Abstract
Hydra vulgaris is considered an effectively immortal freshwater cnidarian. As a sister group to all Bilateria, studies of cnidarian genomes offer valuable evolutionary insight into a range of genetic mechanisms. Over 280 years of interdisciplinary study have paved the way for the use of Hydra as a model organism; however, the development of transgenic methods has been inhibited by their facultative reproduction. Hydra default to asexual reproduction in a lab setting, and despite being understood as hermaphroditic, the conditions to induce egg production were famously cryptic. We overcame this limitation by selecting for and isolating polyps capable of producing eggs and fostering those individuals into a large “KSO” colony, and designing a starvation care protocol to rapidly induce gametogenesis. To validate that this approach led to increased egg production, we set up five replicate populations (100 polyps each) of KSO and a control AEP group and then conducted a 4-week-long starvation trial. We found overwhelming evidence (−1.023 median difference, 95% credible interval: −1.580 to −0.466) that selection for “egg-bearing” as a trait resulted in significant sexual bias toward egg production in a population, supporting previous claims of distinct stable sexes. Overall, the combined effect of starvation and selection resulted in a 5-fold increase in egg production over the control AEP. Our study not only strengthens our understanding of H. vulgaris sexual reproduction but also provides a repeatable, effective approach for establishing stable egg-producing populations for the advancement of transgenic methodology.
Introduction
The freshwater hydrozoan genus Hydra comprises model organisms for studies of stem cells, neurosignaling, senescence, and eumetazoan evolution over the last 280 years making them an accessible and well-studied group for developing genomic research (Galliot, 2012). As a member of the phylum Cnidaria, an early branching sister group to Bilateria, Hydra’s phylogenetic position offers valuable insight on the evolution of biological processes. Historically, genetic manipulation in Hydra was constrained by methodological limitations, relying primarily on chemical or inbreeding induced mutations and electroporation (Bosch et al., 2002; Marcum and Campbell, 1978). A pivotal advance came in 2006 with the development of transgenesis via embryonic microinjection creating a robust foundation for functional genetic studies (Juliano et al., 2014; Wittlieb et al., 2006).
Developing gene perturbation methodologies frequently cite the bottleneck limiting advancements in the field is the inefficient generation of stable transgenic lines. As is the case with other model organisms, in order for a change to be heritable, it must be present in the germ cells. In Hydra, this requires integration of the transgene into interstitial stem cells (i-cells) from which the germline differentiates (Nishimiya-Fujisawa and Kobayashi, 2018). Hydra embryonic microinjection protocols report that approximately 3% of injected embryos produce stable i-cell-integrated polyps. In contrast, a similar protocol using the model organism Caenorhabditis elegans cited successful germline integration rates of approximately 8.3% (Klimovich et al., 2019; Pan et al., 2024; Wittlieb et al., 2006). When it comes to brood size, one C. elegans nematode can consistently produce over 1,000 offspring within 72 h, and this efficiency stands in stark contrast to microinjection protocols reporting a mass culture of over 4,000 Hydra polyps only bearing 10–12 zygotes per day after 6 weeks (Corsi et al., 2015; Klimovich et al., 2019). This supports the claim from multiple studies that the greatest limiting factor in this field is the acquisition of large quantities of Hydra embryos (Juliano et al., 2014; Klimovich et al., 2019).
Hydra are a facultatively sexual freshwater Cnidaria with a simple body plan, capable of both sexual and asexual reproduction (Figure 1A). In a lab environment, Hydra generally exhibit preference for budding, the organism’s method of asexual reproduction, and sexual reproduction can be induced through exposure to environmental stimuli (Reisa, 1973). Sexual reproduction in Hydra involves polyps undergoing oogenesis to develop an egg or spermatogenesis to develop testis (Figures 1C, D). However, due to the ancient origins, expansive distribution, and broad ecological diversity of Hydra, there is unfortunately no known sexual inducer applicable to the entire genus. Some species like H. oligactis are known to respond to cooling, while others, like H. littoralis, respond to warming. However, these stimuli are not always consistent in a lab setting, and some studies provide conflicting results, potentially due to misclassification of species (Kaliszewicz and Lipińska, 2013; Littlefield, 1994; Martínez et al., 2010; Schwentner and Bosch, 2015; Steele et al., 2019; Sugiyama and Fujisawa, 1978; Tökölyi et al., 2021; Reisa, 1973). Despite the ambiguity, one consensus remains that the mechanisms of sex in Hydra, both induction and production, typically involve sickness, stress, or metabolic crisis (Reisa, 1973).
