Did group II intron proliferation in an endosymbiont-bearing archaeon create eukaryotes?

Martin & Koonin recently proposed that the eukaryote nucleus evolved as a quality control mechanism to prevent ribosome readthrough into introns. In their scenario, the bacterial ancestor of mitochondria was resident in an archaeal cell, and group II introns (carried by the fledgling mitochondrion) inserted into coding regions in the archaeal host genome. They suggest that if transcription and translation were coupled, and because splicing is expected to have been slower than translation, the effect of insertion would have been ribosome readthrough into introns, resulting in production of aberrant proteins. The emergence of the nuclear compartment would thus have served to separate transcription and splicing from translation, thereby alleviating this problem. In this article, I argue that Martin & Koonin's model is not compatible with current knowledge. The model requires that group II introns would spread aggressively through an archaeal genome. It is well known that selfish elements can spread through an outbreeding sexual population despite a substantial fitness cost to the host. The same is not true for asexual lineages however, where both theory and observation argue that such elements will be under pressure to reduce proliferation, and may be lost completely. The recent introduction of group II introns into archaea by horizontal transfer provides a natural test case with which to evaluate Martin & Koonin's model. The distribution and behaviour of these introns fits prior theoretical expectations, not the scenario of aggressive proliferation advocated by Martin & Koonin. I therefore conclude that the mitochondrial seed hypothesis for the origin of eukaryote introns, on which their model is based, better explains the early expansion of introns in eukaryotes. The mitochondrial seed hypothesis has the capacity to separate the origin of eukaryotes from the origin of introns, leaving open the possibility that the cell that engulfed the ancestor of mitochondria was a sexually outcrossing eukaryote cell

Manifold Routes to a Nucleus

It is widely assumed that there is a clear distinction between eukaryotes, with cell nuclei, and prokaryotes, which lack nuclei. This suggests the evolution of nuclear compartmentation is a singular event. However, emerging knowledge of the diversity of bacterial internal cell structures suggests the picture may not be as black-and-white as previously thought. For instance, some members of the bacterial PVC superphylum appear to have nucleus-like compartmentation, where transcription and translation are physically separated, and some jumbophages have recently been shown to create nucleus-like structures within their Pseudomonad hosts. Moreover, there is also tantalizing metagenomic identification of new Archaea that carry homologs of genes associated with internal cell membrane structure in eukaryotes. All these cases invite comparison with eukaryote cell biology. While the bacterial cases of genetic compartmentation are likely convergent, and thus viewed by many as not germane to the question of eukaryote origins, we argue here that, in addressing the broader question of the evolution of compartmentation, other instances are at least as important: they provide us with a point of comparison which is critical for a more general understanding of both the conditions favoring the emergence of intracellular compartmentation of DNA and the evolutionary consequences of such cellular architecture. Finally, we consider three classes of explanation for the emergence of compartmentation: physical protection, crosstalk avoidance and nonadaptive origins

Comparative Genomic Evidence for a Complete Nuclear Pore Complex in the Last Eukaryotic Common Ancestor

BackgroundThe Nuclear Pore Complex (NPC) facilitates molecular trafficking between nucleus and cytoplasm and is an integral feature of the eukaryote cell. It exhibits eight-fold rotational symmetry and is comprised of approximately 30 nucleoporins (Nups) in different stoichiometries. Nups are broadly conserved between yeast, vertebrates and plants, but few have been identified among other major eukaryotic groups. Methodology/Principal FindingsWe screened for Nups across 60 eukaryote genomes and report that 19 Nups (spanning all major protein subcomplexes) are found in all eukaryote supergroups represented in our study (Opisthokonts, Amoebozoa, Viridiplantae, Chromalveolates and Excavates). Based on parsimony, between 23 and 26 of 31 Nups can be placed in LECA. Notably, they include central components of the anchoring system (Ndc1 and Gp210) indicating that the anchoring system did not evolve by convergence, as has previously been suggested. These results significantly extend earlier results and, importantly, unambiguously place a fully-fledged NPC in LECA. We also test the proposal that transmembrane Pom proteins in vertebrates and yeasts may account for their variant forms of mitosis (open mitoses in vertebrates, closed among yeasts). The distribution of homologues of vertebrate Pom121 and yeast Pom152 is not consistent with this suggestion, but the distribution of fungal Pom34 fits a scenario wherein it was integral to the evolution of closed mitosis in ascomycetes. We also report an updated screen for vesicle coating complexes, which share a common evolutionary origin with Nups, and can be traced back to LECA. Surprisingly, we find only three supergroup-level differences (one gain and two losses) between the constituents of COPI, COPII and Clathrin complexes. Conclusions/SignificanceOur results indicate that all major protein subcomplexes in the Nuclear Pore Complex are traceable to the Last Eukaryotic Common Ancestor (LECA). In contrast to previous screens, we demonstrate that our conclusions hold regardless of the position of the root of the eukaryote tree

