From Hydra to rotifer and beyond: implications for human aging and delayed senescence

Hypothesis

Hydra and rotifers can therefore be considered as representatives of the opposite ends of the invertebrate lifecycle and aging spectrum, with the negligibly senescent, continuous replicating Hydra at one end and the short-lived, rapidly aging, and eutelic rotifers at the other end. Both Hydra and rotifers are tractable for genetic manipulation via exogenous gene expression including overexpression and genetic knockdown [1, 9–11, 13–15, 20]. Therefore, these two organisms represent an opportunity to test the hypothesis that Hydra-like patterns of gene expression, including but not limited to levels of FoxO, can progressively repress senescence in aging animal models.

We hypothesize that delayed senescence at the organismal level is possible through recapitulation of Hydra-like patterns of gene expression in rotifers, and that data obtained may help generate hypotheses for somatic interventions and prioritize pathways for mammalian validation in future studies.

Hydra-like gene expression patterns refer to the molecular signature that supports continuous stem cell self-renewal, multipotency, and negligible senescence in Hydra vulgaris [1, 10]. This includes: (a) sustained high activity of the transcription factor FoxO, which maintains stem cell proliferation and suppresses terminal differentiation; (b) upregulation of stemness genes (e.g., orthologs of piwi, nanos, vasa-like, and other interstitial/stem cell markers) in stem and progenitor populations; (c) transcriptomic profiles enriched in pathways for DNA repair, proteostasis, mitochondrial function, and stress resistance, with minimal expression of aging-associated genes (e.g., those involved in proteotoxic stress or inflammation). These patterns could be assessed quantitatively via RNA-seq, comparing pre- and post-perturbation rotifer transcriptomes to Hydra stem/progenitor cell clusters [10]. “More Hydra-like” could be defined operationally as achieving ≥0.7 Pearson correlation with Hydra stem cell states or significant upregulation of conserved stemness markers without major fitness costs.

The rotifer's eutelic adult soma (fixed post-hatching cell number with minimal somatic proliferation) fundamentally limits the potential for stem cell compartment expansion or progenitor replenishment [13, 20]. Therefore, the primary mechanism by which Hydra-like gene expression is expected to delay rotifer senescence is through enhanced resilience, proteostasis, stress resistance, and damage repair in existing post-mitotic somatic cells, rather than renewed tissue turnover. Candidate proliferative populations in rotifers are limited to germline-associated cells or rare, tissue-specific progenitors (if any); somatic stem cell activity is not a feature of the adult. Accordingly, success will be measured by functional readouts such as extended post-mitotic cell lifespan, improved mitochondrial function, reduced proteotoxic stress, and delayed aging phenotypes (e.g., reproductive decline, locomotion impairment), rather than increased cell numbers or proliferation markers. Attempts to override eutely (e.g., via forced expression of proliferation regulators) would likely disrupt developmental constraints, leading to malformations, reduced fitness, or tumor-like overgrowth, and will be avoided or tightly controlled in initial experiments. Thus, if increased cell numbers or proliferation markers do occur, this would likely be associated with the aforementioned negative outcomes that will be subject to careful monitoring.

The hypothesis presented here builds on existing data from single-cell RNA-seq in Hydra [10] and aging transcriptomics in Brachionus manjavacas [11]. Recapitulating such patterns for lifespan extension has already been conceptually discussed in reviews [1, 21, 22]. However, the bidirectional approach (Hydra to rotifer overexpression/knockdown) and the reverse (imposing rotifer aging signatures on Hydra) is presented here as a novel integration for iterative refinement. It is not merely a logical extension of prior FoxO-focused Hydra work or of general longevity pathway reviews. The current proposal systematically tests causality in both directions, moving from the extreme of negligible senescence and continuous stem cell renewal (Hydra) to the opposite extreme of short lifespan, eutelic soma, and rapid aging (rotifer), and then reverses the flow (rotifer to Hydra). Thus, this strategy allows for a more rigorous evaluation of which Hydra-like expression patterns are truly causal for delaying senescence vs. which are context-specific or are “bystander” or “passenger” features.

No previous proposal has systematically combined these opposite lifecycle endpoints in an iterative, bidirectional framework to refine targets and validate mechanisms. This dual-direction testing adds a layer of causal inference that single-direction studies (e.g., Hydra-to-other models only) or purely correlative reviews do not provide. It allows for clearer discrimination between necessary vs. sufficient elements of the Hydra genetic program and can in theory reduce the risk of transferring non-causal or maladaptive features into aging-prone organisms. The bidirectional design is therefore a distinctive strength of the current hypothesis; it outlines a structured strategy for progressive iterative refinement that has not been explicitly discussed in previous work on Hydra-inspired longevity pathways.