Figure 1
The lab strain Hydra vulgaris (H. vulgaris) AEP has been the predominant model for transgenesis, owing to its relatively higher, though still limited, propensity for producing gametes in a lab setting (Juliano et al., 2014; Wittlieb et al., 2006). Previous studies reported that feeding changes may result in an increase in gametogenesis; however, previously published protocols only investigated the effect of subtle dietary changes resulting in minute egg production over extended periods of time (Klimovich et al., 2019). In this study, we aimed to optimize this process by designing a starvation care routine that induces sexual reproduction more rapidly and predictably, resulting in a significantly greater quantity of eggs for acquisition without creating lethal conditions for the population.
Though H. vulgaris AEP circumnavigates some barriers to inducing sexual reproduction, core misunderstandings of sexual identities still limit opportunities to optimize embryo collection. Hydra vulgaris has been claimed to be gonochoristic, hermaphroditic, and occasionally both (Campbell, 1989; El-Bawab, 2020; Kaliszewicz and Lipińska, 2013). However, one study criticizes the initial classification of H. vulgaris as a hermaphroditic species, suggesting that well-known evidence supporting distinct sexual identities was ignored (Campbell, 1989). A more recent study supported this critique by validating the stability of egg-forming stem cells and identifying the existence of H. vulgaris “phenotypic males” which are polyps proven capable of oogenesis but observed temporarily bearing testes (Nishimiya-Fujisawa and Kobayashi, 2012, 2018). Previous studies even support that sperm-restricted stem cells in Hydra produce an egg-suppressing molecule (ESM) that forces oogenesis into a stasis-like state (Littlefield, 1994; Nishimiya-Fujisawa and Kobayashi, 2012).
Prior to this experiment, we observed several “masculinization” events within our own lab populations of H. vulgaris AEP where upwards of 80% of polyps within a population appeared to be simultaneously undergoing spermatogenesis with no egg development. Given the proposed existence of ESM, we proposed that on a population scale, the high concentration of polyps expressing testis may result in the suppression of egg development. This would result in any polyp capable of egg production to be disguised in its “phenotypic male” or asexual state. Due to this phenomenon of transient testis formation and the infrequency of egg development, we theorized that all polyps may be capable of producing testis, but only certain individuals were capable of oogenesis. This theory would simultaneously explain previous observations of sequential hermaphroditism and suggests some form of distinct sexual identity that is not universal across the H. vulgaris AEP strain.
With fertilization being highly successful in the presence of testis-bearing polyps, we identified the greatest limitation to embryo acquisition as overall egg production which we proposed was onset by some form of mechanistic suppression (Reisa, 1973). In an effort to identify and overcome this limitation, we implemented an animal husbandry protocol for inducing gametogenesis through intentional food restriction and isolated egg-bearing polyps to investigate the stability and frequency of oogenesis in H. vulgaris AEP. To test our observations, we reared two H. vulgaris AEP populations, one from polyps selected for egg capability and a control group. We then set up five replicate populations (100 polyps each) per group and conducted a 4-week-long starvation trial to quantify and compare gametogenesis in both groups. We analyzed our data using Bayesian methodology to account for Hydra’s tendency to rapidly asexually proliferate; by controlling for population growth over time in the experiment, we were able to isolate the effect of our treatment from potential temporal confounding factors. The results of this study demonstrate that these strategies significantly increase egg production and successfully establish an egg-producing biased colony for repeatedly stimulating egg development.