SnoPatrol: how many snoRNA genes are there?

Small nucleolar RNAs (snoRNAs) are among the most evolutionarily ancient classes of small RNA. Two experimental screens published in BMC Genomics expand the eukaryotic snoRNA catalog, but many more snoRNAs remain to be found

Above and belowground community strategies respond to different global change drivers

Environmental changes alter the diversity and structure of communities. By shifting the range of species traits that will be successful under new conditions, environmental drivers can also dramatically impact ecosystem functioning and resilience. Above and belowground communities jointly regulate whole-ecosystem processes and responses to change, yet they are frequently studied separately. To determine whether these communities respond similarly to environmental changes, we measured taxonomic and trait-based responses of plant and soil microbial communities to four years of experimental warming and nitrogen deposition in a temperate grassland. Plant diversity responded strongly to N addition, whereas soil microbial communities responded primarily to warming, likely via an associated decrease in soil moisture. These above and belowground changes were associated with selection for more resource-conservative plant and microbe growth strategies, which reduced community functional diversity. Functional characteristics of plant and soil microbial communities were weakly correlated (P = 0.07) under control conditions, but not when above or belowground communities were altered by either global change driver. These results highlight the potential for global change drivers operating simultaneously to have asynchronous impacts on above and belowground components of ecosystems. Assessment of a single ecosystem component may therefore greatly underestimate the whole-system impact of global environmental changes

The evolution of strand preference in simulated RNA replicators with strand displacement: Implications for the origin of transcription

<p>Abstract</p> <p>Background</p> <p>The simplest conceivable example of evolving systems is RNA molecules that can replicate themselves. Since replication produces a new RNA strand complementary to a template, all templates would eventually become double-stranded and, hence, become unavailable for replication. Thus the problem of how to separate the two strands is considered a major issue for the early evolution of self-replicating RNA. One biologically plausible way to copy a double-stranded RNA is to displace a preexisting strand by a newly synthesized strand. Such copying can in principle be initiated from either the (+) or (-) strand of a double-stranded RNA. Assuming that only one of them, say (+), can act as replicase when single-stranded, strand displacement produces a new replicase if the (-) strand is the template. If, however, the (+) strand is the template, it produces a new template (but no replicase). Modern transcription exhibits extreme strand preference wherein anti-sense strands are always the template. Likewise, replication by strand displacement seems optimal if it also exhibits extreme strand preference wherein (-) strands are always the template, favoring replicase production. Here we investigate whether such strand preference can evolve in a simple RNA replicator system with strand displacement.</p> <p>Results</p> <p>We first studied a simple mathematical model of the replicator dynamics. Our results indicated that if the system is well-mixed, there is no selective force acting upon strand preference per se. Next, we studied an individual-based simulation model to investigate the evolution of strand preference under finite diffusion. Interestingly, the results showed that selective forces "emerge" because of finite diffusion. Strikingly, the direction of the strand preference that evolves [i.e. (+) or (-) strand excess] is a complex non-monotonic function of the diffusion intensity. The mechanism underlying this behavior is elucidated. Furthermore, a speciation-like phenomenon is observed under certain conditions: two extreme replication strategies, namely replicase producers and template producers, emerge and coexist among competing replicators.</p> <p>Conclusion</p> <p>Finite diffusion enables the evolution of strand preference, the direction of which is a non-monotonic function of the diffusion intensity. By identifying the conditions under which strand preference evolves, this study provides an insight into how a rudimentary transcription-like pattern might have emerged in an RNA-based replicator system.</p> <p>Reviewers</p> <p>This article was reviewed by Eugene V Koonin, Rob Kinght and István Scheuring (nominated by David H Ardell). For the full reviews, please go to the Reviewers' comments section.</p

