Biology subjects

Seah, B. K. B.

Publications and source records attributed to Seah, B. K. B..

11 recordsLinked to original sources

bioRxiv2026

Spliceosome loss in the red tide ciliate Mesodinium rubrum presents a symbiotic cul-de-sac

Mesodinium rubrum, a marine microbial eukaryote associated with some of the largest red tides on Earth, has the remarkable ability to commandeer the plastids, mitochondria and nuclei from the alga Teleaulax amphioxeia for photosynthesis. Here we report analyses of assemblies of M. rubrums two nuclear genomes. Unexpectedly, M. rubrum appears to have completely lost its spliceosomal introns, most spliceosomal molecules, and the key genes for an intron splicing-associated process, Nonsense-mediated mRNA Decay (NMD). In contrast, non-spliceosomal tRNA introns have been retained, as have thousands of intron analogs spliced out of DNA during ciliate somatic genome development (internal eliminated sequences - IESs). Intron-containing genes, especially from intron-rich species like T. amphioxeia, would likely be defunct if horizontally transferred to a host without a spliceosome like M. rubrum, and thus we propose that introns can be a roadblock to progressive endosymbiotic genomic integration.

genomics↗

bioRxiv2025

Emergent metabolic parasitism driven by organelle sequestration

Stable acquisitions of metabolism, such as the endosymbiotic incorporation of eukaryotic chloroplasts, are thought to proceed through mechanisms that increase the genetic repertoire of the host and allow for vertical integration of new metabolism. Here we test these predictions using the chloroplast-stealing marine ciliate genus Mesodinium by comparing transcriptomes from species that represent a spectrum from full heterotrophy to nearly full phototrophy. In contrast to theory, we find a striking divestment in metabolic autonomy with increased reliance on acquired photosynthesis. Indeed, the highly photosynthetic, red tide-forming Mesodinium rubrum appears to have lost the capacity to synthesize amino acids, metabolize fatty acids, and produce peroxisomes. Our results portray a metabolic parasite, masquerading as a free-living ciliate, yet incapable of satisfying most of its basic anabolic needs.

evolutionary biology↗

bioRxiv2023

Nuclear dualism without extensive DNA elimination in the ciliate Loxodes magnus

Ciliates are unicellular eukaryotes with two distinct kinds of nuclei in each cell: transcriptionally active somatic macronuclei (MAC) and silent germline micronuclei (MIC). In the best-studied model species, both nuclei can divide asexually, but only germline MICs participate in meiosis, karyogamy, and development into new MACs. During MIC-to-MAC development, thousands of mobile element relics in the germline, called internally eliminated sequences (IESs), are excised. This genome editing enables IESs to persist by shielding them from somatic natural selection. Editing itself is a costly, time-consuming process, hypothetically maintained by evolutionary addiction. Loxodes magnus and its relatives (class Karyorelictea) are cytologically unusual because their MACs do not divide asexually, but must develop anew from mitotically generated MIC copies every cell division. Here, we report that Loxodes genome development is also unconventional. We found no canonical germline-limited IESs in Loxodes despite careful purification and long-read sequencing of MICs and MACs. The k-mer content of these nuclei overlapped, and indels found by read mapping were consistent with allele variants rather than IESs. Two other hallmarks of genome editing--domesticated DDE-family transposases and editing-associated small RNAs--were also absent. Nonetheless, histone marks, nucleosome and DNA N6-methyladenosine distributions in vegetative Loxodes cells are consistent with actively transcribed MACs and inactive MICs, like other ciliates. Both genomes, not only the MIC, were large and replete with retrotransposon sequences. Given the costs associated with genome editing, we hypothesize that karyorelicteans like Loxodes have lost or streamlined editing during MIC-to-MAC development, and have found a way out of the addictive cycle.

genomics↗

bioRxiv2023

How did UGA codon translation as tryptophan evolve in certain ciliates? A critique of Kachale et al. 2023 Nature