Methods and materials
Animal rearing
Ten H. vulgaris AEP individuals were acquired from the Juliano Lab at UC Davis in June 2024. Members of the genus Hydra are not subject to any regulation or care standards by the Institutional Animal Care and Use Committee (IACUC), and thus, no approval was required for our study. Lab-reared H. vulgaris were sustained on a diet consisting of the saltwater invertebrate Artemia franciscana sourced from Brine Shrimp Direct (similarly excluded from IACUC regulation). The default care routine included four weekly feeding events to encourage asexual reproduction; in addition to food preparation and cleaning, all husbandry methods were conducted as described in a previously published protocol (Cochran, 2025). Our working population of H. vulgaris was grown to a functional size of approximately 2,000 individual polyps over 3 months prior to our study.
Selection for egg-capable polyps
To begin selection, our working population was divided randomly into two equal groups. One group was set aside as a control and was maintained on the aforementioned default care routine. Referred to here as “AEP,” this population acted as our representative of H. vulgaris AEP’s normal sexual distribution. After 2 weeks of default care, the experimental group of H. vulgaris was intentionally stressed through 4 weeks of starvation trials to induce sexual traits. Starvation trials were conducted by reducing feeding days from 4 per week to only once. To control for the infrequency of cleaning caused by this change, we maintained water quality by replacing approximately 20% of the culture medium volume with clean Hydra medium on non-feeding days.
Starved H. vulgaris polyps were regularly observed for signs of gametogenesis. Any polyp exhibiting a visible developing oocyte was classified as egg-capable and relocated to a new population designated as “KSO.” Concluding the starvation trials, we had established two isolated populations: the control (AEP) and the egg-capable (KSO) population. The KSO population was propagated through budding from a starting group of approximately 10 egg-capable individuals and was allowed to propagate asexually to a colony comparable to that of the starting AEP, roughly 2,000 polyps, over the course of 3 months prior to our experiment.
Classification of sexual traits
Hydra vulgaris AEP polyps undergo one of two possible forms of gametogenesis at a time. Spermatogenesis results in the development of multiple testes along the body column; these distinct structures are identifiable by their curly brace shape and movement of live sperm within the tissue (Figure 1C). A polyp would be classified as “testis-bearing” given the presence of at least one testis. In contrast, oogenesis results in the development of a single egg field along the body column. Early stages of oogenesis can be cryptic and look similar to developing asexual buds (Figure 1B). To avoid misclassification, we adopted the defined stages of oocyte development described in Miller et al. (2000). Identification of “egg-bearing” polyps was restricted until stage 4 of growth when the oocyte developed its finger-like pseudopod extensions. By this stage, the ectoderm has visibly thickened over the oocyte mass resulting in a distinct buckling effect between the egg field and the body column (Figures 1D, E). By restricting identification to this stage, we eliminate the risk of misidentifying budding as egg development ensuring accurate daily counts prior to egg breakthrough. All sexual traits were first identified by eye and then confirmed under a Zeiss Stemi 508 dissection microscope with Axiocam 208 color.
Starvation trials to quantify sexual traits
To determine whether KSO produced more eggs than AEP, we conducted comparative experimental trials to quantify sexual trait development between AEP and KSO. All H. vulgaris were maintained on the default care routine for 2 weeks prior to these trials. A total of 500 AEP and 500 KSO polyps were randomly selected and divided into five dishes of 100 polyps each. Dishes were assigned identifiers based on strain and replicate number (e.g., AEP-1, KSO-3). All polyps were then subjected to a starvation routine of feeding once per week for 4 weeks (days −3, 4, 11, and 18).
During these trials, dishes were observed daily, excluding weekends, and data were recorded regarding sexual distribution and population size. First, dishes were checked for any H. vulgaris having undergone either spermatogenesis or oogenesis. The number of each would be recorded, and these polyps would be relocated to a secondary “SD” (sexually differentiated) dish to avoid recounting. Hydra vulgaris in SD dishes were then checked for sexual trait loss, the number of asexual polyps identified in these dishes was recorded, and the polyps were returned to their dish of origin. Total population was calculated as the combined number of individual polyps within the main and SD dishes for that ID. Budding H. vulgaris were counted as an individual polyp until the bud detached. Population counts were done by taking a photo of each dish daily and quantified using the cell-counter function in the biological-image analysis software, Fiji (v2.16.0, Schindelin et al., 2012).