Evaluating the hypothesis

The basic experimental approach is to recapitulate elements of relevant Hydra gene expression programs in rotifers. Genetic manipulation, in an iterative fashion, can be utilized to progressively modify Brachionus manjavacas. Subsequently, more complex organisms (Daphnia to mice, including the MRL mouse) would be utilized, with the aim of repressing aging at the mammalian level. Data obtained from these studies can be used to evaluate geroprotective approaches to repress aging in humans.

Thus, the primary strategy (Figure 1) is to modulate endogenous rotifer orthologs (e.g., FoxO and stemness-related genes) to mimic Hydra expression levels and dynamics via CRISPR-based overexpression or knockdown. Limited heterologous expression of select Hydra genes (e.g., FoxO or piwi orthologs) may be tested in parallel where ortholog function is uncertain. Cross-species differences in cell types and developmental programs will be addressed by focusing on conserved functional modules (e.g., FoxO-mediated stemness) rather than direct cell-type matching. Given the rotifer's eutelic adult soma, success is expected to manifest primarily as enhanced resilience, proteostasis, and stress resistance in existing cells rather than new stem cell compartment expansion. Any attempt to override eutely will be monitored for developmental defects or fitness loss.

Transgenic rotifers exhibiting more Hydra-like patterns of gene expression (e.g., enhanced FoxO activity and stemness marker upregulation in existing post-mitotic cells) will be evaluated for changes in age-related mortality/senescence, health, and function, with emphasis on post-mitotic cell maintenance (e.g., proteostasis, mitochondrial integrity, stress resistance assays) rather than proliferation. Proliferation markers (e.g., BrdU incorporation, phospho-histone H3) will be monitored primarily to detect unintended overgrowth or neoplastic risk rather than to confirm stem cell expansion. Given eutely, any measurable delay in senescence is expected to arise from improved damage resistance/repair in the fixed somatic cell population, with germline effects potentially contributing to transgenerational healthspan benefits.

More specifically, the testing design could be as follows. The proposed experimental approach is iterative and begins with Brachionus manjavacas as the primary target. Rotifers will be subjected to CRISPR-mediated overexpression or knockdown of rotifer orthologs of key Hydra genes (primarily FoxO and 3–5 stemness markers such as piwi/nanos orthologs), with limited heterologous expression of Hydra genes where ortholog function is uncertain. Controls will include wild-type rotifers, mock-transfected animals, and negative-target manipulations (e.g., non-conserved genes). Primary endpoints for delayed senescence include: (a) lifespan and mortality hazard (survival curves, Kaplan-Meier analysis, hazard ratios); (b) fertility and reproduction (fecundity curves, lifetime offspring count, reproductive lifespan); (c) locomotion and functional decline (swimming speed, escape response assays over age); (d) stress resistance (oxidative, thermal, UV challenge survival), and (e) molecular markers (proteostasis via proteasome activity, mitochondrial function via ATP/respiration, transcriptomic aging signatures).

Neoplasia-like changes in microscopic eutelic rotifers will be detected via (a) morphological abnormalities (microscopy for tissue overgrowth, asymmetry, or cysts); (b) proliferation markers (BrdU incorporation, phospho-histone H3 immunostaining, etc.) to identify unintended cell division; and (c) transcriptomic deviation toward Hydra tumor profiles, with RNA-seq comparison to previous work [7], so that high similarity score indicates risk.

Iteration will be guided by predefined decision criteria: (a) advance to next target (e.g., from rotifer to Daphnia) if: ≥20% healthspan extension (median lifespan increase); (b) significant transcriptomic shift toward Hydra stem/progenitor states (e.g., Pearson correlation ≥0.7 with Hydra scRNA-seq clusters) [10]; (c) and no major fitness cost (fecundity decline <10%, no developmental defects).

Iterate or mitigate if: (a) no healthspan benefit, high neoplasia signal (proliferation markers >2-fold baseline), or (b) fitness costs (fecundity <80% control, locomotion decline >30%).

Additional trade-offs that can be monitored include metabolic costs (oxygen consumption, ATP levels), stress sensitivity, and developmental defects (morphological scoring at hatching).