Ciliates are a widespread clade of microbial eukaryotes with the greatest diversity of nuclear genetic codes (at least eight) following a recent addition1. All non-standard ciliate genetic codes involve stop codon reassignments1,2,3. Two of these codes are ambiguous1-3, with "stop" codons either translated or terminating translation depending on their context2,3. Ambiguous genetic codes have arisen not only in ciliates, but also independently in trypanosomatids from the genus Blastocrithidia4 and an alveolate species from the genus Amoebophrya5. Two ambiguous genetic codes in ciliates share translation of UGA "stop" codons as tryptophan with Blastocrithidia and the Amoebophrya species. tRNA genes with complementary anticodons to reassigned UAA and UAG stop codons have invariably been found in ciliate species that translate these codons1,2. Furthermore, though a UGA-cognate tRNACysUCA was reported in Euplotes6, a ciliate genus that translates UGA as cysteine, vexingly, no nuclear genome-encoded tRNATrpUCA has been found in ciliate species with UGA tryptophan codons. Recently, Kachale et al. provided evidence for UGA translation as tryptophan in Blastocrithidia nonstop and the ciliate Condylostoma magnum using 4 base pair anticodon stem (AS) near-cognate tryptophan tRNATrpCCAs, rather than the typical 5 base pair stem tRNAs7. New tRNA data we report from additional ciliates bolsters this hypothesis. Kachale et al. also hypothesised that a particular amino acid substitution in the key stop codon recognition protein, eRF1 (eukaryotic Release Factor 1), favours translation of UGA as tryptophan instead of termination7. Contrary to Kachale et al, we propose such substitutions favouring reduced eRF1 competition enhancing "stop" codon translation do not need to occur concomitantly with tRNA alterations or acquisitions to evolve new genetic codes via stop codon reassignment. We report multiple instances of the substitution investigated in Kachale et al. 2023 that have not led to UGA translation, and multiple ciliate species with UGA tryptophan translation but without the substitution, indicating it is not necessary. Consistent with the ambiguous intermediate hypothesis for genetic code evolution, experimental evidence and our observations suggest continued potential ciliate eRF1-tRNA competition.

evolutionary biology↗

bioRxiv2022

Improved methods for bulk cultivation and fixation of Loxodes ciliates for fluorescence microscopy

Loxodes is one of the best ecologically characterized ciliate genera with numerous intriguing physiological abilities, including gravity-sensing organelles and nitrate respiration. However, these cells have been considered challenging to cultivate in bulk, and are poorly preserved by conventional fixatives used for fluorescence microscopy. Here we describe methods to grow and harvest Loxodes cells in bulk with liquid soil extract medium, as well as a new fixative called ZFAE (zinc sulfate, formaldehyde, acetic acid, ethanol) that can fix Loxodes cells more effectively than buffered formaldehyde or methanol. We show that ZFAE is compatible with immunofluorescence and the nuclear stain DAPI. Loxodes is thus now amenable to long-term maintenance, large-scale growth, and modern cell biology investigations of monoclonal strains in laboratory conditions.

cell biology↗

bioRxiv2022

MITE infestation of germline accommodated by genome editing in Blepharisma

During a sophisticated developmental process, ciliates excise numerous internally eliminated sequences (IESs) from a germline genome copy, producing a functional somatic genome. Most IESs ultimately originate from transposons but homology is obscured by sequence decay. To obtain more representative perspectives on ciliate genome editing, we assembled forty thousand IESs of Blepharisma stoltei, from a much earlier-diverging lineage than existing models. Short IESs (< 115 bp) were largely non-repetitive, with a pronounced ~10 bp length periodicity, whereas longer IESs (max 7 kbp) were non-periodic and contained abundant interspersed repeats. Contrary to current models, the Blepharisma germline genome encodes few transposases. Instead, its most abundant repeat (8000 copies) was a Miniature Inverted-repeat Transposable Element (MITE), apparently a deletion derivative of a germline-limited Pogo-family transposon. We propose MITEs as an important and eventually self-limiting IES source. Rather than defending germline genomes against mobile elements, we argue that transposase domestication actually facilitates junk DNA accumulation.

genomics↗

bioRxiv2022

Karyorelict ciliates use an ambiguous genetic code with context-dependent stop/sense codons

AO_SCPLOWBSTRACTC_SCPLOWIn ambiguous stop/sense genetic codes, the stop codon(s) not only terminate translation but can also encode amino acids. Such codes have evolved at least four times in eukaryotes, twice among ciliates (Condylostoma magnum and Parduczia sp.). These have appeared to be isolated cases whose next closest relatives use conventional stop codons. However, little genomic data have been published for the Karyorelictea, the ciliate class that contains Parduczia sp., and previous studies may have overlooked ambiguous codes because of their apparent rarity. We therefore analyzed single-cell transcriptomes from four of the six karyorelict families to determine their genetic codes. Reassignment of canonical stops to sense codons was inferred from codon frequencies in conserved protein domains, while the actual stop codon was predicted from full-length transcripts with intact 3-untranslated regions (3-UTRs). We found that all available karyorelicts use the Parduczia code, where canonical stops UAA and UAG are reassigned to glutamine, and UGA encodes either tryptophan or stop. Furthermore, a small minority of transcripts may use an ambiguous stop-UAA instead of stop-UGA. Given the ubiquity of karyorelicts in marine coastal sediments, ambiguous genetic codes are not mere marginal curiosities but a defining feature of a globally distributed and diverse group of eukaryotes.

genomics↗

bioRxiv2021

The Blepharisma stoltei macronuclear genome: towards the origins of whole genome reorganization

