Biology subjects

Rink, J. C.

Publications and source records attributed to Rink, J. C..

13 recordsLinked to original sources

bioRxiv2026

Trans-regenerational RNAi Memory in Planarians

How cells and organisms preserve molecular memory is a central question in biology. Chromatin states can maintain cellular memory in animals, yet DNA and histone modifications are typically reset in the germ line and early embryo. In Caenorhabditis elegans, RNA-dependent RNA polymerases (RdRPs) amplify small RNAs and enable transgenerational gene regulation. However, many organisms, including mammals, lack canonical RdRPs. Here we find that planarians, highly regenerative flatworms that lack canonical RdRPs and critical nuclear RNAi machinery, mount long-term heritable gene regulatory responses. We show that ingestion of double-stranded RNA induces potent, sequence-specific silencing that persists for months, withstands repeated cycles of whole-body regeneration, and can be transferred between animals. This long-term silencing, heritable during asexual reproduction, follows two phases: an early, transient response to exogenous RNA, and a stable, cell-autonomous memory phase that maintains repression long after the RNA trigger is gone. The memory phase is maintained post-transcriptionally, as transcription and chromatin accessibility remain unchanged while mature mRNA is depleted. This phase coincides with the emergence of A-tailed 21-23-nt antisense small RNAs and with silencing responses that act in trans and spread across the length of the target transcript. These results reveal a two-step, canonical RdRP-independent mechanism for persistent RNAi memory in a regenerating animal and establish planarians as a model for RNA-based inheritance, illuminating a novel mode by which RNA can encode durable gene-silencing states in metazoans.

developmental biology↗

bioRxiv2025

Stepwise emergence of recombination suppression precedes fissiparous asexuality in the planarian Schmidtea mediterranea

A central paradox in evolutionary biology is the rarity of asexual reproduction, which is often attributed to developmental constraints and long-term costs. Yet, fissiparous asexuality--where animals split and regenerate--is widespread among planarians, hinting at genomic features predisposing them to asexuality. We investigated the genomic underpinnings and evolutionary consequences of asexuality in the planarian Schmidtea mediterranea, which exists as both obligately fissiparous and sexual strains. We generated a haplotype-phased genome assembly of the asexual strain and collected population genomic data from laboratory and wild populations to uncover extensive heterozygous chromosomal rearrangements affecting all chromosomes. We show that these rearrangements arose in a sexually reproducing ancestor without directly disrupting reproductive genes but instead progressively suppressing recombination across the genome. The asexual genome exhibits minimal deleterious mutation accumulation, indicating a low cost of asexuality. Population-genomic data show this strain lacks any detectable sexual reproduction and originated recently (0.17-0.4 Ma), but the young age is insufficient to explain the low mutational burden. Instead, planarians may be able to exploit the lack of a single-cell bottleneck in fissiparous reproduction to mitigate the costs of asexuality. Altogether, our results support a model in which stepwise recombination suppression due to structural rearrangements eroded the benefits of sex and enabled the emergence of fissiparous asexuality in S. mediterranea.

evolutionary biology↗

bioRxiv2025

An integrative taxonomic approach reveals unexplored diversity in Croatian planarians

BackgroundFreshwater ecosystems are among the most endangered habitats on Earth, with approximately one-fourth of aquatic species at risk of extinction. Effective conservation efforts require comprehensive monitoring and accurate species identification, including often overlooked groups. DNA barcoding promises rapid and accessible species identification but requires the availability of "universal" primer pairs and robust, taxonomically curated reference libraries. Planarian flatworms are one such group for which these resources are currently lacking, even though they are common constituents of freshwater ecosystems worldwide. As a result, the true extent of planarian diversity remains underappreciated in many areas. ResultsMotivated by the highly skewed representation of planarian diversity in current GenBank records of the barcoding gene mitochondrial cytochrome c oxidase subunit I (COI), we optimised existing barcoding primers and tested them on planarians collected in a species-rich but, so far, poorly surveyed region of Croatia. Using an integrative approach combining the new primers, traditional taxonomy and karyology, we generated new taxonomically curated COI barcode sequences for European dugesiids, dendrocoelids and planariids, which, in the case of the latter two groups, represent a substantial increase of the number of entries for those families. Our efforts resulted in several significant findings, including the description of a new pigmented Dendrocoelum species, Dendrocoelum pigmentatum sp. nov, the discovery of two highly differentiated haplotypic clades in Schmidtea lugubris, and the rediscovery of Polycladodes alba in Croatia after a century. ConclusionsOverall, our work extends the utility of DNA barcoding for species-level identification of planarians to previously inaccessible groups. Additionally, it integrates Croatia as an underexplored but species-rich region into the endeavour to systematically describe the planarian fauna of Europe. The expansion of a known number of Croatian planarian species from eight to sixteen and the discovery of a new, large planarian species in continental Europe, Dendrocoelum pigmentatum, demonstrate the effectiveness of our integrative approach. Overall, our work highlights the underappreciated diversity of planarians, even in continental Europe and supports practical conservation efforts to preserve aquatic biodiversity.

