Endosymbiosis and Eukaryotic Cell Evolution

Understanding the evolution of eukaryotic cellular complexity is one of the grand challenges of modern biology. It has now been firmly established that mitochondria and plastids, the classical membrane-bound organelles of eukaryotic cells, evolved from bacteria by endosymbiosis. In the case of mitochondria, evidence points very clearly to an endosymbiont of α-proteobacterial ancestry. The precise nature of the host cell that partnered with this endosymbiont is, however, very much an open question. And while the host for the cyanobacterial progenitor of the plastid was undoubtedly a fully-fledged eukaryote, how — and how often — plastids moved from one eukaryote to another during algal diversification is vigorously debated. In this article I frame modern views on endosymbiotic theory in a historical context, highlighting the transformative role DNA sequencing played in solving early problems in eukaryotic cell evolution, and posing key unanswered questions emerging from the age of comparative genomics

A spliceosomal intron of mitochondrial DNA origin

SummaryThe origin of spliceosomal introns is one of the most enduring mysteries in molecular biology. In nuclear genomes such as our own, the protein-coding regions of genes (exons) can be separated from one another by hundreds of thousands of base pairs (bp) of intronic (non-coding) DNA, and while they are often considered ‘junk’, introns are increasingly ascribed important regulatory functions [1]. Here we present evidence that an intron in a GTPase superfamily gene in the unicellular alga Bigelowiella natans is derived from — and was created by — the insertion of a fragment of mitochondrial DNA. Organelle-to-nucleus DNA transfer is an increasingly well-understood phenomenon, one that has the potential to greatly influence genome structure [2,3]. Our data suggest that such transfers could represent a hitherto underappreciated source of new spliceosomal introns

Sdss J102347.6+003841: A Millisecond Radio Pulsar Binary that Had a Hot Disk During 2000-2001

The Sloan Digital Sky Survey (SDSS) source J102347.6+003841 was recently revealed to be a binary 1.69 ms radio pulsar with a 4.75 hr orbital period and a ~0.2 M ☉ companion. Here, we analyze the SDSS spectrum of the source in detail. The spectrum was taken on 2001 February 1, when the source was in a bright state and showed broad, double-peaked hydrogen and helium lines—dramatically different from the G-type absorption spectrum seen from 2002 May onward. The lines are consistent with emission from a disk around the compact primary. We derive properties of the disk by fitting the SDSS continuum with a simple disk model, and find a temperature range of 2000-34,000 K from the outer to inner edge of the disk. The disk inner and outer radii were approximately 109 and 5.7×1010 cm, respectively. These results further emphasize the unique feature of the source: it is a system likely at the end of its transition from an X-ray binary to a recycled radio pulsar. The disk mass is estimated to have been ~1023 g, most of which would have been lost due to pulsar wind ablation (or due to the propeller effect if the disk had extended inside the light cylinder of the pulsar) before the final disk disruption event. The system could undergo repeated episodes of disk formation. Close monitoring of the source is needed to catch the system in its bright state again, so that this unusual example of a pulsar-disk interaction can be studied in much finer detail

Eukaryote-to-eukaryote gene transfer gives rise to genome mosaicism in euglenids

<p>Abstract</p> <p>Background</p> <p>Euglenophytes are a group of photosynthetic flagellates possessing a plastid derived from a green algal endosymbiont, which was incorporated into an ancestral host cell via secondary endosymbiosis. However, the impact of endosymbiosis on the euglenophyte nuclear genome is not fully understood due to its complex nature as a 'hybrid' of a non-photosynthetic host cell and a secondary endosymbiont.</p> <p>Results</p> <p>We analyzed an EST dataset of the model euglenophyte <it>Euglena gracilis </it>using a gene mining program designed to detect laterally transferred genes. We found <it>E. gracilis </it>genes showing affinity not only with green algae, from which the secondary plastid in euglenophytes evolved, but also red algae and/or secondary algae containing red algal-derived plastids. Phylogenetic analyses of these 'red lineage' genes suggest that <it>E. gracilis </it>acquired at least 14 genes via eukaryote-to-eukaryote lateral gene transfer from algal sources other than the green algal endosymbiont that gave rise to its current plastid. We constructed an EST library of the aplastidic euglenid <it>Peranema trichophorum</it>, which is a eukaryovorous relative of euglenophytes, and also identified 'red lineage' genes in its genome.</p> <p>Conclusions</p> <p>Our data show genome mosaicism in <it>E. gracilis </it>and <it>P. trichophorum</it>. One possible explanation for the presence of these genes in these organisms is that some or all of them were independently acquired by lateral gene transfer and contributed to the successful integration and functioning of the green algal endosymbiont as a secondary plastid. Alternative hypotheses include the presence of a phagocytosed alga as the single source of those genes, or a cryptic tertiary endosymbiont harboring secondary plastid of red algal origin, which the eukaryovorous ancestor of euglenophytes had acquired prior to the secondary endosymbiosis of a green alga.</p

