Biology subjects

Swart, E.

Publications and source records attributed to Swart, E..

7 recordsLinked to original sources

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↗

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

Assessing the efficacy of antibiotic treatment for the creation of axenic earthworms

Earthworms are an integral part of soil ecosystems, especially for their role in soil functions such as organic matter (OM) decomposition and nutrient cycling. Earthworms and microorganisms are interdependent, and a considerable portion of the contribution earthworms make to influencing OM fate is through interactions with microorganisms. However, the importance of the earthworm-associated microbiome is not fully understood, because it is difficult to separate the direct influence of the earthworms from the indirect influence of their microbiome. Here, we evaluated an antibiotic-based procedure for producing axenic ecologically-contrasting earthworm species (E. fetida, L. terrestris, A. chlorotica) as the first step towards soil studies aimed at understanding the importance of the earthworm microbiome for host health and function. Individual earthworms were exposed to antibiotics: cycloheximide (150 g ml-1), ampicillin (100 g ml-1), ciprofloxacin (50 g ml-1), nalidixic acid (50 g ml-1), and gentamicin (50 g ml-1) either singly or in a cocktail via culture (96 h) in a semi-solid agar carrier. Compared to the non-antibiotic treated control, the cocktail (for all three species) and ciprofloxacin (for E. fetida and A. chlorotica) treatments significantly reduced (P<0.05) culturable microbial abundance on nutrient agar and potato dextrose agar. The microbial counts were reduced to below detection (<50 CFU individual-1) for E. fetida and A. chlorotica receiving the cocktail. Illumina 16S rDNA amplicon sequence analysis of culturable L. terrestris -associated bacteria showed that antibiotic treatment influenced community composition revealing putative sensitive (Comomonas, Kosakonia and Sphingobacterium) and insensitive (Aeromonas, Pseudochrobactrum) taxa. Overall, we report a rapid, with minimal earthworm-handling, process of creating axenic E. fetida and A. chlorotica individuals or L. terrestris with a suppressed microbiome as a tool to be used in future ecological studies of earthworm microbial interactions affecting host health and function.

ecology↗

bioRxiv2020

Limitations of DNA barcoding in determining the origin of smuggled seahorses and pipefishes

Seahorses and pipefishes are heavily exploited for use in Traditional Chinese Medicine (TCM), and less frequently for curio markets or as aquarium fish. A number of recent studies have used DNA barcoding to identify species sold at TCM markets in East Asia, but the usefulness of this approach in determining the region of origin remains poorly explored. Here, we generated DNA barcodes of dried seahorses and pipefishes destined for TCM that were confiscated at South Africas largest airport because they lacked the export permits required for the CITES-listed seahorses. These were compared with published sequences and new sequences generated for Mozambican seahorses, with the aim of determining whether it is possible to identify their country of origin. All pipefishes were identified as Syngnathoides biaculeatus, a widespread Indo-Pacific species, but the published sequence data did not provide sufficient resolution to identify the region of origin. The same was true of the majority of seahorses, which could not even be identified to species level because they clustered among an unresolved species complex whose sequences were published under the names Hippocampus kuda, H. fuscus and H. capensis. The presence of a few specimens of a second seahorse, H. camelopardalis, suggests that the shipment originated from East Africa because the range of this seahorse is centred around this region, but again, it was not possible to determine their country of origin. Even though seahorses and pipefishes have high levels of genetic population structure because of their low dispersal potential, DNA barcoding was only suitable to tentatively identify species, but not their region of origin. DNA barcoding is increasingly used to identify illegally traded wildlife, but our results show that more sophisticated methods are needed to monitor and police the trade in seahorses and pipefishes.

genetics↗