zoology↗

bioRxiv2024

A comparative analysis of planarian regeneration specificity reveals tissue polarity contributions of the axial cWnt signalling gradient.

Planarians exhibit remarkable whole-body regeneration abilities. The formation of heads at forward-facing wounds and tails at rearward-facing wounds suggests an intrinsic tissue polarity guiding regeneration. While the underlying mechanisms remain unclear, reports of double-headed regenerates from increasingly narrow tissue fragments have long been hypothesised to reflect gradient-based polarity specification. Here, we systematically re-examine this hypothesis in the modern model species Schmidtea mediterranea and a representative of the genus likely used in the original studies, Girardia sinensis. While we never observed double-heads in S. mediterranea, G. sinensis readily regenerated double-heads in a manner dependent on piece length, anatomical position and body size. We found that the reduced regeneration robustness of G. sinensis was partially explained by wound site-symmetric expression of the head determinant notum, which is highly anterior-specific in S. mediterranea. Exploring what else might mediate head/tail regeneration specificity in G. sinensis, we examined the role of the conserved tail-to-head cWnt signalling gradient. By developing a time-resolved pharmacological approach to reduce the cWnt gradient slope without affecting wound-induced cWnt signalling dynamics, we observed an increased incidence of double-headed regenerates. In addition, the body size-dependence of double-head formation correlated with the decreasing steepness of the cWnt gradient due to scaling. Taken together, our results indicate that the slope of the cWnt gradient may contribute to planarian head/tail regeneration specificity. Furthermore, they suggest that planarian tissue polarity is composed of multiple parallely-acting polarity cues, the differential reliance on which contributes to the observed interspecies variation in regeneration specificity.

developmental biology↗

bioRxiv2024

Regeneration in the absence of canonical neoblasts in an early branching flatworm

The remarkable regenerative abilities of flatworms are closely linked to neoblasts - adult pluripotent stem cells that are the only division-competent cell type outside of the reproductive system. Although the presence of neoblast-like cells and whole-body regeneration in other animals has led to the idea that these features may represent the ancestral metazoan state, the evolutionary origin of both remains unclear. Here we show that the catenulid Stenostomum brevipharyngium, a member of the earliest-branching flatworm lineage, lacks conventional neoblasts despite being capable of whole-body regeneration and asexual reproduction. Using a combination of single-nuclei transcriptomics, in situ gene expression analysis, and functional experiments, we find that cell divisions are not restricted to a single cell type and are associated with multiple fully differentiated somatic tissues. Furthermore, the cohort of germline multipotency genes, which are considered canonical neoblast markers, are not expressed in dividing cells, but in the germline instead, and we experimentally show that they are neither necessary for proliferation nor regeneration. Overall, our results challenge the notion that canonical neoblasts are necessary for flatworm regeneration and open up the possibility that neoblast-like cells may have evolved convergently in different animals, independent of their regenerative capacity.

zoology↗

bioRxiv2023

A comparative analysis of planarian genomes reveals regulatory conservation in the face of rapid structural divergence

