The volvocine algae, which include the single-celled species Chlamydomonas reinhardtii and the colonial species Volvox carteri, serve as a model in which to study the evolution of multicellularity and cellular differentiation. Studies reconstructing the history of this group have by and large relied on datasets of one to a few genes for phylogenetic inference and ancestral character state reconstruction. As a result, volvocine phylogenies lack concordance depending on the number and/or type of genes (i.e., chloroplast vs nuclear) chosen for phylogenetic inference. While multiple studies suggest that multicellularity evolved only once in the volvocine algae, that each of its three colonial families is monophyletic, and that there have been at least three independent origins of cellular differentiation in the group, other studies call into question one or more of these conclusions. An accurate assessment of the evolutionary history of the volvocine algae requires inference of a more robust phylogeny. We performed RNA sequencing (RNA-seq) on 55 strains representing 47 volvocine algal species and obtained similar data from curated databases on 13 additional strains. We then compiled a dataset consisting of transcripts for 40 single-copy, protein-coding, nuclear genes and subjected the predicted amino acid sequences of these genes to maximum likelihood, Bayesian inference, and coalescent-based analyses. These analyses show that multicellularity independently evolved at least twice in the volvocine algae and that the colonial family Goniaceae is not monophyletic. Our data further indicate that cellular differentiation arose independently at least four, and possibly as many as six times, within the volvocine algae. Altogether, our results demonstrate that multicellularity and cellular differentiation are evolutionarily labile in the volvocine algae, affirming the importance of this group as a model system for the study of major transitions in the history of life.

中文翻译：

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系统转录组学指出 volocine 藻类多细胞性和细胞分化的多个独立起源


藻类，包括单细胞物种莱茵衣藻和群落物种沃尔沃斯卡特里，可以作为研究多细胞进化和细胞分化的模型。重建该群体历史的研究基本上依赖于一到几个基因的数据集来进行系统发育推断和祖先特征状态重建。因此，根据选择用于系统发育推断的基因的数量和/或类型（即叶绿体与核），vollocine 系统发育缺乏一致性。虽然多项研究表明，多细胞性在 volocine 藻类中只进化了一次，其三个群体家族中的每一个都是单系的，并且该群体中至少存在三个独立的细胞分化起源，但其他研究对以下一个或多个方面提出了质疑：这些结论。准确评估 volocine 藻类的进化史需要推断更稳健的系统发育。我们对代表 47 个 volocine 藻类物种的 55 个菌株进行了 RNA 测序 (RNA-seq)，并从另外 13 个菌株的精选数据库中获得了类似的数据。然后，我们编译了一个由 40 个单拷贝、蛋白质编码、核基因的转录本组成的数据集，并对这些基因的预测氨基酸序列进行了最大似然、贝叶斯推理和基于合并的分析。这些分析表明，多细胞性在 volocine 藻类中至少独立进化了两次，并且群落科 Goniaceae 不是单系的。我们的数据进一步表明，在 volocine 藻类中，细胞分化独立发生至少四次，可能多达六次。 总而言之，我们的结果表明，卷藻中的多细胞性和细胞分化在进化上不稳定，证实了该群体作为研究生命历史重大转变的模型系统的重要性。