Comparative Analysis of RNA Families Reveals Distinct Repertoires for Each Domain of Life

The RNA world hypothesis, that RNA genomes and catalysts preceded DNA genomes and genetically-encoded protein catalysts, has been central to models for the early evolution of life on Earth. A key part of such models is continuity between the earliest stages in the evolution of life and the RNA repertoires of extant lineages. Some assessments seem consistent with a diverse RNA world, yet direct continuity between modern RNAs and an RNA world has not been demonstrated for the majority of RNA families, and, anecdotally, many RNA functions appear restricted in their distribution. Despite much discussion of the possible antiquity of RNA families, no systematic analyses of RNA family distribution have been performed. To chart the broad evolutionary history of known RNA families, we performed comparative genomic analysis of over 3 million RNA annotations spanning 1446 families from the Rfam 10 database. We report that 99% of known RNA families are restricted to a single domain of life, revealing discrete repertoires for each domain. For the 1% of RNA families/clans present in more than one domain, over half show evidence of horizontal gene transfer, and the rest show a vertical trace, indicating the presence of a complex protein synthesis machinery in the Last Universal Common Ancestor (LUCA) and consistent with the evolutionary history of the most ancient protein-coding genes. However, with limited interdomain transfer and few RNA families exhibiting demonstrable antiquity as predicted under RNA world continuity, our results indicate that the majority of modern cellular RNA repertoires have primarily evolved in a domain-specific manner.Comment: 47 pages, 4 main figures, 3 supplementary figures, 4 supplementary tables. Submitted to PLOS Computational Biolog

Fisher: a program for the detection of H/ACA snoRNAs using MFE secondary structure prediction and comparative genomics – assessment and update

<p>Abstract</p> <p>Background</p> <p>The H/ACA family of small nucleolar RNAs (snoRNAs) plays a central role in guiding the pseudouridylation of ribosomal RNA (rRNA). In an effort to systematically identify the complete set of rRNA-modifying H/ACA snoRNAs from the genome sequence of the budding yeast, <it>Saccharomyces cerevisiae</it>, we developed a program – Fisher – and previously presented several candidate snoRNAs based on our analysis <abbrgrp><abbr bid="B1">1</abbr></abbrgrp>.</p> <p>Findings</p> <p>In this report, we provide a brief update of this work, which was aborted after the publication of experimentally-identified snoRNAs <abbrgrp><abbr bid="B2">2</abbr></abbrgrp> identical to candidates we had identified bioinformatically using Fisher. Our motivation for revisiting this work is to report on the status of the candidate snoRNAs described in <abbrgrp><abbr bid="B1">1</abbr></abbrgrp>, and secondly, to report that a modified version of Fisher together with the available multiple yeast genome sequences was able to correctly identify several H/ACA snoRNAs for modification sites not identified by the snoGPS program <abbrgrp><abbr bid="B3">3</abbr></abbrgrp>. While we are no longer developing Fisher, we briefly consider the merits of the Fisher algorithm relative to snoGPS, which may be of use for workers considering pursuing a similar search strategy for the identification of small RNAs. The modified source code for Fisher is made available as supplementary material.</p> <p>Conclusion</p> <p>Our results confirm the validity of using minimum free energy (MFE) secondary structure prediction to guide comparative genomic screening for RNA families with few sequence constraints.</p