The planarian Schmidtea mediterranea can regenerate its entire body from small tissue fragments and is studied as regeneration model species. The assembly and functional analysis of planarian genomes has proven challenging due its high A/T content (70% A/T), repetitive nature, and limited transferability of routine laboratory protocols due to their divergent biochemistry. Only few and often fragmented genome assemblies are currently available, and open challenges include the provision of well-annotated chromosome-scale reference assemblies of the model species and other planarians for a comparative genome evolution perspective. Here we report a haplotype-phased, chromosome-scale genome assembly and high-quality gene annotations of the sexual S2 strain of S. mediterranea and provide putative regulatory region annotations via optimized ATAC-seq and ChIP-seq protocols. To additionally leverage sequence conservation for regulatory element annotations, we generated chromosome-scale genome assemblies and chromatin accessibility data for the three closest relatives of S. mediterranea: S. polychroa, S. nova, and S. lugubris. We find substantial divergence in protein-coding sequences and regulatory regions, yet reveal remarkable conservation in ChIP-mark bearing open chromatin regions identified as promoters and enhancers in S. mediterranea. The resulting high-confidence set of evolutionary conserved enhancers and promoters provides a valuable resource for the analysis of gene regulatory circuits and their evolution within the taxon. In addition, our four chromosome-scale genome assemblies provide a first comparative perspective on planarian genome evolution. Our analyses reveal frequent retrotransposon-associated chromosomal inversions and inter-chromosomal translocations that lead to a degradation of synteny across the genus. Interestingly, we further find independent and near-complete losses of the ancestral metazoan synteny across Schmidtea and two other flatworm groups, indicating that platyhelminth genomes largely evolve without syntenic constraints. Our work provides valuable genome resources for the planarian research community and sets a foundation for the comparative genomics of planarians. We reveal a contrast between the fast structural evolution of planarian genomes and the conservation of their regulatory elements, suggesting a unique genome evolution in flatworms where gene positioning may not be essential.

genomics↗

bioRxiv2023

Epidermal turnover in the planarian Schmidtea mediterranea involves basal cell extrusion and intestinal digestion.

Planarian flatworms undergo continuous internal turnover, wherein old cells are replaced by the division progeny of adult pluripotent stem cells known as neoblasts. How dynamic cell turnover is executed at the organismal scale remains an intriguing question in planarians and biological systems in general. While previous studies have predominantly focused on neoblast proliferation, little is known about the processes that mediate cell loss during tissue homeostasis. Here, we use the planarian epidermis as a model to study the mechanisms of cell removal in Schmidtea mediterranea. We established a covalent dye-labeling assay and image analysis pipeline to quantify the cell turnover rate in the planarian epidermis. Our findings indicate that the ventral epidermis is highly dynamic, with a half-life of the constituent cells of approximately 4.5 days. Using live-imaging and pulse-chase assays, we find that epidermal cells undergo internalization via basal extrusion, followed by a migration towards the intestine and ultimately digestion by intestinal phagocytes. Overall, our studies reveal an intricate homeostatic cell clearance process that may reduce the metabolic costs of high turnover tissues in planarians.

developmental biology↗

bioRxiv2022

Probing the evolutionary dynamics of whole-body regeneration within planarian flatworms

Why some animals can regenerate while many others cannot remains a fascinating question. Even amongst planarian flatworms, well-known for their ability to regenerate complete animals from small body fragments, species exist that have restricted regeneration abilities or are entirely regeneration incompetent. Towards the goal of probing the evolutionary dynamics of regeneration, we have assembled a diverse live collection of planarian species from around the world. The combined quantification of species-specific head regeneration abilities and comprehensive transcriptome-based phylogeny reconstructions reveals multiple independent transitions between robust whole-body regeneration and restricted regeneration in the freshwater species. Our demonstration that the RNAi-mediated inhibition of canonical Wnt signalling can nevertheless bypass all experimentally tractable head regeneration defects in the current collection indicates that the pathway may represent a hot spot in the evolution of planarian regeneration defects. Combined with our finding that Wnt signalling has multiple roles in the reproductive system of the model species S. mediterranea, this raises the possibility of a trade-off between egg-laying and asexual reproduction by fission/regeneration as a driver of regenerative trait evolution. Although initial quantitative comparisons of Wnt signalling levels, reproductive investment, and regenerative abilities across the collection confirm some of the models predictions, they also highlight the diversification of molecular mechanisms amongst the divergent planarian lineages. Overall, our study establishes a framework for the mechanistic evolution of regenerative abilities and planarians as model taxon for comparative regeneration research.

evolutionary biology↗

bioRxiv2022

Content-aware image restoration improves spatiotemporal resolution in luminescence microscopy.

