Throughout its nearly four-billion-year history, life has undergone evolutionary transitions in which simpler subunits have become integrated to form a more complex whole. Many of these transitions opened the door to innovations that resulted in increased biodiversity and/or organismal efficiency. The evolution of multicellularity from unicellular forms represents one such transition, one that paved the way for cellular differentiation, including differentiation of male and female gametes. A useful model for studying the evolution of multicellularity and cellular differentiation is the volvocine algae, a clade of freshwater green algae whose members range from unicellular to colonial, from undifferentiated to completely differentiated, and whose gamete types can be isogamous, anisogamous, or oogamous. To better understand how multicellularity, differentiation, and gametes evolved in this group, we used comparative genomics and fossil data to establish a geologically calibrated roadmap of when these innovations occurred. Our ancestral-state reconstructions, show that multicellularity arose independently twice in the volvocine algae. Our chronograms indicate multicellularity evolved during the Carboniferous-Triassic periods in Goniaceae + Volvocaceae, and possibly as early as the Cretaceous in Tetrabaenaceae. Using divergence time estimates we inferred when, and in what order, specific developmental changes occurred that led to differentiated multicellularity and oogamy. We find that in the volvocine algae the temporal sequence of developmental changes leading to differentiated multicellularity is much as proposed by David Kirk, and that multicellularity is correlated with the acquisition of anisogamy and oogamy. Lastly, morphological, molecular, and divergence time data suggest the possibility of cryptic species in Tetrabaenaceae. Large molecular datasets and robust phylogenetic methods are bringing the evolutionary history of the volvocine algae more sharply into focus. Mounting evidence suggests that extant species in this group are the result of two independent origins of multicellularity and multiple independent origins of cell differentiation. Also, the origin of the Tetrabaenaceae-Goniaceae-Volvocaceae clade may be much older than previously thought. Finally, the possibility of cryptic species in the Tetrabaenaceae provides an exciting opportunity to study the recent divergence of lineages adapted to live in very different thermal environments.

中文翻译：

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化石校准的分子钟数据能够重建导致沃尔沃辛藻中分化的多细胞性和异配性的步骤

在其近四十亿年的历史中，生命经历了进化转变，其中较简单的亚基已整合形成一个更复杂的整体。其中许多转变打开了创新之门，从而提高了生物多样性和/或有机体效率。从单细胞形式进化到多细胞就代表了这样一种转变，这种转变为细胞分化（包括雄性和雌性配子的分化）铺平了道路。研究多细胞性和细胞分化进化的一个有用模型是 volocine 藻类，它是淡水绿藻的一个分支，其成员范围从单细胞到群体，从未分化到完全分化，其配子类型可以是同配、异配或卵配。为了更好地了解多细胞性、分化和配子在这个群体中是如何进化的，我们使用比较基因组学和化石数据来建立这些创新何时发生的地质校准路线图。我们的祖先状态重建表明，多细胞性在 volocine 藻类中独立出现了两次。我们的年代记录表明，海角藻科 + 沃尔沃科的多细胞性在石炭纪-三叠纪时期就已进化，而四甲藻科的多细胞性可能早在白垩纪就已进化。使用分歧时间估计，我们推断出特定的发育变化发生的时间和顺序，从而导致分化的多细胞性和卵配性。我们发现，在 volocine 藻类中，导致分化的多细胞性的发育变化的时间顺序与 David Kirk 所提出的非常相似，并且多细胞性与异配性和异配性的获得相关。最后，形态、分子和分歧时间数据表明四甲虫科中存在隐秘物种的可能性。大型分子数据集和强大的系统发育方法使 volocine 藻类的进化史更加引人注目。越来越多的证据表明，该类群中的现存物种是两个独立的多细胞起源和多个独立的细胞分化起源的结果。此外，Tetrabaenaceae-Goniaceae-Volvocaceae 分支的起源可能比之前认为的要古老得多。最后，四甲虫科中隐藏物种的可能性为研究最近适应生活在截然不同的热环境中的谱系的分歧提供了令人兴奋的机会。