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Parra-Acero et al. investigate cell-substrate adhesion in the filasterean C. owczarzaki, a close unicellular relative of animals. They show that cell adhesion relies on actin-dependent filopodia and is mediated by the integrin adhesome. This suggests that the role of integrins in cell-matrix adhesion was established before the emergence of animals.In animals, cell-matrix adhesions are essential for cell migration, tissue organization, and differentiation, which have central roles in embryonic development [1–6]. Integrins are the major cell surface adhesion receptors mediating cell-matrix adhesion in animals. They are heterodimeric transmembrane proteins that bind extracellular matrix (ECM) molecules on one side and connect to the actin cytoskeleton on the other [7]. Given the importance of integrin-mediated cell-matrix adhesion in development of multicellular animals, it is of interest to discover when and how this machinery arose during evolution. Comparative genomic analyses have shown that core components of the integrin adhesome pre-date the emergence of animals [8–11]; however, whether it mediates cell adhesion in non-metazoan taxa remains unknown. Here, we investigate cell-substrate adhesion in Capsaspora owczarzaki, the closest unicellular relative of animals with the most complete integrin adhesome [11, 12]. Previous work described that the life cycle of C. owczarzaki (hereafter, Capsaspora) includes three distinct life stages: adherent; cystic; and aggregative [13]. Using an adhesion assay, we show that, during the adherent life stage, C. owczarzaki adheres to surfaces using actin-dependent filopodia. We show that integrin β2 and its associated protein vinculin localize as distinct patches in the filopodia. We also demonstrate that substrate adhesion and integrin localization are enhanced by mammalian fibronectin. Finally, using a specific antibody for integrin β2, we inhibited cell adhesion to a fibronectin-coated surface. Our results suggest that adhesion to the substrate in C. owczarzaki is mediated by integrins. We thus propose that integrin-mediated adhesion pre-dates the emergence of animals. ; This work was funded by European Research Council Consolidator Grant (ERC-2012-Co-616960) to I.R.-T.; O.D. was supported by a Swiss National Science Foundation Early PostDoc Mobility fellowship (P2LAP3_171815) and a Marie Sklodowska-Curie individual fellowship (MSCA-IF 746044). M.H. received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/ 2007-2013/ under REA grant agreement no. 330925. N.H.B.'s work on this project was supported by a Royal Society International Exchange Grant (IE141189). The CRG/UPF Proteomics Unit is part of the of Proteored, PRB3 and is supported by grant PT17/0019 of the PE I+D+i 2013-2016, funded by ISCIII and ERDF.

The unicellular ancestor of animals had a complex repertoire of genes linked to multicellular processes. This suggests that changes in the regulatory genome, rather than in gene innovation, were key to the origin of animals. Here, we carry out multiple functional genomic assays in Capsaspora owczarzaki, the unicellular relative of animals with the largest known gene repertoire for transcriptional regulation. We show that changing chromatin states, differential lincRNA expression, and dynamic cis-regulatory sites are associated with life cycle transitions in Capsaspora. Moreover, we demonstrate conservation of animal developmental transcription-factor networks and extensive network interconnection in this premetazoan organism. In contrast, however, Capsaspora lacks animal promoter types, and its regulatory sites are small, proximal, and lack signatures of animal enhancers. Overall, our results indicate that the emergence of animal multicellularity was linked to a major shift in genome cis-regulatory complexity, most notably the appearance of distal enhancer regulation. ; This work was supported by an Institució Catalana de Recerca i Estudis Avançats contract, a European Research Council Consolidator Grant (ERC-2012-Co-616960), and a grant from Ministerio de Economía y Competitividad (MINECO) (BFU-2011-23434) (to I.R.-T.). We also acknowledge financial support from Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat de Catalunya (project 2014 SGR 619). The work in L.D.'s laboratory was supported by grants from the Spanish "Ministerio de Educación y Ciencia" (SAF2013-48926-P), AGAUR, and the European Commission's 7th Framework Program 4DCellFate (277899). A.S.-P. is supported by an EMBO Long-Term Fellowship (ALTF 841-2014). J.L.G.-S. was funded by grants from Ministerio de Economía y Competitividad (BFU2013-41322-P) and the Andalusian Government (BIO-396). J.J.T. has a postdoctoral grant from the University Pablo de Olavide. The CRG/UPF Proteomics Unit is part of the "Plataforma de Recursos Biomoleculares y Bioinformáticos (ProteoRed)" supported by a grant from Instituto de Salud Carlos III (ISCIII) (PT13/0001). ; Peer reviewed

