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et al., The draft genome of the moss model, Physcomitrella patens, comprised approximately 2000 unordered scaffolds. In order to enable analyses of genome structure and evolution we generated a chromosome-scale genome assembly using genetic linkage as well as (end) sequencing of long DNA fragments. We find that 57% of the genome comprises transposable elements (TEs), some of which may be actively transposing during the life cycle. Unlike in flowering plant genomes, gene- and TE-rich regions show an overall even distribution along the chromosomes. However, the chromosomes are mono-centric with peaks of a class of Copia elements potentially coinciding with centromeres. Gene body methylation is evident in 5.7% of the protein-coding genes, typically coinciding with low GC and low expression. Some giant virus insertions are transcriptionally active and might protect gametes from viral infection via siRNA mediated silencing. Structure-based detection methods show that the genome evolved via two rounds of whole genome duplications (WGDs), apparently common in mosses but not in liverworts and hornworts. Several hundred genes are present in colinear regions conserved since the last common ancestor of plants. These syntenic regions are enriched for functions related to plant-specific cell growth and tissue organization. The P. patens genome lacks the TE-rich pericentromeric and gene-rich distal regions typical for most flowering plant genomes. More non-seed plant genomes are needed to unravel how plant genomes evolve, and to understand whether the P. patens genome structure is typical for mosses or bryophytes., The work conducted by the US Department of Energy Joint Genome Institute is supported by the Office of Science of the US Department of Energy under Contract No. DE-AC02-05CH11231. Support to RR and SAR by the German Research Foundation (DFG RE 837/10-2), the Excellence Initiative of the German Federal and State Governments (EXC 294), and by the German Federal Ministry of Education and Research (BMBF FRISYS), is highly appreciated. CoGe is supported by the US National Science Foundation under Award Numbers IOS-339156 and IOS-1444490, CyVerse is supported by the U.S. National Science Foundation under Award Numbers DBI-0735191 and DBI-1265383. YK and ACC are grateful for support from the UK Biological Sciences and Biotechnology Research Council (Grant BB/F001797/1). KV acknowledges the Multidisciplinary Research Partnership ‘Bioinformatics: from nucleotides to networks’ Project (no 01MR0410W) of Ghent University. JC is grateful for support from the Spanish Ministerio de Economía y Competitividad (Grant AGL2013-43244-R). RSQ is grateful to Monsanto (St. Louis, MO, USA) for sequencing genomic DNA of P. patens accession Kaskaskia. The 1000 Plants (1 KP) initiative, led by GKSW, is funded by the Alberta Ministry of Innovation and Advanced Education, Alberta Innovates Technology Futures (AITF), Innovates Centres of Research Excellence (iCORE), Musea Ventures, BGI-Shenzhen and China National Genebank (CNGB). TW was supported by EMBO Long-Term Fellowships (ALTF 1166-2011) and by Marie Curie Actions (European Commission EMBOCOFUND2010, GA-2010-267146). The work conducted at PGSB was supported by the German Research Foundation (SFB924) and German Ministry of Education and Research (BMBF, 031A536/de.NBI).