Note that genetic information from the Hydra oligactis inducible aging phenotype [3–6] can be used in a secondary extension of evaluating the main hypothesis. This can include, e.g., genetic manipulation to “swap” aging phenotypes between Hydra vulgaris and Hydra oligactis, ascertain effects of Hydra oligactis gene expression patterns in rotifers and other organisms, as well as to explore whether the potentiality for inducible aging makes Hydra oligactis more susceptible to neoplasia. That could be follow-up work succeeding the results of the main Hydra vulgaris-based experiments outlined here.

The expected outcome of the main experimental protocols (involving Hydra vulgaris) would be decreased age-related mortality/senescence of Brachionus manjavacas, resulting in an increased lifespan. Further, organism function (e.g., locomotion, reproduction) is expected to demonstrate a prolonged “youthful” period (increased healthspan). At the cellular level, these changes would be reflected in increased post-mitotic cell maintenance as described above. At the molecular level, we expect to observe gene expression patterns consistent with earlier, more “vigorous,” stages of the rotifer lifecycle being maintained for longer periods of time. It is possible we may observe a particular emphasis on maintenance of gene expression patterns of stem cell self-renewal, coupled to continuing replenishment of progenitor cells, which may cause neoplastic overgrowth and/or developmental defects.

While not a practical objective, or an expected outcome, from a speculative standpoint the experiments could provide a definitive answer to (a) whether full suppression of senescence in Brachionus manjavacas is at all possible, and (b) if possible whether it can be achieved without radically altering the phenotype of the organism and its normal lifecycle. The answer to both questions is almost certainly “no” and this can be empirically demonstrated.

Success with this foundational model organism can be utilized to identify gene expression profiles to be used to progressively repress senescence and enhance healthspan in ever more complex organisms, starting with the genetically tractable Daphnia, another invertebrate model of aging [16, 23]. The studies could then further extend to mouse models, including the more rapidly aging MRL mouse [17–19], with the ultimate objective of repressing aging at the mammalian level. Various other mouse models of aging [19] would no doubt be useful targets once the experimental strategy reaches the mammalian level of genetic modification. We would expect similar age-repressing effects in these model organisms as with the rotifer.

Of course, the opposite strategy can also be employed; gene expression patterns reflecting aging of Brachionus manjavacas can be recapitulated into Hydra to ascertain whether age-related increased mortality/senescence is thus induced. Findings from these reverse experiments can identify more genetic targets for modification required for the main approach for suppression of senescence. This iterative bidirectional process can in theory be utilized as the experimental strategy moves up through increasing organism complexity. Epigenetic alterations of interest [24] can also be analyzed.

Considering evolutionary trade-offs

Considering evolutionary trade-offs, the model of Nelson and Masel [25] posits that multicellular aging is inevitable due to three key tenets: (1) inevitable cellular degradation over time, (2) long-term uncoupling of organismal vigor from developmental outcomes (e.g., body size), and (3) lack of selective coordination between cellular vigor and cooperation, leading to intercellular competition where “cheater' cells” that evade cooperation gain selective advantage and drive neoplasia. In Hydra, constant stem cell renewal appears to maintain positive correlation between vigor and cooperation, enabling negative senescence [26]. However, even in Hydra (Hydra oligactis and Pelmatohydra robusta), spontaneous tumors arise when this coordination is disrupted [7]. Differences in Hydra species as regards, e.g., inducible aging of Hydra oligactis [3–6] could be useful for interpreting evolutionary trade-offs involving senescence vs. neoplasia.

However, Hydra vulgaris is multicellular and seemingly lacking in senescence. One “out” to explain this phenotype in terms of intercellular competition is that a positive correlation between cellular vigor and cellular cooperation is possible if there is a continuous supply of “nondegraded” cells (i.e., those derived from stem cells) that can compete against both nonfunctional and neoplastic cells, concomitantly selecting for function and cooperation [26]. It is believed that this is an unrealistic scenario since stem cells are expected to also exhibit degradation and decreased function over time [26].

However, since Hydra vulgaris apparently can evade this fate, the importance of this organism for senescence research is underscored. This brings up the question of neoplasia as a trade-off for suppressed senescence: does Hydra exhibit sufficient positive correlation between cellular vigor and cell cooperation to evade neoplasia, or does neoplasia in this organism exist with implications for a strategy of enforced genetic suppression of senescence? As has been noted, certain Hydra species can exhibit neoplasia and develop transplantable tumors that exhibit altered gene expression, similar to vertebrate neoplasms [7]. These Hydra tumors reduce the fitness of the animals but typically do not kill nor completely prevent reproduction [7]. Further, the tumors seem to derive from differentiation-arrested female gametes, and while they do not exhibit increased proliferative potential, they do exhibit resistance to apoptosis, enhanced migration, and autonomy from the cellular microenvironment [7]. Thus, Hydra neoplasia displays some similarities to that of vertebrates. On the other hand, Hydra tumors are not universal, nor do they necessarily lead to mortality. Thus, consequences for an suppression of senescence program as outlined in Figure 1 would be the possibility of neoplasia, which would have to be carefully monitored as the anti-senescence program progresses, particularly with higher organisms (Figure 2).

