Biology subjects

Dagan, T.

Publications and source records attributed to Dagan, T..

28 records · Page 2Linked to original sources

bioRxiv2019

Somatic genetic drift and multi-level selection in modular species

Cells in multicellular organisms are genetically heterogeneous owing to somatic mutations. The accumulation of somatic genetic variation in species undergoing asexual (or clonal) reproduction (termed modular species) may lead to phenotypic heterogeneity among modules. However, abundance and dynamics of somatic genetic variation under clonal growth, a widespread life history in nature, remain poorly understood. Here we show that branching events in a seagrass clone or genet leads to population bottlenecks at the cellular level and hence the evolution of genetically differentiated modules. Studying inter-module somatic genetic variation, we uncovered thousands of SNPs that segregated among modules. The strength of purifying selection on mosaic genetic variation was greater at the intra-module comparing with the inter-module level. Our study provides evidence for the operation of selection at multiple levels, of cell population and modules. Somatic genetic drift leads to the emergence of genetically unique modules; hence, modules in long-lived clonal species constitute an appropriate elementary level of selection and individuality.

evolutionary biology↗

bioRxiv2019

Phylogenomic Testing of Root Hypotheses Without a Species Tree

AO_SCPLOWBSTRACTC_SCPLOWThe determination of the last common ancestor (LCA) of a group of species plays a vital role in evolutionary theory. Traditionally, an LCA is inferred by the rooting of a fully resolved species tree. From a theoretical perspective, however, inference of the LCA amounts to the reconstruction of just one branch - the root branch - of the true species tree, and should therefore be a much easier task than the full resolution of the species tree. Discarding the reliance on a hypothesised species tree and its rooting leads us to re-evaluate what phylogenetic signal is directly relevant to LCA inference, and to recast the task as that of sampling the total evidence from all gene families at the genomic scope. Here we reformulate LCA and root inference in the framework of statistical hypothesis testing and outline an analytical procedure to formally test competing a-priori LCA hypotheses and to infer confidence sets for the earliest speciation events in the history of a group of species. Applying our methods to two demonstrative datasets we show that our inference of the opisthokonta LCA is well in agreement with the common knowledge. Inference of the proteobacteria LCA shows that it is most closely related to modern Epsilonproteobacteria, raising the possibility that it may have been characterized by a chemolithoautotrophic and anaerobic life-style. Our inference is based on data comprising between 43% (opisthokonta) and 86% (proteobacteria) of all gene families. Approaching LCA inference within a statistical framework renders the phylogenomic inference powerful and robust.

evolutionary biology↗

bioRxiv2019

The effect of population bottleneck size and selective regime on genetic diversity and evolvability in bacteria

Population bottlenecks leading to a drastic reduction of the population size are common in the evolutionary dynamics of natural populations; their occurrence is known to have implications for genome evolution due to genetic drift, the consequent reduction in genetic diversity and the rate of adaptation. Nevertheless, an empirical characterization of the effect of population bottleneck size on evolutionary dynamics of bacteria is currently lacking. Here we show that selective conditions have a stronger effect on the evolutionary history of bacteria in comparison to genetic drift following population bottlenecks. We evolved Escherichia coli populations under three different population bottlenecks (small, medium, large) in two temperature regimes (37{degrees}C and 20{degrees}C). We find a high genetic diversity in the large in comparison to the small bottleneck size. Nonetheless, the cold temperature led to reduced genetic diversity in all bottleneck sizes, hence, the temperature has a stronger effect on the genetic diversity in comparison to the bottleneck size. A comparison of the fitness gain among the evolved populations reveals a similar pattern where the temperature has a significant effect on the fitness. Our study demonstrates that population bottlenecks are an important determinant of the evolvability in bacteria; their consequences depend on the selective conditions and are best understood via their effect on the standing genetic variation.

evolutionary biology↗

bioRxiv2019

Higher stochasticity of microbiota composition in seedlings of domesticated wheat compared to wild wheat

Plants constitute an ecological niche for microbial communities that colonize different plant tissues and explore the plant habitat for reproduction and dispersal. The association of microbiota and plant may be altered by ecological and evolutionary changes in the host population. Seedborne microbiota, expected to be largely vertically-transferred, have the potential to co-adapt with their host over generations. Reduced host diversity because of strong directional selection and polyploidization during plant domestication and cultivation may have impacted the assembly and transmission of seed-associated microbiota. Nonetheless, the effect of plant domestication on the diversity of their associated microbes is poorly understood. Here we show that microbial communities in domesticated wheat, Triticum aestivum, are less diverse but more inconsistent among individual plants compared to the wild wheat species, T. dicoccoides. We found that diversity of microbes in seeds overall is low, but comparable in different wheat species, independent of their genetic and geographic origin. However, the diversity of seedborne microbiota that colonize the roots and leaves of the young seedling is significantly reduced in domesticated wheat genotypes. Moreover, we observe a higher variability between replicates of T. aestivum suggesting a stronger effect of chance events in microbial colonization and assembly. We also propagated wild and domesticated wheat in two different soils and found that different factors govern the assembly of soil-derived and seedborne microbial communities. Overall, our results demonstrate that the role of stochastic processes in seedborne microbial community assembly is larger in domesticated wheat compared to the wild wheat. We suggest that the directional selection on the plant host and polyploidization events during domestication may have decreased the degree of wheat-microbiota interactions and consequently led to a decreased stable core microbiota.