Statistical analysis methods
The dataset consisted of 380 observations over 25 days (19 days of data collection), with 5 replicate subgroups per treatment group (AEP and KSO). Each observation recorded the number of new individual polyps per dish each day that were either egg-bearing (EB) or testis-bearing (TB) as well as the total subgroup population. Using these data, we conducted two complementary Bayesian tests to analyze the effects of our protocol on H. vulgaris sexual differentiation. The first test was a longitudinal analysis of daily sexual expression counts to examine temporal dynamics, and the second test was a cumulative analysis of sexual expression counts to examine total sexual reproductive output at the end of the experiment.
Longitudinal count analysis
To conduct our first test, the dataset was restructured from wide to long format with sexual trait (EB or TB) as a categorical predictor. We fitted a series of negative binomial generalized linear models with increasing complexity using the rstanarm package in R (R Core Team, 2024; Goodrich et al., 2024). The negative binomial distribution was selected to account for the overdispersion inherent in count-based response data. The general model structure was:
yi ~ NegativeBinomial(μi, φ).
log(μi) = β0 + βsex · Isexi=TB + βgroup · Igroupi=KSO + βsex:group · Isexi=TB · Igroupi=KSO +.
βday · dayi + random effects.
where yi represents the observed count, μi, is the expected count, φ is the dispersion parameter, and I denotes indicator functions for categorical predictors: sex and day.
The following four candidate models were evaluated for best fit using leave-one-out cross-validation (LOO-CV) with the loo package (Vehtari et al., 2025).
Model 1: Sex by group interaction with no time (day) effects.
Model 2: Sex by group interaction with time (day) effect (linear time trend).
Model 3: Model 2 + random intercepts by dish.
Model 4: Model 2 + random slopes for day by dish.
The best-performing model based on expected log pointwise predictive density (ELPD) was selected for inference.
Cumulative count analysis
To conduct our second test to assess total sexual trait output, day 0 counts were subtracted from the day 25 count to obtain the cumulative growth per dish. We fitted negative binomial generalized linear models with the following structure:
log(μi) = β0 + βsex · Isexi=TB + βtreatment · Itreatmenti=KSO + βsex:treatment · Isexi=TB · Itreatmenti=KSO + random effects.
The following five candidate models were similarly evaluated for best fit using LOO-CV.
Model 5: Null model (intercept only).
Model 6: Main effects of sex and treatment.
Model 7: Sex by treatment interaction.
Model 8: Model 6 + random intercept by dish.
Model 9: Model 7 + random intercept by dish.
Prior specifications, posterior sampling, and diagnostics
For both analyses, weakly informative priors were specified to regularize parameter estimates while allowing the data to dominate inference: intercept: β0 ~ Normal(0, 10), fixed effects: βj ~ Normal(0, 2.5), dispersion parameter: φ ~ exponential(1), random effect variance (when applicable): σ² ~ exponential(1).
Posterior distributions were sampled using Hamiltonian Monte Carlo (HMC) via Stan, implemented through rstanarm (Goodrich et al., 2024). For each model, we ran four chains of 2,000 iterations each (1,000 warmup, 1,000 sampling), yielding 4,000 posterior samples. Convergence was assessed using R statistics (all values <1.1 indicate convergence) and visual inspection of trace plots. Model fit was evaluated using posterior predictive checks, comparing observed data to replicated datasets drawn from the posterior predictive distribution.
Hypothesis testing
We conducted pairwise Bayesian hypothesis tests for the following comparisons: 1) the effect of treatment group (KSO vs. AEP) on the expression of each sexual trait (EB and TB), 2) the effect of sexual traits (TB vs. EB) within each treatment group (AEP and KSO), and 3) the effect of time in the longitudinal analysis. For each hypothesis, we computed posterior probabilities of directional effects [P(difference > 0) and P(difference < 0)] and 95% credible intervals. We also calculated probabilities of practically significant effects, defined as a >50% increase (ratio > 1.5) or >33% decrease (ratio < 0.67) on the multiplicative scale for the longitudinal analysis and >10-unit increase or >10-unit decrease on the count scale for the cumulative analysis. We interpreted P(effect) > 0.95 as strong evidence for an effect, following standard Bayesian decision criteria.
