Evolution of Multicellular Complexity in The Dictyostelid Social Amoebas

This article belongs to the Special Issue Evolution of Multicellularity. Academic Editor: J. Mark Cock.Multicellularity evolved repeatedly in the history of life, but how it unfolded varies greatly between different lineages. Dictyostelid social amoebas offer a good system to study the evolution of multicellular complexity, with a well-resolved phylogeny and molecular genetic tools being available. We compare the life cycles of the Dictyostelids with closely related amoebozoans to show that complex life cycles were already present in the unicellular common ancestor of Dictyostelids. We propose frost resistance as an early driver of multicellular evolution in Dictyostelids and show that the cell signalling pathways for differentiating spore and stalk cells evolved from that for encystation. The stalk cell differentiation program was further modified, possibly through gene duplication, to evolve a new cell type, cup cells, in Group 4 Dictyostelids. Studies in various multicellular organisms, including Dictyostelids, volvocine algae, and metazoans, suggest as a common principle in the evolution of multicellular complexity that unicellular regulatory programs for adapting to environmental change serve as “proto-cell types” for subsequent evolution of multicellular organisms. Later, new cell types could further evolve by duplicating and diversifying the “proto-cell type” gene regulatory networks.This work was supported by the European Research Council [742288], The Wellcome Trust [100293/Z/12/Z], the European Molecular Biology Organisation [ALTF 295-2015], and the Japanese Organisation for the Promotion of Science [H28-1002].Peer reviewe

Cell-type phylogenetics and the origin of endometrial stromal cells

SummaryA challenge of genome annotation is the identification of genes performing specific biological functions. Here, we propose a phylogenetic approach that utilizes RNA-seq data to infer the historical relationships among cell types and to trace the pattern of gene-expression changes on the tree. The hypothesis is that gene-expression changes coincidental with the origin of a cell type will be important for the function of the derived cell type. We apply this approach to the endometrial stromal cells (ESCs), which are critical for the initiation and maintenance of pregnancy. Our approach identified well-known regulators of ESCs, PGR and FOXO1, as well as genes not yet implicated in female fertility, including GATA2 and TFAP2C. Knockdown analysis confirmed that they are essential for ESC differentiation. We conclude that phylogenetic analysis of cell transcriptomes is a powerful tool for discovery of genes performing cell-type-specific functions

Novel RNAseq-informed cell-type markers and their regulation alter paradigms of Dictyostelium developmental control

Cell differentiation is traditionally monitored with a few marker genes, which may bias results. To understand the evolution and regulation of the spore, stalk, cup and basal disc cells in Dictyostelia, we previously performed RNAseq on purified cell-types of taxon-group representative dictyostelids. Using promoter-lacZ constructs in D. discoideum, we here investigate the spatio-temporal expression pattern of 29 cell-type specific genes. Genes selected for spore- or cup-specificity in RNAseq were validated as such by lacZ expression, but genes selected for stalk-specificity showed variable additional expression in basal disc, early cup or prestalk populations. We measured responses of 25 genes to 15 single or combined regimes of induction by stimuli known to regulate cell differentiation. The outcomes of these experiments were subjected to hierarchical clustering to identify whether common modes of regulation were correlated with specific expression patterns. The analysis identified a cluster combining the spore and cup genes, which shared upregulation by 8-bromo cyclic AMP and down-regulation by Differentiation Inducing Factor 1 (DIF-1). Most stalk-expressed genes combined into a single cluster and shared strong upregulation by cyclic di-guanylate (c-di-GMP), and synergistic upregulation by combined DIF-1 and c-di-GMP. There was no clustering of genes expressed in other soma besides the stalk, but two genes that were only expressed in the stalk did not respond to any stimuli. In contrast to current models, the study indicates the existence of a stem-cell like soma population in slugs, whose members only acquire ultimate cell fate after progressing to their terminal location during fruiting body morphogenesis.This research was funded by ERC advanced grant 742288. Z-HC. was additionally funded by Wellcome grant 100293/Z/12/Z and K.K. was additionally supported by EMBO long-term fellowship ALTF 295–2015 and by JSPS Overseas Research Fellowship H28–1002.Peer reviewe