microbiology↗

bioRxiv2019

Identification and characterization of novel filament-forming proteins in cyanobacteria

Filament-forming proteins in bacteria function in stabilization and localization of proteinaceous complexes and replicons; hence they are instrumental for myriad cellular processes such as cell division and growth. Here we present two novel filament-forming proteins in cyanobacteria. Surveying cyanobacterial genomes for coiled-coil-rich proteins (CCRPs) that are predicted as putative filament-forming proteins, we observed a higher proportion of CCRPs in filamentous cyanobacteria in comparison to unicellular cyanobacteria. Using our predictions, we identified nine protein families with putative intermediate filament (IF) properties. Polymerization assays revealed four proteins that formed polymers in vitro and three proteins that formed polymers in vivo. Fm7001 from Fischerella muscicola PCC 7414 polymerized in vitro and formed filaments in vivo in several organisms. Additionally, we identified a tetratricopeptide repeat protein - All4981 - in Anabaena sp. PCC 7120 that polymerized into filaments in vitro and in vivo. All4981 interacts with known cytoskeletal proteins and is indispensable for Anabaena viability. Although it did not form filaments in vitro, Syc2039 from Synechococcus elongatus PCC 7942 assembled into filaments in vivo and a {Delta}syc2039 mutant was characterized by an impaired cytokinesis. Our results expand the repertoire of known prokaryotic filament-forming CCRPs and demonstrate that cyanobacterial CCRPs are involved in cell morphology, motility, cytokinesis and colony integrity.

microbiology↗

bioRxiv2019

Comparative analysis of amplicon and metagenomic sequencing methods reveals key features in the evolution of animal metaorganisms

BackgroundThe interplay between hosts and their associated microbiome is now recognized as a fundamental basis of the ecology, evolution and development of both players. These interdependencies inspired a new view of multicellular organisms as \"metaorganisms\". The goal of the Collaborative Research Center \"Origin and Function of Metaorganisms\" is to understand why and how microbial communities form long-term associations with hosts from diverse taxonomic groups, ranging from sponges to humans in addition to plants.\n\nMethodsIn order to optimize the choice of analysis procedures, which may differ according to the host organism and question at hand, we systematically compared the two main technical approaches for profiling microbial communities, 16S rRNA gene amplicon- and metagenomic shotgun sequencing across our panel of ten host taxa. This includes two commonly used 16S rRNA gene regions and two amplification procedures, thus totaling five different microbial profiles per host sample.\n\nConclusionWhile 16S rRNA gene-based analyses are subject to much skepticism, we demonstrate that many aspects of bacterial community characterization are consistent across methods and that metagenomic shotgun results are largely dependent on the employed pipeline. The resulting insight facilitates the selection of appropriate methods across a wide range of host taxa. Finally, by contrasting taxonomic and functional profiles and performing phylogenetic analysis, we provide important and novel insight into broad evolutionary patterns among metaorganisms, whereby the transition of animals from an aquatic to a terrestrial habitat marks a major event in the evolution of host-associated microbial composition.

microbiology↗

bioRxiv2019

A chronology of multicellularity evolution in cyanobacteria

The transition from unicellular to multicellular organisms is one of the most significant events in the history of life. Key to this process is the emergence of Darwinian individuality at the higher level: groups must become single entities capable of reproduction for selection to shape their evolution. Evolutionary transitions in individuality are characterized by cooperation between the lower level entities and by division of labor. Theory suggests that division of labor may drive the transition to multicellularity by eliminating the trade-off between two incompatible processes that cannot be performed simultaneously in one cell. Here we examine the evolution of the most ancient multicellular transition known today, that of cyanobacteria, where we reconstruct the sequence of ecological and phenotypic trait evolution. Our results show that the prime driver of multicellularity in cyanobacteria was the expansion in metabolic capacity offered by nitrogen fixation, which was accompanied by the emergence of the filamentous morphology and succeeded by a reproductive life cycle. This was followed by the progression of multicellularity into higher complexity in the form of differentiated cells and patterned multicellularity. Significance StatementThe emergence of multicellularity is a major evolutionary transition. The oldest transition, that of cyanobacteria, happened more than 3 to 3.5 billion years ago. We find N2 fixation to be the prime driver of multicellularity in cyanobacteria. This innovation faced the challenge of incompatible metabolic processes since the N2 fixing enzyme (nitrogenase) is sensitive to oxygen, which is abundantly found in cyanobacteria cells performing photosynthesis. At the same time, N2-fixation conferred an adaptive benefit to the filamentous morphology as cells could divide their labour into performing either N2-fixation or photosynthesis. This was followed by the culmination of complex multicellularity in the form of differentiated cells and patterned multicellularity.