Massive DNA excision occurs regularly in ciliates, ubiquitous microbial eukaryotes with somatic and germline nuclei in the same cell. Tens of thousands of internally eliminated sequences (IESs) scattered throughout a copy of the ciliate germline genome are deleted during development of the streamlined somatic genome. Blepharisma represents one of the two earliest diverging ciliate classes, and, unusually, has dual pathways of somatic nuclear development, making it ideal for investigating the functioning and evolution of these processes. Here, we report the somatic genome assembly of Blepharisma stoltei strain ATCC 30299 (41 Mb), arranged as numerous alternative telomere-capped minichromosomes. This genome encodes eight PiggyBac transposase homologs liberated from transposons. All are subject to purifying selection, but just one, the putative IES excisase, has a complete catalytic triad. We propose PiggyBac homologs were ancestral excisases that enabled evolution of extensive, natural genome editing.

genomics↗

bioRxiv2021

BleTIES: Annotation of natural genome editing in ciliates using long read sequencing

SummaryCiliates are single-celled eukaryotes that eliminate specific, interspersed DNA sequences (internally eliminated sequences, IESs) from their genomes during development. These are challenging to annotate and assemble because IES-containing sequences are much less abundant in the cell than those without, and IES sequences themselves often contain repetitive and low-complexity sequences. Long read sequencing technologies from Pacific Biosciences and Oxford Nanopore have the potential to reconstruct longer IESs than has been possible with short reads, and also the ability to detect correlations of neighboring element elimination. Here we present BleTIES, a software toolkit for detecting, assembling, and analyzing IESs using mapped long reads. Availability and implementationBleTIES is implemented in Python 3. Source code is available at https://github.com/Swart-lab/bleties (MIT license), and also distributed via Bioconda. ContactContact: kb.seah@tuebingen.mpg.de Supplementary informationBenchmarking of BleTIES with published sequence data.

bioinformatics↗

bioRxiv2020

Desulfovibrio diazotrophica sp. nov., a sulphate reducing bacterium from the human gut capable of nitrogen fixation

Sulphate-reducing bacteria (SRB) are widespread in human guts, yet their expansion has been linked to colonic diseases. We report the isolation, genome sequencing, and physiological characterisation of a novel SRB species belonging to the class Deltaproteobacteria (QI0027T). Phylogenomic analysis revealed that the QI0027T strain belongs to the genus Desulfovibrio with its closest relative being Desulfovibrio legallii. Metagenomic sequencing of stool samples from 45 individuals, as well as comparison with 1690 Desulfovibrionaceae metagenome-assembled genomes, revealed the presence of QI0027T in at least 22 further individuals. QI0027T encoded nitrogen fixation genes and based on the acetylene reduction assay, actively fixed nitrogen. Transcriptomics revealed that QI0027T overexpressed 45 genes in nitrogen limiting conditions as compared to cultures supplemented with ammonia, including nitrogenases, an urea uptake system and the urease enzyme complex. To the best of our knowledge, this is the first Desulfovibrio human isolate for which nitrogen fixation has been demonstrated. This isolate was named Desulfovibrio diazotrophica sp. nov., referring to its ability to fix nitrogen ( diazotroph). ImportanceAnimals are often nitrogen limited and have evolved diverse strategies to capture biologically active nitrogen. These strategies range from amino acid transporters to stable associations with beneficial microbes that can provide fixed nitrogen. Although frequently thought as a nutrient-rich environment, nitrogen fixation can occur in the human gut of some populations, but so far it has been attributed mainly to Clostridia and Klebsiella based on sequencing. We have cultivated a novel Desulfovibrio from human gut origin which encoded, expressed and actively used nitrogen fixation genes, suggesting that some sulphate reducing bacteria could also play a role in the availability of nitrogen in the gut.

microbiology↗

bioRxiv2020

Kentrophoros magnus sp. nov. (Ciliophora, Karyorelictea), a new flagship species of marine interstitial ciliates

The karyorelictean ciliate Kentrophoros lacks a defined oral apparatus but has a dense coat of symbiotic bacteria that it consumes by phagocytosis. Body size, shape, and nuclear characters are variable in this genus. We formally describe a new species, K. magnus from Elba (Italy), which has unusual folding of its symbiont-bearing surface into pouch-like compartments, a body form that we term "pseudotrophosomal". K. magnus cells are large (2100 {+/-} 700 x 170 {+/-} 23 m in vivo), but contain only one micronucleus and two macronuclei, although these are much bigger than other Kentrophoros (widths 20 {+/-} 2.5 and 31 {+/-} 4.0 m respectively in K. magnus). We also present morphological observations on a close relative from Twin Cayes (Belize), which also has relatively large nuclei (micronuclei 13 {+/-} 1.5 m, mature macronuclei 20 {+/-} 2.8 m), but unlike K. magnus, it has on average 22 nuclei per cell, with different developmental stages of the macronuclei present simultaneously, and lacks pouch-like folding. Nuclear number and arrangement are important characters for karyorelicts. We suggest the use of a "nuclear formula" to simplify descriptions. Our discovery of large and morphologically distinctive new species underlines the incompleteness of our knowledge about meiofaunal ciliates.

microbiology↗