Luminescence microscopy is a powerful tool in biomedical imaging applications due to its intrinsically high signal to noise ratio. However, luminescence signal detection requires longer exposure times than fluorescence imaging and is consequently less suited for applications requiring high temporal resolution or throughput. Here we demonstrate that content-aware image restoration can drastically reduce the exposure time requirements in luminescence imaging, thus overcoming one of the major limitations of the technique.

cell biology↗

bioRxiv2022

Long-range morphogen gradient formation by cell-to-cell signal propagation

Morphogen gradients are a central concept in developmental biology. Their formation often involves the secretion of morphogens from a local source, that spread by diffusion in the cell field, where molecules eventually get degraded. This implies limits to both the time and length scales over which morphogen gradients can form which are set by diffusion coefficients and degradation rates. Towards the goal of identifying plausible mechanisms capable of extending the gradient range, we here use theory to explore properties of a cell-to-cell signaling relay. Inspired by the millimeter-scale Wnt-expression and signaling gradients in flatworms, we consider morphogen-mediated morphogen production in the cell field. We show that such a relay can generate stable morphogen and signaling gradients that emanate from a source of morphogen at a boundary. This gradient formation can be related to an effective diffusion and an effective degradation that result from morphogen production due to signaling relay. If the secretion of morphogen produced in response to the relay is polarized, it further gives rise to an effective drift. We find that signaling relay can generate long-ranged gradients in relevant times without relying on extreme choices of diffusion coefficients or degradation rates, thus exceeding the limits set by physiological diffusion coefficients and degradation rates. A signaling relay is hence an attractive principle to conceptualize long-ranged gradient formation by slowly diffusing morphogens that are relevant for patterning in adult contexts such as regeneration and tissue turn-over.

biophysics↗

bioRxiv2021

Heterologous reporter expression in the planarian Schmidtea mediterranea through somatic mRNA transfection

Planarians have long been studied for their regenerative abilities. Moving forward, tools for ectopic expression of non-native proteins will be of substantial value. Using a luminescent reporter to overcome the strong autofluorescence background of planarian tissues, we demonstrate heterologous protein expression in planarian cells and live animals. Our approach is based on the introduction of mRNA through several nanotechnological and chemical transfection methods. We improve reporter expression by altering untranslated region (UTR) sequences and codon bias, facilitating measurement of expression kinetics both in isolated cells and in whole planarians using luminescence imaging. We also examine protein expression as a function of variations in the UTRs of delivered mRNA, demonstrating a framework to investigate gene regulation at the post-transcriptional level. Together, these advances expand the toolbox for the mechanistic analysis of planarian biology and establish a strong foundation for the development and expansion of transgenic techniques in this unique model system. MotivationThe study of planarians has contributed to advances in our understanding of regeneration, stem cell dynamics, and many other fundamental biological processes. However, the persistent challenge of expressing transgenes in planarians has led to the speculation that they may be resistant to transfection. In this work, we develop methods to express exogenous mRNAs in both isolated planarian cells and whole animals by optimizing delivery techniques, genetic constructs, and detection methods. These methods allow us to study transfection kinetics and post-transcriptional regulation of gene expression in a quantitative manner. Beyond planarian research, this work should also provide a broadly applicable strategy to develop similar tools for animals that are also challenging to modify genetically.

bioengineering↗

bioRxiv2018

Body size-dependent energy storage causes Kleibers law scaling of the metabolic rate in planarians

Kleibers law, or the [3/4]-power law scaling of the metabolic rate with body mass, is considered one of the few quantitative laws in biology, yet its physiological basis remains unknown. Here, we report Kleibers law scaling in the planarian Schmidtea mediterranea. Its reversible and life history-independent changes in adult body size over 2 orders of magnitude reveal that Kleibers law does not emerge from the size-dependent decrease in cellular metabolic rate, but from a size-dependent increase in mass per cell. Through a combination of experiment and theoretical analysis of the organismal energy balance, we further show that the mass allometry is caused by body size dependent energy storage. Our results reveal the physiological origins of Kleibers law in planarians and thus have general implications for understanding a fundamental scaling law in biology.

biophysics↗

bioRxiv2018

Multi-scale coordination of planar cell polarity in planarians

Polarity is a universal design principle of biological systems that manifests at all organizational scales. Although well understood at the cellular level, the mechanisms that coordinate polarity at the tissue or organismal scale remain poorly understood. Here, we make use of the extreme body plan plasticity of planarian flatworms to probe the multi-scale coordination of polarity. Quantitative analysis of ciliary rootlet orientation in the epidermis reveals a global polarization field with head and tail as independent mediators of anteroposterior (A/P) polarization and the body margin influencing mediolateral (M/L) polarization. Mathematical modeling demonstrates that superposition of separate A/P- and M/L-fields can explain the global polarity field and we identify the core planar cell polarity (PCP) and Ft/Ds pathways as their specific mediators. Overall, our study establishes a mechanistic framework for the multi-scale coordination of planar polarity in planarians and establishes the core PCP and Ft/Ds pathways as evolutionarily conserved 2D-polarization module.

cell biology↗