evolutionary biology↗

bioRxiv2019

A cytoskeletal network maintains filament shape in the multicellular cyanobacterium Anabaena sp. PCC 7120

Polymerizing and filament-forming proteins are instrumental for numerous cellular processes such as cell division and growth. Their function in stabilization and localization of protein complexes and replicons is achieved by a filamentous structure. Known filamentous proteins assemble into homopolymers consisting of single subunits - e.g. MreB and FtsZ in bacteria - or heteropolymers that are composed of two subunits, e.g. keratin and /{beta} tubulin in eukaryotes. Here, we describe two novel coiled-coil-rich proteins (CCRPs) in the filament forming cyanobacterium Anabaena sp. PCC 7120 (hereafter Anabaena) that assemble into a heteropolymer and function in the maintenance of the Anabaena multicellular shape (termed trichome). The two CCRPs - Alr4504 and Alr4505 (named ZicK and ZacK) - are strictly interdependent for the assembly of protein filaments in vivo and polymerize nucleotide-independently in vitro, similar to known intermediate filament (IF) proteins. A {Delta}zicK{Delta}zacK double mutant is characterized by a zigzagged cell arrangement and hence a loss of the typical linear Anabaena trichome shape. ZicK and ZacK interact with themselves, with each other, with the elongasome protein MreB, the septal junction protein SepJ and the divisome associate septal protein SepI. Our results suggest that ZicK and ZacK function in cooperation with SepJ and MreB to stabilize the Anabaena trichome and are likely essential for the manifestation of the multicellular shape in Anabaena. Our study reveals the presence of filament-forming IF-like proteins whose function is achieved through the formation of heteropolymers in cyanobacteria.

microbiology↗

bioRxiv2019

Horizontally transmitted symbiont populations in deep-sea mussels are genetically isolated

Eukaryotes are habitats for bacterial organisms where the host colonization and dispersal among individual hosts have consequences for the bacterial ecology and evolution. Vertical symbiont transmission leads to geographic isolation of the microbial population and consequently to genetic isolation of microbiotas from individual hosts. In contrast, the extent of geographic and genetic isolation of horizontally transmitted microbiota is poorly characterized. Here we show that chemosynthetic symbionts of individual Bathymodiolus brooksi mussels constitute genetically isolated populations. The reconstruction of core genome-wide strain sequences from high-resolution metagenomes revealed distinct phylogenetic clades. Nucleotide diversity and strain composition vary along the mussel lifespan and individual hosts show a high degree of genetic isolation. Our results suggest that the uptake of environmental bacteria is a restricted process in B. brooksi, where self-infection of the gill tissue results in serial founder effects during symbiont evolution. We conclude that bacterial colonization dynamics over the host life-cycle is thus an important determinant of population structure and genome evolution of horizontally transmitted symbionts.

microbiology↗

bioRxiv2018

Segregational drift and the interplay between plasmid copy number and evolvability

The ubiquity of plasmids in all prokaryotic phyla and habitats and their ability to transfer between cells marks them as prominent constituents of prokaryotic genomes. Many plasmids are found in their host cell in multiple copies. This leads to an increased mutational supply of plasmid-encoded genes and genetically heterogeneous plasmid genomes. Nonetheless, the segregation of plasmid copies into daughter cells during cell division is considered to occur in the absence of selection on the plasmid alleles. We investigate the implications of random genetic drift of multicopy plasmids during cell division - termed here segregational drift - to plasmid evolution. Performing experimental evolution of low- and high-copy non-mobile plasmids in Escherichia coli, we find that the evolutionary rate of multicopy plasmids does not reflect the increased mutational supply expected according to their copy number. In addition, simulated evolution of multicopy plasmid alleles demonstrates that segregational drift leads to increased loss frequency and extended fixation time of plasmid mutations in comparison to haploid chromosomes. Furthermore, an examination of the experimentally evolved hosts reveals a significant impact of the plasmid type on the host chromosome evolution. Our study demonstrates that segregational drift of multicopy plasmids interferes with the retention and fixation of novel plasmid variants. Depending on the selection pressure on newly emerging variants, plasmid genomes may evolve slower than haploid chromosomes, regardless of their higher mutational supply. We suggest that plasmid copy number is an important determinant of plasmid evolvability due to the manifestation of segregational drift.

microbiology↗