Transposable elements (TEs) are a major driver of plant genome evolution. A part from being a rich source of new genes and regulatory sequences, TEs can also affect plant genome evolution by modifying genome size and shaping chromosome structure. TEs tend to concentrate in heterochromatic pericentromeric regions and their proliferation may expand these regions. Here, we show that after the split of melon and cucumber, TEs have expanded the pericentromeric regions of melon chromosomes that, probably as a consequence, show a very low recombination frequency. In contrast, TEs have not proliferated to a high extent in cucumber, which has small TE-dense pericentromeric regions and shows a relatively constant recombination rate along chromosomes. These differences in chromosome structure also translate in differences in gene nucleotide diversity. Although gene nucleotide diversity is essentially constant along cucumber chromosomes, melon chromosomes show a bimodal pattern of genetic variability, with a gene-poor region where variability is negatively correlated with gene density. Interestingly, genes are not homogeneously distributed in melon, and the high variable low-recombining pericentromeric regions show a higher concentration of melon-specific genes whereas genes shared with cucumber and other plants are essentially found in gene-rich chromosomal arms. The results presented here suggest that melon pericentromeric regions may allow gene sequences to evolve more freely than in other chromosomal compartments which may allow new ORFs to arise and eventually be selected. These results show that TEs can drastically change the structure of chromosomes creating different chromosomal compartments imposing different constraints for gene evolution., This work was supported by Ministerio de Economia y Competitividad grants AGL2013-43244-R and AGL2016-78992-R to J.C., Ministerio de Economia y Competitividad grant AGL2015-64625-C2-1-R to J.G.-M. and Centro de Excelencia Severo Ochoa 2016–2020, and the CERCA Programme/Generalitat de Catalunya to J.C., J.G.-M. and S.R.-O.

The availability of extensive databases of crop genome sequences should allow analysis of crop variability at an unprecedented scale, which should have an important impact in plant breeding. However, up to now the analysis of genetic variability at the whole-genome scale has been mainly restricted to single nucleotide polymorphisms (SNPs). This is a strong limitation as structural variation (SV) and transposon insertion polymorphisms are frequent in plant species and have had an important mutational role in crop domestication and breeding. Here, we present the first comprehensive analysis of melon genetic diversity, which includes a detailed analysis of SNPs, SV, and transposon insertion polymorphisms. The variability found among seven melon varieties representing the species diversity and including wild accessions and highly breed lines, is relatively high due in part to the marked divergence of some lineages. The diversity is distributed nonuniformly across the genome, being lower at the extremes of the chromosomes and higher in the pericentromeric regions, which is compatible with the effect of purifying selection and recombination forces over functional regions. Additionally, this variability is greatly reduced among elite varieties, probably due to selection during breeding. We have found some chromosomal regions showing a high differentiation of the elite varieties versus the rest, which could be considered as strongly selected candidate regions. Our data also suggest that transposons and SV may be at the origin of an important fraction of the variability in melon, which highlights the importance of analyzing all types of genetic variability to understand crop genome evolution., This work was supported by Ministerio de Ciencia e Innovación (grant BFU2009-11932 to J.M.C. and grant AGL2012-40130-C02-01 to J.G.-M.); Ministerio de Economía y Competitividad (grant AGL2013-43244-R to J.M.C., grant AGL2013-41834-R to S.E.R.-O., and grant AGL2009-12698-C02-01 to J.G.-M.); and Fundación Genoma España to J.G.-M.

Transposable elements (TEs) are major players in genome evolution. The effects of their movement vary from gene knockouts to more subtle effects such as changes in gene expression. It has recently been shown that TEs may contain transcription factor binding sites (TFBSs), and it has been proposed that they may rewire new genes into existing transcriptional networks. However, little is known about the dynamics of this process and its effect on transcription factor binding. Here we show that TEs have extensively amplified the number of sequences that match the E2F TFBS during Brassica speciation, and, as a result, as many as 85% of the sequences that fit the E2F TFBS consensus are within TEs in some Brassica species. We show that these sequences found within TEs bind E2Fa in vivo, which indicates a direct effect of these TEs on E2F-mediated gene regulation. Our results suggest that the TEs located close to genes may directly participate in gene promoters, whereas those located far from genes may have an indirect effect by diluting the effective amount of E2F protein able to bind to its cognate promoters. These results illustrate an extreme case of the effect of TEs in TFBS evolution, and suggest a singular way by which they affect host genes by modulating essential transcriptional networks., This work was supported by the Ministerio de Ciencia e Innovación (grant BFU2009-11932 to J.M.C. and grants BFU2009-9783 and BFU2012-34821 to C.G.), and by an institutional grant from the Fundacion Ramon Areces to Centro de Biologia Molecular Severo Ochoa (to C.G.).