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Ancient whole-genome duplications (WGDs), also referred to as paleopolyploidizations, have been reported in most evolutionary lineages. Their attributed role remains a major topic of discussion, ranging from an evolutionary dead end to a road toward evolutionary success, with evidence supporting both fates. Previously, based on dating WGDs in a limited number of plant species, we found a clustering of angiosperm paleopolyploidizations around the Cretaceous–Paleogene (K–Pg) extinction event about 66 million years ago. Here we revisit this finding, which has proven controversial, by combining genome sequence information for many more plant lineages and using more sophisticated analyses. We include 38 full genome sequences and three transcriptome assemblies in a Bayesian evolutionary analysis framework that incorporates uncorrelated relaxed clock methods and fossil uncertainty. In accordance with earlier findings, we demonstrate a strongly nonrandom pattern of genome duplications over time with many WGDs clustering around the K–Pg boundary. We interpret these results in the context of recent studies on invasive polyploid plant species, and suggest that polyploid establishment is promoted during times of environmental stress. We argue that considering the evolutionary potential of polyploids in light of the environmental and ecological conditions present around the time of polyploidization could mitigate the stark contrast in the proposed evolutionary fates of polyploids. ; Ghent University (Multidisciplinary Research Partnership ''Bioinformatics: from nucleotides to networks'').The Fund for Scientific Research Flanders (FWO). The European Union Seventh Framework Programme (FP7/2007-2013) under ERC Grant agreement no. 260864.The European Union Seventh Framework Programme (FP7/2007-2013) under ERC Advanced Grant Agreement no. 322739–DOUBLE-UP. ; http://www.genome.org ; 2015-02-28 ; hb2014

Ferns are the closest sister group to all seed plants, yet little is known about their genomes other than that they are generally colossal. Here, we report on the genomes of Azolla filiculoides and Salvinia cucullata (Salviniales) and present evidence for episodic whole-genome duplication in ferns—one at the base of 'core leptosporangiates' and one specific to Azolla. One fernspecific gene that we identified, recently shown to confer high insect resistance, seems to have been derived from bacteria through horizontal gene transfer. Azolla coexists in a unique symbiosis with N2-fixing cyanobacteria, and we demonstrate a clear pattern of cospeciation between the two partners. Furthermore, the Azolla genome lacks genes that are common to arbuscular mycorrhizal and root nodule symbioses, and we identify several putative transporter genes specific to Azolla–cyanobacterial symbiosis. These genomic resources will help in exploring the biotechnological potential of Azolla and address fundamental questions in the evolution of plant life. ; Partly supported by the Shenzhen Municipal Government of China (no. JCYJ20150529150409546), the National Science Foundation Doctoral Dissertation Improvement Grant DEB-1407158 (to K.M.P. and F.-W.L.) and the German Research Foundation Research Fellowship VR132/1-1 (to J.d.V.). ; http://www.nature.com/nplants ; am2018 ; Biochemistry ; Genetics ; Microbiology and Plant Pathology