Best fit models
We used LOO-CV to determine the best-fit models for both our longitudinal and cumulative datasets. For the longitudinal analysis, the LOO-CV determined that model 2: sex by group interaction + day effect (linear time trend) was the best fit to the data. All R-hat statistics were <1.1 (ranging from 1.000 to 1.002), indicating successful convergence of the Markov chain Monte Carlo (MCMC) chains. Posterior predictive checks demonstrated good model fit, with simulated datasets closely matching the observed data distribution (Supplementary Material, Supplementary Figures 6, 7). For the cumulative analysis, LOO-CV determined model 7: sex by treatment interaction to be the best fit for the dataset. Convergence diagnostics confirmed successful MCMC sampling (R-hat < 1.1 for all parameters), and posterior predictive checks indicated adequate model fit (Supplementary Material, Supplementary Figure 9). Additionally, pairwise comparisons of both longitudinal and cumulative datasets revealed consistency with both analyses (Supplementary Material, Supplementary Figures 8, 10).
All statistical analyses were conducted in R version 4.4.0 (R Core Team, 2024) using the rstanarm package, which implements Bayesian inference through the MCMC sampling (Goodrich et al., 2024). Additional packages used included tidyverse for data manipulation (Wickham et al., 2019), bayesplot for model diagnostics (Gabry and Mahr, 2025), and loo for model comparison (Vehtari et al., 2025).
Results
Distribution of sexual traits
The distribution of sexual traits differed markedly between the KSO and AEP groups, as shown by the group averages of total EB and TB polyps expressed as a percentage of the final population (Figure 2A). The proportion of polyps that developed eggs was substantially higher in the KSO group (34.94%) than in the AEP group (7.08%) (Supplementary Figure 1). Conversely, 53.11% of AEP populations had differentiated to testis-bearing while seeing only 11.60% in KSO (Supplementary Figure 2). From these data, we calculated that on average 39.80% of polyps in AEP and 53.45% of polyps in KSO never sexually differentiated during the experiment. However, a significant variable contributing to undifferentiated proportions is continuous asexual recruitment. Replicate populations on average grew by 60.8% in AEP and 53.4% in KSO from budding alone over the experiment period (Supplementary Figure 3).
Figure 2
The interaction of treatment group and sexual traits
Our analyses found that the treatment to select for EB polyps for establishing KSO colonies had a substantial effect on sexual expression counts. Overall, when compared to AEP, KSO populations comprised 4.67 times more EB expressing polyps (model 2; median log-difference = 1.541; 95% credible interval: 1.142–1.936) (Supplementary Materials, Supplementary Figures 5, 8). Additionally, the KSO groups showed a 77% reduction in TB expression polyps (median log difference = −1.490; 95% CI: −1.847 to −1.131). This pattern was supported in the cumulative data set providing additional overwhelming evidence that the KSO selection treatment increased total egg-bearing expression (model 7; 1.502 median log difference; 95% CI: 0.924–2.064) and decreased total testis-bearing expression (−1.237 median log difference; 95% CI: −1.794 to −0.685) (Figure 2B). Furthermore, in model 2, the probability of practically significant effects under the selection was close to 100% for both increasing EB expression (>50% increase) and decreasing TB expression (>33% decrease) demonstrating strong evidence of trait selection for egg-capable polyps as a valid method for increasing egg production.
