An international consortium of researchers generated an extraordinary trove of data: 771 multiomics datasets spanning embryonic development and adult tissues in Atlantic salmon and rainbow trout, totalling 77.7 billion read pairs. The work was carried out under the European AQUA-FAANG project, part of the Functional Annotation of Animal Genomes initiative, and combined RNA sequencing to measure gene expression with ATAC-seq and ChIP-seq to map open chromatin and epigenetic marks. From these data, the team identified 319,718 robust open chromatin regions in salmon and 194,002 in trout, and used a computational approach called ChromHMM to classify the genome into functional states, pinpointing tens of thousands of active promoters and enhancers in each species.

The reason salmonids are so valuable for this kind of work lies in their peculiar genomic history. Roughly 80 to 100 million years ago, the ancestor of all salmon and trout experienced a whole genome duplication, an autotetraploidization event that doubled the entire genome within a single species. Unlike the ancient duplications that shaped all vertebrates more than half a billion years ago, this salmonid event is recent enough that the genome is still in the midst of rediploidization, the slow return to normal diploid inheritance. Around half of salmonid genes are still retained as functional duplicate pairs, known as ohnologues, sitting in large collinear blocks on duplicated chromosomes.

Crucially, rediploidization did not happen uniformly across the genome. Some duplicated regions resolved back into diploid inheritance before salmon and trout lineages split roughly 25 million years ago, giving their genes a long head start on divergence. Other regions resolved independently in each species, meaning their duplicate pairs are evolutionarily much younger. This stratification, called asynchronous rediploidization, allowed the researchers to compare duplicate genes at different stages of post-duplication evolution within the same genomes, effectively providing a series of snapshots of the same evolutionary process at different ages.

The central finding is striking: the pace of regulatory evolution after genome duplication is not random but is governed by the same developmental constraints that shape gene evolution across all vertebrates. Measuring expression divergence between duplicate pairs using Jensen-Shannon distance, the team found that divergence was highest in early embryos, before the maternal-to-zygotic transition, and fell steadily through embryogenesis, reaching its minimum at the pharyngula stage, which corresponds to the vertebrate phylotypic period. This is the stage, long recognized in evolutionary developmental biology, when embryos of different species look most alike and when pleiotropic genes are recruited across many developing tissues, creating a densely interconnected regulatory environment that resists change.

The correlation was remarkably tight. The pattern of decreasing expression divergence across embryogenesis was nearly identical between early and late rediploidization pairs, with a Pearson correlation of 0.97, and it also matched the divergence of orthologous genes between salmon and trout. In other words, the constraints acting on duplicate genes within a single genome mirror those acting on the same genes across species. The authors argue that these pleiotropic constraints do not merely limit divergence after duplication; they actively drive which duplicates are retained, because genes expressed during mid-to-late segmentation tend to be dosage-sensitive, and keeping both copies of such genes buffers against harmful imbalances in gene dose.

Adult tissues told a different and equally revealing story. Expression divergence between duplicates varied enormously across the body, with the brain showing the lowest divergence, consistent with its status as the tissue under the strongest purifying selection, while liver, ovary and testis showed the highest. Sexual maturation had little effect in most tissues, but mature testis displayed elevated divergence compared with immature testis. When the researchers compared orthologous genes between salmon and trout, they found that duplicates retained from the genome duplication had evolved more slowly than single-copy genes in gill, head kidney and distal intestine, though in the brain even single-copy genes evolved slowly, and in the ovary, maturation stage mattered more than duplication status.

Turning from genes to their regulatory elements, the team classified every active promoter and enhancer according to its fate after duplication: shared, meaning both duplicated copies remained active; alignable only, meaning the sequence was conserved but activity was lost from one copy; or exclusive, meaning the active element existed on only one chromosome. Promoters proved conservative, with 53.5 percent shared, closely matching the estimated 55 percent retention rate of duplicate gene pairs. Enhancers were far more dynamic, with only 33.3 percent shared and 43.8 percent falling into the alignable-only category, confirming that enhancers turn over faster than promoters after genome duplication, a pattern also seen when comparing regulatory elements across species.

