期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：47
页码：E7367-E7374
DOI：10.1073/pnas.1605202113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe free-living cyanobacterium Trichodesmium is an important nitrogen-fixer in the global oceans, yet virtually nothing is known about its molecular evolution to increased CO2. Here we show that Trichodesmium can fix a plastic, short-term response upon long-term adaptation, potentially through genetic assimilation. We provide transcriptional evidence for molecular mechanisms that parallel the fixation of the plastic phenotype, thereby demonstrating an important evolutionary capability in Trichodesmium CO2 adaptation. Transcriptional shifts involve transposition and other regulatory mechanisms (sigma factors) that control a variety of metabolic pathways, suggesting alterations in upstream regulation to be important under genetic assimilation. Together, these data highlight potential biochemical evidence of genetic assimilation in a keystone marine N2-fixer, with broad implications for microbial evolution and biogeochemistry. Most investigations of biogeochemically important microbes have focused on plastic (short-term) phenotypic responses in the absence of genetic change, whereas few have investigated adaptive (long-term) responses. However, no studies to date have investigated the molecular progression underlying the transition from plasticity to adaptation under elevated CO2 for a marine nitrogen-fixer. To address this gap, we cultured the globally important cyanobacterium Trichodesmium at both low and high CO2 for 4.5 y, followed by reciprocal transplantation experiments to test for adaptation. Intriguingly, fitness actually increased in all high-CO2 adapted cell lines in the ancestral environment upon reciprocal transplantation. By leveraging coordinated phenotypic and transcriptomic profiles, we identified expression changes and pathway enrichments that rapidly responded to elevated CO2 and were maintained upon adaptation, providing strong evidence for genetic assimilation. These candidate genes and pathways included those involved in photosystems, transcriptional regulation, cell signaling, carbon/nitrogen storage, and energy metabolism. Conversely, significant changes in specific sigma factor expression were only observed upon adaptation. These data reveal genetic assimilation as a potentially adaptive response of Trichodesmium and importantly elucidate underlying metabolic pathways paralleling the fixation of the plastic phenotype upon adaptation, thereby contributing to the few available data demonstrating genetic assimilation in microbial photoautotrophs. These molecular insights are thus critical for identifying pathways under selection as drivers in plasticity and adaptation.
关键词：diazotroph ; evolution ; CO2 ; genetic assimilation ; plasticity