It should be noted that senescence in mammals plays a dual role: it acts as a tumor-suppressive barrier early in life by arresting damaged cells and preventing neoplastic progression, but in later life the accumulation of senescent cells promotes chronic inflammation through the SASP and contributes to age-related tissue dysfunction, immune dysregulation, and metabolic decline. The proposed Hydra-like renewal strategy aims to bypass this arrest-based senescence altogether, potentially avoiding SASP-driven late-life deterioration while still requiring careful monitoring for dysregulation-induced neoplasia.

To test the Nelson and Masel tenets in the rotifer system, we propose the following measurements: (a) cellular degradation to assess proteostasis (proteasome activity, aggregate clearance), mitochondrial function (ATP/respiration), and oxidative damage markers over time in wild-type vs. manipulated rotifers; (b) vigor-cooperation correlation in which the experimenter will monitor fitness proxies (fecundity, locomotion) alongside cellular cooperation markers (e.g., cell-cell adhesion gene expression, apoptosis rates in damaged cells); (c) intercellular competition and cheater emergence, for which the approach would be to detect neoplasia-like overgrowth via proliferation markers (BrdU incorporation, phospho-histone H3) and transcriptomic deviation toward Hydra tumor profiles, keeping in mind Hydra species differences [7].

Cheater dynamics will be inferred from disproportionate expansion of any proliferating subpopulations (if induced), or fitness decline despite molecular renewal signatures. Further, there would also be an examination of (d) negative senescence potential, in which the experimenter would quantify whether manipulated rotifers show flat or declining mortality hazard (survival curves) compared to controls, alongside reduced senescence hallmarks (e.g., lower inflammatory gene expression or SASP-like signatures). These readouts will be integrated into the iterative experimental framework (Figure 1), with iteration decisions guided by whether the manipulations shift the balance toward vigor-cooperation correlation (e.g., extended healthspan without increased neoplasia) or toward competition-driven dysfunction (e.g., tumor-like overgrowth or fitness cost). This approach allows direct testing of the model's tenets in the rotifer, a simple, eutelic metazoan.

Sustained FoxO activity or stemness promotion in a eutelic organism like the rotifer could promote aberrant proliferation rather than the desired result of benign coordinated renewal, potentially resulting in neoplasia-like changes analogous to those observed in Hydra [7]. In mammals, forced maintenance of stem cell compartments frequently elevates tumorigenesis risk, particularly when p53 function is compromised and FoxO-p53 crosstalk is disrupted [27]. Studies in model systems have provided quantitative measures of the relevant trade-offs. Thus, FoxO activation in Drosophila extends lifespan by ~20–30% but increases cancer-like phenotypes in intestinal stem cells [28]. In mice, inducible FOXO3a overexpression extends median and maximum lifespan by ~30% through enhanced DNA repair and radiation resistance [29], whereas skeletal muscle-specific FOXO1 overexpression yields no lifespan extension and is associated with atrophy and proteostatic dysregulation [30]. These mixed results underscore that potential delayed senescence and/or healthspan gains are context-dependent and often accompanied by risks. Further, the associated risks can include therapy resistance or metastasis promotion in cancer models in which high FOXO expression offsets its suppressive effects [31]. For our proposed bidirectional experiments (e.g., enforcing rotifer-like aging gene expression patterns on Hydra), we predict outcomes such as a 20–50% increase in age-related mortality based on FoxO knockdown data [1]. To mitigate the aforementioned risks, we can, e.g., propose reasonable safeguards including inducible expression systems (e.g., Tet-On for reversible activation) and co-expression of tumor suppressors (e.g., p53 orthologs). Thus, if neoplasia is identified, genetic modulation to suppress neoplasia and enhance intercellular cooperation will be attempted.