As for the influence of sex on the treatment group, we found support that the selection of EB polyps has a substantial effect on KSO group expression biases. In the control group AEP, TB polyps were 7.31 times more abundant than EB polyps (model 2; median log difference = 1.989; 95% CI: 1.596–2.388). The posterior probability of TB > EB was 1.000, with a probability of 100% for a practically significant increase. These results demonstrate overwhelming evidence that the control AEP populations have testis-bearing-biased sexual expression dynamics. Contrastingly, in KSO populations, TB counts only made up 35% of the expression seen in EB polyps (model 2; median log difference = −1.049, 95% CI: −1.406 to −0.678). The posterior probability that TB < EB was 1.000, which means a practically significant decrease. This provides overwhelming evidence that our selection protocol results in egg-bearing-biased sexual expression dynamics. Model 7 further supports this pattern in the cumulative dataset, confirming an egg-bearing-biased reproductive output in the KSO treatment (median difference = −1.023; 95% CI: −1.580 to −0.466) and a testes-bearing-biased output in the AEP controls (median difference = 1.721; 95% CI) (see Figure 2B).
The temporal effect
Time had a slight but reliable negative effect on overall sexual trait expression in both groups. We found that trait counts (EB and TB) were reduced by approximately 2.2% per day over the course of the starvation trial (model 2; median log difference = −0.022 per day; 95% CI: −0.038 to −0.007) (Figure 2C). The posterior probability of a negative time effect was 0.998, providing strong evidence for a gradual decline in sexual trait counts over time. However, the probability of a practically significant effect was close to 0, indicating that while the trend was reliably negative, its magnitude was not large enough to be considered practically significant by our predefined thresholds. This likely suggests the gradual onset of starvation side effects, reducing a polyp’s capacity to undergo gametogenesis after long periods of time without food.
Discussion
We have developed and validated technical approaches that significantly increase the efficiency of egg acquisition in the lab strain H. vulgaris AEP and can expand our understanding of the mechanisms underlying sexual differentiation in a Medusozoan species. Our data support the hypothesis proposed by previous studies that H. vulgaris may not be true sequentially hermaphroditic animals and “egg-capable” is a stable identity passed on to offspring produced through budding (Nishimiya-Fujisawa and Kobayashi, 2018). By selecting for egg capability, we established a population (KSO) with nearly 5-fold rate of egg production than the control (AEP) population. Previous protocols have reported attempts to acquire eggs using populations of approximately 4,000 polyps producing 10–12 zygotes per day 6 weeks after inducing sexual traits, a rate of approximately 1.92% per week (Klimovich et al., 2019). Comparatively, our approach was able to produce an average of 77.5 eggs per week from only 500 polyps, a weekly rate of 15.5%. Significant effects (>50% change in daily counts) were observed for all sexual traits (TB, EB) and treatment comparisons (KSO, AEP). This confirms that our selection protocol profoundly reshaped the sexual differentiation landscape of H. vulgaris, providing a robust and efficient means to enhance egg production for research.
The masculinization problem
Previous studies have reported that H. vulgaris populations used for egg collection eventually all succumb to full masculinization over the course of a year causing a significant reduction in egg production (Klimovich et al., 2019). In contrast, our KSO population challenges this paradigm. Established 10 months prior to the current experiment and subjected to three previous rounds of starvation trials, this population has maintained consistent egg production rates. Additionally, ongoing experiments tracking individuals’ post-oogenesis have shown polyps developing eggs again after only 2 weeks (ongoing studies, Supplementary Data File 1).
We propose that masculinization reports are due to a number of complications in animal husbandry methodology. An undetected egg-incapable polyp can dramatically impact strain integrity through asexual reproduction. Given that polyps frequently attach to tools during care, strain-specific equipment can help avoid accidental cross-contamination of sensitive populations. Additionally, damaging or mishandling H. vulgaris can also result in polyps regenerating as a different sexual identity. As confirmed by another study, if regenerating polyp tissue does not contain any germline stem cells, it is possible for the polyp to regrow without egg capability (Nishimiya-Fujisawa and Kobayashi, 2018). The previous two explanations combined with egg-bearer removal without replacement would inevitably result in an increased presence of males over time.
Limitations
Data for our experiment were collected at a population level, meaning we were unable to track individual polyps. Sexual trait count data from three replicates in AEP indicate that sexual expression occurred in more than 100 polyps. This could mean either some of the initial 100 polyps became sexual more than once or new clones that arose during the course of the experiment were capable of sexual differentiation by the end of the experiment. Both explanations could contribute to this observation; however, the lack of individual tracking data prevents us from discerning the cause.