Complete Sequence and Analysis of the Mitochondrial Genome of Hemiselmis andersenii CCMP644 (Cryptophyceae)

<p>Abstract</p> <p>Background</p> <p>Cryptophytes are an enigmatic group of unicellular eukaryotes with plastids derived by secondary (i.e., eukaryote-eukaryote) endosymbiosis. Cryptophytes are unusual in that they possess four genomes–a host cell-derived nuclear and mitochondrial genome and an endosymbiont-derived plastid and 'nucleomorph' genome. The evolutionary origins of the host and endosymbiont components of cryptophyte algae are at present poorly understood. Thus far, a single complete mitochondrial genome sequence has been determined for the cryptophyte <it>Rhodomonas salina</it>. Here, the second complete mitochondrial genome of the cryptophyte alga <it>Hemiselmis andersenii </it>CCMP644 is presented.</p> <p>Results</p> <p>The <it>H. andersenii </it>mtDNA is 60,553 bp in size and encodes 30 structural RNAs and 36 protein-coding genes, all located on the same strand. A prominent feature of the genome is the presence of a ~20 Kbp long intergenic region comprised of numerous tandem and dispersed repeat units of between 22–336 bp. Adjacent to these repeats are 27 copies of palindromic sequences predicted to form stable DNA stem-loop structures. One such stem-loop is located near a GC-rich and GC-poor region and may have a regulatory function in replication or transcription. The <it>H. andersenii </it>mtDNA shares a number of features in common with the genome of the cryptophyte <it>Rhodomonas salina</it>, including general architecture, gene content, and the presence of a large repeat region. However, the <it>H. andersenii </it>mtDNA is devoid of inverted repeats and introns, which are present in <it>R. salina</it>. Comparative analyses of the suite of tRNAs encoded in the two genomes reveal that the <it>H. andersenii </it>mtDNA has lost or converted its original <it>trnK(uuu) </it>gene and possesses a <it>trnS</it>-derived '<it>trnK(uuu)</it>', which appears unable to produce a functional tRNA. Mitochondrial protein coding gene phylogenies strongly support a variety of previously established eukaryotic groups, but fail to resolve the relationships among higher-order eukaryotic lineages.</p> <p>Conclusion</p> <p>Comparison of the <it>H. andersenii </it>and <it>R. salina </it>mitochondrial genomes reveals a number of cryptophyte-specific genomic features, most notably the presence of a large repeat-rich intergenic region. However, unlike <it>R. salina</it>, the <it>H. andersenii </it>mtDNA does not possess introns and lacks a Lys-tRNA, which is presumably imported from the cytosol.</p

Estimating the Incidence of Typhoid Fever and Other Febrile Illnesses in Developing Countries

To measure the incidence of typhoid fever and other febrile illnesses in Bilbeis District, Egypt, we conducted a household survey to determine patterns of health seeking among persons with fever. Then we established surveillance for 4 months among a representative sample of health providers who saw febrile patients. Health providers collected epidemiologic information and blood (for culture and serologic testing) from eligible patients. After adjusting for the provider sampling scheme, test sensitivity, and seasonality, we estimated that the incidence of typhoid fever was 13/100,000 persons per year and the incidence of brucellosis was 18/100,000 persons per year in the district. This surveillance tool could have wide applications for surveillance for febrile illness in developing countries

Atmospheric composition and climate impacts of a future hydrogen economy

Hydrogen is expected to play a key role in the global energy transition to net zero emissions in many scenarios. However, fugitive emissions of hydrogen into the atmosphere during its production, storage, distribution and use could reduce the climate benefit and also have implications for air quality. Here we explore the atmospheric composition and climate impacts of increases in atmospheric hydrogen abundance using the UKESM1 chemistry-climate model. We find that increases in hydrogen result in increases in methane, tropospheric ozone and stratospheric water vapour, resulting in a positive radiative forcing. However, some of the impacts of hydrogen leakage are partially offset by potential reductions in emissions of methane, carbon monoxide, nitrogen oxides and volatile organic compounds from the consumption of fossil fuels. We derive a new methodology for determining indirect Global Warming Potentials from steady-state simulations which is applicable to both shorter-lived species and those with intermediate and longer lifetimes, such as hydrogen. Using this methodology, we determine a 100-year Global Warming Potential for hydrogen of 12 &plusmn; 6. To maximise the benefit of hydrogen as an energy source, emissions associated with hydrogen leakage and emissions of the ozone precursor gases need to be minimised.</p

Evolving a photosynthetic organelle

The evolution of plastids from cyanobacteria is believed to represent a singularity in the history of life. The enigmatic amoeba Paulinella and its 'recently' acquired photosynthetic inclusions provide a fascinating system through which to gain fresh insight into how endosymbionts become organelles