Inventing and implementing future-ready archival education

The Archival / Preservation Education SIG panel engages with community-responsive master's-level archival education. Seven ten-minute individual presentations and audience discussion traverse the decision points in managing curricular change; presenters bring perspectives from multiple states. "Audio Preservation as Metacognitive Archival Education" by Sarah Buchanan discusses how audiovisual archiving experiences support the continual development of students' metacognitive skills during their graduate program. Based on community collaboration, the activity progressions provide students with digital experiences, faculty with curricular guidance, and online audiences with more representative primary sources. "LIS Students Contributing to Building a Sustainable Digital Community Archive" by Krystyna Matusiak describes a community-based two-year project aimed at preserving and promoting the Park County Local History Archive in rural Colorado, now available at https://pclha.cvlcollections.org/. The presentation illustrates students' many contributions: organizing materials and assessing their copyright status, digitizing photographs, converting oral histories, creating metadata records, building exhibits, and showcasing community resilience. "Changing Horses Midstream: Revising Curriculum and Student Engagement to Ensure a Resilient Future" by Edward Benoit, III and Amanda Lima discusses the revision process for transitioning two programs to LSU Online, compares assessments from the traditional and LSU Online programs, and reflects on completing the first year. Additionally, the presenters will highlight the use of student-run Slack channels and virtual coffee hours as online student community building tools for the new LSU Online students, and discuss the school's future in the platform. "Producing Practical Professionals with Curriculum for Equity, Diversity, and Inclusion" by Aisha Johnson acknowledges that cultural heritage programs should address the need for cultural preservation and reflection, for archivists of Black, Indigenous, and Persons of Color (BIPOC) heritage. The presentation will review a reestablished Archives and Records Management concentration, with core archival and complementary knowledge curriculum, as a case study for exploring new approaches to pedagogy on the purpose, value, and importance of archives in society. "Learning from Experience: Lessons from a Virtual Service-Learning Experiment" by Colin Post discusses a service-learning project documenting an artist's performance as well as their artwork archives. While such projects place even greater pressure on the instructor as a project manager, they enhance connections between theory and practice in online courses. "Lessons Learned from the Digital Preservation Outreach and Education Network" by Anthony Cocciolo and Erin Barsan discusses the types of needs they have uncovered, the communities served, and the lessons learned over the course of a year running DPOE-N. The Network's response to the COVID-19 pandemic comprises microfunding for professional development and emergency hardware support for cultural heritage professionals. "National Forum Grant Project: Exploring New Frontiers in 21st Century Archival Education" by Alex Poole and Jane Zhang discusses the environmental scan, National Forum event, and final outputs of their year-long project. The presentation addresses motivation and need, historical and current context, research components, and intended results and impact. The moderator will facilitate Q&A within and across the presentations

Characterisation of an Escherichia coli line that completely lacks ribonucleotide reduction yields insights into the evolution of parasitism and endosymbiosis

Life requires ribonucleotide reduction for de novo synthesis of deoxyribonucleotides. As ribonucleotide reduction has on occasion been lost in parasites and endosymbionts, which are instead dependent on their host for deoxyribonucleotide synthesis, it should in principle be possible to knock this process out if growth media are supplemented with deoxyribonucleosides. We report the creation of a strain of Escherichia coli where all three ribonucleotide reductase operons have been deleted following introduction of a broad spectrum deoxyribonucleoside kinase from Mycoplasma mycoides. Our strain shows slowed but substantial growth in the presence of deoxyribonucleosides. Under limiting deoxyribonucleoside levels, we observe a distinctive filamentous cell morphology, where cells grow but do not appear to divide regularly. Finally, we examined whether our lines can adapt to limited supplies of deoxyribonucleosides, as might occur in the switch from de novo synthesis to dependence on host production during the evolution of parasitism or endosymbiosis. Over the course of an evolution experiment, we observe a 25-fold reduction in the minimum concentration of exogenous deoxyribonucleosides necessary for growth. Genome analysis reveals that several replicate lines carry mutations in deoB and cdd. deoB codes for phosphopentomutase, a key part of the deoxyriboaldolase pathway, which has been hypothesised as an alternative to ribonucleotide reduction for deoxyribonucleotide synthesis. Rather than complementing the loss of ribonucleotide reduction, our experiments reveal that mutations appear that reduce or eliminate the capacity for this pathway to catabolise deoxyribonucleotides, thus preventing their loss via central metabolism. Mutational inactivation of both deoB and cdd is also observed in a number of obligate intracellular bacteria that have lost ribonucleotide reduction. We conclude that our experiments recapitulate key evolutionary steps in the adaptation to life without ribonucleotide reduction