The link between enhancer fate and gene divergence was direct. Duplicate gene pairs whose expression remained conserved had significantly more shared enhancers, while pairs with diverged expression had more exclusive enhancers, and their enhancer-usage profiles across tissues were measurably farther apart. In adult tissues, enhancer-usage distance tracked expression distance closely, whereas during embryogenesis it did not, suggesting that promoter changes and a specific subset of shared enhancers dominate regulatory divergence in embryos, while wholesale remodelling of the enhancer repertoire drives divergence in adult organs. Transposable elements added another layer: exclusive enhancers overlapped transposon sequences more than twice as often as shared enhancers, and DNA transposons of the TcMar class, the most active group in the salmon genome, were specifically enriched in enhancers of diverged duplicate pairs, implicating these jumping genes both in disabling old enhancers and in creating new ones.

Perhaps the most evocative result concerns conserved non-coding elements, ancient DNA sequences under purifying selection across hundreds of millions of years of vertebrate evolution. Among 24,007 enhancer-associated conserved elements in salmon, 61.4 percent were retained as shared duplicate pairs, and these were strongly enriched for the oldest categories, dating back to the ancestor of all bony vertebrates. The shared elements active during late somitogenesis, the phylotypic period, were packed with motifs for master developmental transcription factors such as POU5F1, Sox and Pax family proteins, precisely the regulators that orchestrate embryonic patterning. This suggests that duplicated enhancers carrying deeply conserved developmental instructions are functionally retained in duplicate, reinforcing the developmental lock that limits evolutionary change at the hourglass waist. The study, whose data are openly available through the FAANG portal and SalmoBase, not only illuminates how genomes recover from duplication but also provides a resource for breeding improved aquaculture strains and for conserving wild salmonid populations whose genomes carry this extraordinary evolutionary archive.

Subject of Research: Gene-regulatory evolution following whole genome duplication in salmonid fishes

Article Title: Developmental contexts of gene-regulatory evolution following whole genome duplication in salmonids

Article References: Baudement, M.-O., Morata, D. P., Gillard, G. B., Dewari, P. S., Gundappa, M. K., Podgorniak, T., Grønvold, L., Baranasic, D., Laurent, A., Giudicelli, F., Zunar, B., Carrera-García, E., Perquis, A., Brionne, A., Thi Nguyen, T., Ruiz Daniels, R., Merino, G. A., Thybert, D., Ilsley, G. R., … Macqueen, D. J. (2026). Developmental contexts of gene-regulatory evolution following whole genome duplication in salmonids. Nature Ecology & Evolution, 10(10), 1959-1981. https://doi.org/10.1038/s41559-026-03171-6

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03171-6

Keywords: whole genome duplication, salmonids, Atlantic salmon, rainbow trout, ohnologues, rediploidization, enhancers, gene regulation, phylotypic period, transposable elements, conserved non-coding elements, comparative genomics

The logic behind ICE models is deceptively simple. When two species respond similarly to a wide range of chemicals, the toxicity of a new compound in one species can be estimated from its measured toxicity in the other. The challenge lies in demonstrating that the correlation is strong enough, and based on enough matched data points, to be trustworthy. For this model, the authors gathered acute toxicity data for both species from publicly available databases, pairing 96-hour LC50 values, the concentrations lethal to half of the exposed animals over four days, for the same chemicals in frog tadpoles and in juvenile rainbow trout.

Raw database values cannot simply be pooled. The researchers curated the dataset carefully, converting units where necessary so that all values were directly comparable, and calculating geometric mean values whenever multiple LC50 values existed for the same chemical. They also checked the paired data for potential outliers that might distort the regression. After this cleaning process, the final analysis rested on 138 LC50 values covering 69 organic chemicals, a respectable sample size for a model intended to support regulatory hazard assessment.

The statistical outcome was clear. Spearman’s rank correlation coefficient between frog and trout toxicity values was 0.86, with a p-value below 0.0001, indicating a highly significant relationship across the chemicals tested. The regression itself yielded an R-squared of 0.74, also with p below 0.0001, and the model was judged statistically robust according to the evaluation criteria built into the US EPA’s Web-ICE software, the standard tool for this class of interspecies estimation models. In practical terms, knowing how toxic a chemical is to rainbow trout explains roughly three quarters of the variation in its toxicity to African clawed frog tadpoles.