Emerging roles for diguanylate cyclase during the evolution of soma in dictyostelia

Background: Cyclic di-guanylate (c-di-GMP), synthesized by diguanylate cyclase, is a major second messenger in prokaryotes, where it triggers biofilm formation. The dictyostelid social amoebas acquired diguanylate cyclase (dgcA) by horizontal gene transfer. Dictyostelium discoideum (Ddis) in taxon group 4 uses c-di-GMP as a secreted signal to induce differentiation of stalk cells, the ancestral somatic cell type that supports the propagating spores. We here investigated how this role for c-di-GMP evolved in Dictyostelia by exploring dgcA function in the group 2 species Polysphondylium pallidum (Ppal) and in Polysphondylium violaceum (Pvio), which resides in a small sister clade to group 4.Results: Similar to Ddis, dgcA is upregulated after aggregation in Ppal and Pvio and predominantly expressed in the anterior region and stalks of emerging fruiting bodies. DgcA null mutants in Ppal and Pvio made fruiting bodies with very long and thin stalks and only few spores and showed delayed aggregation and larger aggregates, respectively. Ddis dgcAˉ cells cannot form stalks at all, but showed no aggregation defects. The long, thin stalks of Ppal and Pvio dgcAˉ mutants were also observed in acaAˉ mutants in these species. AcaA encodes adenylate cyclase A, which mediates the effects of c-di-GMP on stalk induction in Ddis. Other factors that promote stalk formation in Ddis are DIF-1, produced by the polyketide synthase StlB, low ammonia, facilitated by the ammonia transporter AmtC, and high oxygen, detected by the oxygen sensor PhyA (prolyl 4-hydroxylase). We deleted the single stlB, amtC and phyA genes in Pvio wild-type and dgcAˉ cells. Neither of these interventions affected stalk formation in Pvio wild-type and not or very mildly exacerbated the long thin stalk phenotype of Pvio dgcAˉ cells.Conclusions: The study reveals a novel role for c-di-GMP in aggregation, while the reduced spore number in Pvio and Ppal dgcAˉ is likely an indirect effect, due to depletion of the cell pool by the extended stalk formation. The results indicate that in addition to c-di-GMP, Dictyostelia ancestrally used an as yet unknown factor for induction of stalk formation. The activation of AcaA by c-di-GMP is likely conserved throughout Dictyostelia

Interactome and evolutionary conservation of Dictyostelid small GTPases and their direct regulators

The GTP binding proteins known as small GTPases make up one of the largest groups of regulatory proteins and control almost all functions of living cells. Their activity is under respectively positive and negative regulation by guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), which together with their upstream regulators and the downstream targets of the small GTPases form formidable signaling networks. While genomics has revealed the large size of the GTPase, GEF and GAP repertoire, only a small fraction of their interactions and functions have yet been experimentally explored. Dictyostelid social amoebas have been particularly useful in unraveling the roles of many proteins in the Rac-Rho and Ras-Rap families of GTPases in directional cell migration and regulation of the actin cytoskeleton. Almost completely assembled genomes and cell-type specific and developmental transcriptomes are available for Dictyostelium species that span the 0.5 billion years of evolution of the group from their unicellular ancestor. In this work, we identified all GTPases, GEFs and GAP from genomes representative of the four major taxon groups and investigated their phylogenetic relationships and evolutionary conservation and change in their functional domain architecture and in their developmental and cell-type specific expression. We performed hierarchical cluster analysis of the expression profiles of the ~2000 analysed genes to identify putative interacting sets of GTPases, GEFs and GAPs, which highlighted sets known to interact experimentally and many novel combinations. The work represents a valuable resource for research into all fields of cellular regulation