the single-copy orthogroups, supergene analysis and the coalescent-based approach were adopted to perform phylotranscriptomic analysis of the Chaetophorales. The phylogenetic relationships of most species were consistent with those of phylogenetic analyses based on the chloroplast genome data rather than nuclear molecular markers. The Schizomeriaceae and the Aphanochaetaceae clustered into a well-resolved basal clade in the Chaetophorales by either strategy. Evolutionary analyses of divergence time and substitution rate also revealed that the closest relationships existed between the Schizomeriaceae and Aphanochaetaceae. All species in the Chaetophorales exhibited a large number of expanded and contracted gene families, in particular the common ancestor of the Schizomeriaceae and Aphanochaetaceae. The only terrestrial alga, Fritschiella tuberosa, had the greatest number of expanded gene families, which were associated with increased fatty acid biosynthesis. Phylotranscriptomic and evolutionary analyses all robustly identified the unique taxonomic relationship of Chaetophorales consistent with chloroplast genome data, proving the advantages of high-throughput data in phylogeny. Full article

This study determined the transcriptomes of eight Oedogoniales species, including six species from Oedogonium and two species from Oedocladium to conduct phylotranscriptomic and evolutionary analyses. 155,952 gene families and 192 single-copy orthogroups were detected. Phylotranscriptomic analyses based on single-copy orthogroups were conducted using supermatrix and coalescent-based approaches. The phylotranscriptomic analysis results revealed that Oedogonium is polyphyletic, and Oedocladium clustered with Oedogonium. Together with the transcriptomes of the OCC clade in the public database, the phylogenetic relationship of the three orders (Oedogoniales, Chaetophorales, Chaetopeltidales) is discussed. The non-synonymous (dN) to synonymous substitution (dS) ratios of single-copy orthogroups of the terrestrial Oedogoniales species using a branch model of phylogenetic analysis by maximum likelihood were estimated, which showed that 92 single-copy orthogroups were putative rapidly evolving genes. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses results revealed that some of the rapidly evolving genes were associated with photosynthesis, implying that terrestrial Oedogoniales species experienced rapid evolution to adapt to terrestrial habitats. The phylogenetic results combined with evolutionary analyses suggest that the terrestrialization process of Oedogoniales may have occured more than once. Full article