The unicellular ancestor of animals had a complex repertoire of genes linked to multicellular processes. This suggests that changes in the regulatory genome, rather than in gene innovation, were key to the origin of animals. Here, we carry out multiple functional genomic assays in Capsaspora owczarzaki, the unicellular relative of animals with the largest known gene repertoire for transcriptional regulation. We show that changing chromatin states, differential lincRNA expression, and dynamic cis-regulatory sites are associated with life cycle transitions in Capsaspora. Moreover, we demonstrate conservation of animal developmental transcription-factor networks and extensive network interconnection in this premetazoan organism. In contrast, however, Capsaspora lacks animal promoter types, and its regulatory sites are small, proximal, and lack signatures of animal enhancers. Overall, our results indicate that the emergence of animal multicellularity was linked to a major shift in genome cis-regulatory complexity, most notably the appearance of distal enhancer regulation. ; This work was supported by an Institució Catalana de Recerca i Estudis Avançats contract, a European Research Council Consolidator Grant (ERC-2012-Co-616960), and a grant from Ministerio de Economía y Competitividad (MINECO) (BFU-2011-23434) (to I.R.-T.). We also acknowledge financial support from Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat de Catalunya (project 2014 SGR 619). The work in L.D.'s laboratory was supported by grants from the Spanish "Ministerio de Educación y Ciencia" (SAF2013-48926-P), AGAUR, and the European Commission's 7th Framework Program 4DCellFate (277899). A.S.-P. is supported by an EMBO Long-Term Fellowship (ALTF 841-2014). J.L.G.-S. was funded by grants from Ministerio de Economía y Competitividad (BFU2013-41322-P) and the Andalusian Government (BIO-396). J.J.T. has a postdoctoral grant from the University Pablo de Olavide. The CRG/UPF Proteomics Unit is part of the "Plataforma de Recursos Biomoleculares y Bioinformáticos (ProteoRed)" supported by a grant from Instituto de Salud Carlos III (ISCIII) (PT13/0001). We thank Guadalupe Espadas for her support with the histone derivatization protocol and Núria Ros and Meritxell Antó for technical support. Finally, we thank the CRG Genomics Unit for helping with ChIP-seq and RNA-seq sequencing.

This article is distributed exclusively by Cold Spring Harbor Laboratory Press for the first six months. After six months, it is available under a Creative Commons License (Attribution-NonCommercial 4.0 International). ; The complex relationship between ontogeny and phylogeny has been the subject of attention and controversy since von Baer's formulations in the 19th century. The classic concept that embryogenesis progresses from clade general features to species-specific characters has often been revisited. It has become accepted that embryos from a clade show maximum morphological similarity at the so-called phylotypic period (i.e., during mid-embryogenesis). According to the hourglass model, body plan conservation would depend on constrained molecular mechanisms operating at this period. More recently, comparative transcriptomic analyses have provided conclusive evidence that such molecular constraints exist. Examining cis-regulatory architecture during the phylotypic period is essential to understand the evolutionary source of body plan stability. Here we compare transcriptomes and key epigenetic marks (H3K4me3 and H3K27ac) from medaka (Oryzias latipes) and zebrafish (Danio rerio), two distantly related teleosts separated by an evolutionary distance of 115-200 Myr. We show that comparison of transcriptome profiles correlates with anatomical similarities and heterochronies observed at the phylotypic stage. Through comparative epigenomics, we uncover a pool of conserved regulatory regions (≈700), which are active during the vertebrate phylotypic period in both species. Moreover, we show that their neighboring genes encode mainly transcription factors with fundamental roles in tissue specification. We postulate that these regulatory regions, active in both teleost genomes, represent key constrained nodes of the gene networks that sustain the vertebrate body plan. ; The Andalusian government (JA) supported A.F-.M. as scientific manager of the Aquatic Vertebrates Platform at CABD. J.W.C. was supported by a studentship from The Institute of Cancer Research. Spanish and Andalusian government grants BFU2010-14839, CSD2007-00008, and P08-CVI-3488 to J.L.G-.S.; and BFU2011-22916 and P11-CVI-7256 to J.R.M-.M. supported this work. ; Peer Reviewed