Data from: Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils

NOTE: PLEASE ALSO SEE THE CORRIGENDUM TO THE ORIGINAL ARTICLE, PUBLISHED
AT http://dx.doi.org/10.1111/pala.12193. Cyanobacteria are among the most
ancient of evolutionary lineages, oxygenic photosynthesizers that may have
originated before 3.0 Ga, as evidenced by free oxygen levels. Throughout
the Precambrian, cyanobacteria were one of the most important drivers of
biological innovations, strongly impacting early Earth's
environments. At the end of the Archean Eon, they were responsible for the
rapid oxygenation of Earth's atmosphere during an episode referred to
as the Great Oxidation Event (GOE). However, little is known about the
origin and diversity of early cyanobacterial taxa, due to: (1) the
scarceness of Precambrian fossil deposits; (2) limited characteristics for
the identification of taxa; and (3) the poor preservation of ancient
microfossils. Previous studies based on 16S rRNA have suggested that the
origin of multicellularity within cyanobacteria might have been associated
with the GOE. However, single-gene analyses have limitations, particularly
for deep branches. We reconstructed the evolutionary history of
cyanobacteria using genome scale data and re-evaluated the Precambrian
fossil record to get more precise calibrations for a relaxed clock
analysis. For the phylogenomic reconstructions, we identified 756
conserved gene sequences in 65 cyanobacterial taxa, of which eight genomes
have been sequenced in this study. Character state reconstructions based
on maximum likelihood and Bayesian phylogenetic inference confirm previous
findings, of an ancient multicellular cyanobacterial lineage ancestral to
the majority of modern cyanobacteria. Relaxed clock analyses provide firm
support for an origin of cyanobacteria in the Archean and a transition to
multicellularity before the GOE. It is likely that multicellularity had a
greater impact on cyanobacterial fitness and thus abundance, than
previously assumed. Multicellularity, as a major evolutionary innovation,
forming a novel unit for selection to act upon, may have served to
overcome evolutionary constraints and enabled diversification of the
variety of morphotypes seen in cyanobacteria today.

注意：请同时参阅原文的勘误表，该勘误表发布于http://dx.doi.org/10.1111/pala.12193。蓝细菌（Cyanobacteria）是最古老的演化支系之一，属于产氧光合微生物，游离氧水平的相关证据表明其可能起源于30亿年前（3.0 Ga）。在前寒武纪时期，蓝细菌是推动生物革新的核心动力之一，对早期地球环境造成了极为深远的影响。在太古宙（Archean Eon）末期，地球大气在被称为大氧化事件（Great Oxidation Event, GOE）的阶段中快速富氧，这一过程由蓝细菌主导完成。然而，由于以下三方面局限，学界对早期蓝细菌类群的起源与多样性仍知之甚少：(1) 前寒武纪化石沉积物稀缺；(2) 类群鉴定可用的特征有限；(3) 古代微化石的保存状况极差。此前基于16S rRNA的研究曾提出，蓝细菌的多细胞化起源可能与大氧化事件存在关联。但单基因分析存在固有局限，针对深层演化分支的分析更是如此。本研究借助全基因组规模数据重构了蓝细菌的演化历史，并重新评估了前寒武纪化石记录，以期为放宽分子钟分析提供更为精准的时间校准。在系统发育基因组学重构环节中，研究团队在65个蓝细菌类群中鉴定出756个保守基因序列，其中8个基因组为本研究新完成测序。基于最大似然法（maximum likelihood）与贝叶斯系统发育推断（Bayesian phylogenetic inference）的性状状态重构验证了此前的研究结论：存在一个古老的多细胞蓝细菌支系，该支系是绝大多数现代蓝细菌的祖先。放宽分子钟分析有力支持了蓝细菌起源于太古宙，且多细胞化事件发生于大氧化事件之前。相较于此前的认知，多细胞化对蓝细菌适合度乃至种群丰度的影响可能更为显著。作为一项重大演化革新，多细胞化形成了新的选择作用单元，或许帮助蓝细菌突破了演化约束，并促成了现今蓝细菌各类形态型的多样化。