Asexual clones produced during the course of the starvation trials were more susceptible to the negative effects of starvation without the 2-week robust feeding period afforded to the starting 100 polyps. Some newly budded clones were observed to undergo multiple days without food, and this resulted in dramatically smaller body mass and less activity on feeding days; if the polyp was too inactive to catch food on feeding days, the effects of starvation would likely become fatal. Population counts show potential loss of a few polyps, either from dissolving onset by starvation or body size and inactivity making them indistinguishable from food and waste pellets.
Future directions
Given the reported difficulties of pursuing transgenics in H. vulgaris as a result of their limited sexual reproduction rates, our study alleviates much of this difficulty by developing a repeatable and consistent source for embryos. Future directions for transgenics include more complicated techniques such as CRISPR, allowing for complete transgene integration and true genetic modification.
Additionally, our findings provide a novel framework for investigating the long-observed problem of population masculinization. We propose that this phenomenon is not an inevitable fate but rather an artifact of lab conditions. Hydra vulgaris in their natural habitat likely employ molecular signaling to communicate with other polyps throughout a comparatively large body of water that an egg is being developed and will need to be fertilized. We hypothesize that not only do H. vulgaris respond to egg development signaling but that testes or sperm also produce a detectable signal to encourage spermatogenesis in other polyps. This signaling theoretically creates an opportunity for sexual competition and increased genetic diversity in a predominantly asexual species. We believe whole population masculinization may just be the result of these signaling mechanisms occurring in a high-density laboratory population resulting in a spermatogenesis-inducing positive feedback loop. Further studies should aim to identify these putative molecular mechanisms and model the population dynamics underpinning this sexual fate decision-making.
Conclusion
We conclude that the combined effects of trait selection for egg capability during colony establishment and an optimized starvation care protocol result in a statistically significant increase in egg-developing H. vulgaris populations, effectively bypassing previously reported limitations. Our study provides a stepping stone to support the progress of future research utilizing H. vulgaris as a model organism by removing the most prominent inhibiting variable to transgenics and elucidating a pattern of stability in cryptic reproductive traits. This step toward better understanding the sexual reproduction strategies in Hydra within the field will allow for the expansion of genomic studies utilizing H. vulgaris with greater efficiency and less risk as a result of unpredictable facultative reproduction.
Statements
Data availability statement
The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.
Ethics statement
The manuscript presents research on animals that do not require ethical approval for their study.
Author contributions
KC: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing. JD: Data curation, Writing – review & editing. MA-C: Data curation, Formal analysis, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. AM-M: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Validation, Visualization, Writing – review & editing.
Funding
The author(s) declared that financial support was not received for this work and/or its publication.
Acknowledgments
We would like to thank our fellow Cnidolab members at UC Santa Cruz for comments on this manuscript, Dr. Celina Juliano at UC Davis for the Hydra vulgaris AEP strain we conducted this study with, Dr. Mark Phillips at Oregon State University for advising on fecundity study parameters, and Stephen H. Preston-Eto for early revisions.
Conflict of interest
The authors declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo.2026.1761812/full#supplementary-material
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Summary
Keywords
cnidarian reproduction, gametogenesis, Hydra vulgaris, oogenesis, sexual differentiation
Citation
Cochran KN, DeMartino J, Alfaro-Córdoba M and Macias-Muñoz A (2026) Optimizing egg production by trait selection and starvation in Hydra vulgaris. Front. Ecol. Evol. 14:1761812. doi: 10.3389/fevo.2026.1761812
Received
06 December 2025
Revised
18 February 2026
Accepted
16 March 2026
Published
10 April 2026
Volume
14 - 2026
Edited by
André Carrara Morandini, University of São Paulo, Brazil
Reviewed by
Jácint Tökölyi, University of Debrecen, Hungary
Saumitra Dey Choudhury, All India Institute of Medical Sciences, India
Updates
Copyright
© 2026 Cochran, DeMartino, Alfaro-Córdoba and Macias-Muñoz.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Aide Macias-Muñoz, amacia16@ucsc.edu
Disclaimer
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