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Volvocales forms a species‐rich clade with wide morphological variety and is regarded as an ideal model for tracing the evolutionary transitions in multicellularity. The phylogenetic relationships among the colonial volvocine algae and its relatives are important for investigating the origin of multicellularity in the clade Reinhardtinia. Therefore, a robust phylogenetic framework of the unicellular and colonial volvocine algae with broad taxon and gene sampling is essential for illuminating the evolution of multicellularity. Recent chloroplast phylogenomic studies have uncovered five major orders in the Chlorophyceae, but the family‐level relationships within Sphaeropleales and Volvocales remain elusive due to the uncertain positions of some incertae sedis taxa. In this study, we contributed six newly sequenced chloroplast genomes in the Volvocales and analyzed a dataset with 91 chlorophycean taxa and 58 protein‐coding genes. Conflicting phylogenetic signals were detected among chloroplast genes that resulted in discordant tree topologies among different analyses. We compared the phylogenetic trees inferred from original nucleotide, RY‐coding, codon‐degenerate, and amino acid datasets, and improved the robustness of phylogenetic inference in the Chlorophyceae by reducing base compositional bias. Our analyses indicate that the unicellular Chlamydomonas and Vitreochlamys are close to or nested within the colonial taxa, and all the incertae sedis taxa are nested within the monophyletic Sphaeropleales s.l. We propose that the colonial taxa in the Reinhardtinia are paraphyletic and multicellularity evolved once in the volvocine green algae and might be lost in Chlamydomonas and Vitreochlamys. [ABSTRACT FROM AUTHOR]

The harsh environments and high biodiversity of the Qinghai–Tibet Plateau (QTP) provide an ideal natural laboratory for studies on adaptive evolution. Low temperature and intense UV‐B radiation are the main abiotic stresses to plants at high altitudes. The rapid development of RNA sequencing has enabled us to investigate the molecular adaptations of plants that thrive in the QTP at the genomic level. Despite the ecological importance of Oocystis algae on the plateau, the genetic mechanisms of the adaptations of these algae to this high‐altitude environment remain poorly understood. Here, we sequenced and assembled the transcriptomes of Oocystis marina and Oocystis sp. LXD‐20, and undertook comparative transcriptomic and evolutionary analyses to reveal their adaptive strategies. Our results identified 348 positively selected genes in Oocystis algae, and functional analyses indicated that many of these positively selected genes were associated with adaptation to abiotic stresses, such as antioxidative response, DNA repair mechanisms, translation, and post‐translational modifications. We also identified the cold‐responsive and UV‐B‐responsive genes in O. marina and Oocystis sp. LXD‐20, and revealed the transcriptional regulation strategies under stress conditions. Our analyses provided a wealth of sequence data to serve as reference transcriptomes for future studies, and shed light on the adaptive strategies of green algae that live in the QTP. [ABSTRACT FROM AUTHOR]