Data obtained from other invertebrate model organisms, such as C. elegans and Drosophila, can be useful, but most promising are more complex animal models that demonstrate both reduced senescence as well as resistance to neoplasia, such as the naked mole rat [32–34]. Such organisms can assist in the search for strategies to repress neoplasia that may be selected for and promoted via suppression of senescence, particularly in more complex animal models. Using naked mole rat genetics in mouse models as an approach to enhance murine resistance to aging and cancer, as has been proposed [33], and demonstrated with considerable success [34], is an example of this strategy.

Full “organismal physiological immortality,” defined as a lack of senescence and no increase in age-related mortality rates, in bilaterians is not possible due to factors like tissue complexity and evolutionary trade-offs, including resource allocation [35] and neoplasia. Thus, while Hydra achieves negligible senescence via continuous stem cell renewal, full recapitulation in complex bilaterians is biologically implausible. Instead, we can more reasonably expect delayed senescence and/or extended healthspan rather than true non-senescence. Ultimately, in complex animal models, the optimal practical outcome would manifest as relatively negligible senescence (as seen in naked mole-rats) rather than absolute non-senescence [36].

Technical challenges and limitations

While rotifers such as Brachionus manjavacas are tractable for genetic manipulation [20], the proposed iterative manipulation of Hydra-like gene patterns faces substantial biological and technical hurdles.

A primary limitation stems from the eutelic nature of rotifers: adult somatic cell number is fixed post-hatching, with minimal or no post-developmental somatic proliferation [13]. These characteristics contrast sharply with the continuously cycling stem cell compartments of Hydra and therefore restrict the potential for expansion of stem cell-like activity or progenitor cell replenishment as initially envisioned in an earlier version of the experimental plan. Forced promotion of stemness programs (e.g., via FoxO overexpression) may thus manifest primarily as enhanced resilience, proteostasis, or stress resistance in existing post-mitotic cells rather than renewed tissue turnover. Moreover, attempts to significantly alter the eutelic lifecycle, whether through ectopic activation of proliferation pathways or forced stem-like behavior, could disrupt body plan integrity, reproduction, or locomotion. That could result in developmental malformations, reduced fitness, or even lethality in extreme cases. In practice, therefore, the most realistic positive outcome of introducing Hydra-like patterns would be improved functional maintenance of the fixed somatic cell population rather than creation of a renewable stem cell pool. However, in the event that a renewable stem cell population unexpectedly develops, the expected negative consequences of neoplasia and/or developmental defects will be carefully monitored and mitigated (Figure 2).

Stable, heritable, multi-gene manipulations pose further technical challenges. Although CRISPR delivery via microinjection or electroporation is feasible [20], achieving inducible, tissue-specific, or precisely dosage-controlled expression across generations remains technically demanding. Risks include off-target effects, epigenetic silencing of transgenes, variable penetrance, and dosage toxicity from overexpression. These issues are common across genetic manipulation studies in many model organisms but are particularly problematic in a microscopic, eutelic species with limited cell numbers and a rapid life cycle.

Pleiotropic consequences are likely and may complicate desired phenotypic outcomes. In Hydra, FoxO regulates not only stemness but also antimicrobial peptide production and microbiome composition [1, 37]; analogous effects in rotifers could alter microbial interactions that are critical for health and aging, offsetting any gains in cellular maintenance. In parthenogenetic Daphnia, tissue complexity increases the likelihood of off-target effects [23]. Extending these objectives to more complex animal models could amplify these issues. In mice (including MRL strains), viral or transgenic delivery faces immune rejection, epigenetic instability, and heightened potential for neoplasia arising from sustained stemness promotion. Prior FoxO manipulations in mammals and flies demonstrate predominantly mixed outcomes: overexpression can yield no lifespan extension, accelerated aging phenotypes, increased thermal stress sensitivity, or proteostatic deregulation in a dose-dependent manner [30, 38, 39]. Some conditional mouse models show modest healthspan benefits (e.g., ~30% lifespan extension via inducible FOXO3a) [29].

Without pilot data or highly constrained initial targets, the proposed approach remains speculative. Inducible systems and careful monitoring of reproduction, behavior, neoplasia proxies, and other fitness parameters will be essential to (a) mitigate these limitations and (b) refine the strategy iteratively. Note that the iterative nature of the experimental plan is a fundamental aspect of the proposal.

It must also be noted that the hypothesis and the proposed experiments, although highly speculative, are introduced in part for their epistemological value: to provoke discussion and debate within the scientific community on these fundamental issues, perhaps stimulating more optimal or alternative approaches. In this sense, the development of the hypothesis and its testing can itself be approached in an iterative fashion by the broader research field.