Why build a frog model from fish data at all? The answer lies in how the two species are actually used in testing. Rainbow trout are among the fish species most commonly employed for acute toxicity testing worldwide, so trout LC50 values exist for a very large number of chemicals, including many pesticides and industrial organics. The African clawed frog, by contrast, is primarily used in endocrine assays, tests that examine how chemicals interfere with hormones rather than how quickly they kill. Acute lethality data for the frog are therefore comparatively rare, even though the species is a standard laboratory organism with a long history in aquatic toxicology.

That scarcity matters in specific regulatory contexts. Acute toxicity information for Xenopus laevis may be needed to set test concentrations for endocrine assays, ensuring that animals in those experiments are exposed to meaningful but not uniformly lethal doses. Where required, frog LC50 values also serve as inputs for aquatic risk assessments for amphibians. Without a model like this one, obtaining such a value could mean commissioning an additional vertebrate study, with all the ethical, financial and temporal costs that entails. The authors note that by applying this robust ICE model, additional vertebrate studies can be avoided altogether, a point that resonates strongly with ongoing efforts across toxicology to reduce animal testing.

The new model fits into a broader and rapidly maturing framework. The US EPA maintains the Web-ICE platform, which hosts a family of interspecies correlation estimation models linking standard test species to one another, and recent work has focused on uncertainty analysis and updated user guidance for these models, including their behavior with low-toxicity compounds. Other recent evaluations have examined how ICE models can increase the taxonomic diversity of chemical assessments while reducing reliance on animal testing under statutes such as the Toxic Substances Control Act. Adding a fish-to-amphibian model extends this toolkit to a class of vertebrates, amphibians, that sits between fish and reptiles in many assessment schemes but has historically been underrepresented in acute toxicity databases.

Amphibians occupy an awkward position in chemical regulation. Their aquatic larvae share exposure routes with fish, yet their biology differs in ways that could plausibly alter sensitivity, from permeable skin and developmental transitions to distinct metabolic pathways. Earlier comparative studies have examined the relative acute and chronic sensitivity of fish and amphibians, and case studies have asked whether fish assays and standardized frog embryo assays are protective enough for amphibian larvae. The strong correlation reported here between trout and frog acute toxicity suggests that, at least for the organic chemicals covered by the 69-compound dataset, rainbow trout data capture much of the interspecies variation in lethal sensitivity, lending empirical weight to the practice of using fish as a surrogate for amphibian acute hazard.

The underlying data themselves tell a story about the history of ecotoxicology. The matched values compiled for the model draw on decades of published acute toxicity testing, spanning legacy compounds and modern pesticides alike. Amphibian toxicity testing has its own long lineage, from early studies of pesticides on tadpoles of various species to the development of standardized protocols such as the Frog Embryo Teratogenesis Assay-Xenopus, known as FETAX, and the suite of OECD test guidelines covering the amphibian metamorphosis assay, the larval amphibian growth and development assay, and the Xenopus eleutheroembryonic thyroid assay. Each of these endpoints probes a different aspect of chemical hazard, but none generates the simple 96-hour LC50 as routinely as the fish acute toxicity test does, which is precisely the gap the ICE model is designed to bridge.

For the model to be useful in practice, its limitations need to be respected. An ICE model is only as reliable as the chemicals it was trained on; predictions for compounds with modes of action or physicochemical properties far outside the 69-chemical training set carry greater uncertainty, and interspecies correlations generally perform better for chemicals within the range of toxicity values used to fit the regression. The authors describe the model as a non-testing method to generate an amphibian LC50 value when a rainbow trout value is available, for use in hazard and risk assessment. Read that way, the equation does not replace amphibian testing where it is genuinely required, but it offers a defensible, quantitative alternative when the question is whether a frog study is needed at all, or what concentrations a planned assay should cover.

The implications extend beyond the laboratory. Chemical registration programs worldwide routinely require acute fish toxicity data, meaning trout LC50 values already exist for thousands of substances in regulatory dossiers. A validated trout-to-frog model effectively unlocks that existing data trove for amphibian hazard characterization, at zero additional cost to laboratory animals. As environmental agencies continue to confront the question of how well fish-based assessments protect other aquatic vertebrates, models like this one provide the quantitative evidence needed to make those judgments transparent. For a species whose embryos and tadpoles have taught generations of biologists about development, the African clawed frog may now also help redefine how chemical safety is assessed, one regression equation at a time, with fewer animals in the tanks.

Subject of Research: An interspecies correlation estimation model predicting acute chemical toxicity to the African clawed frog using rainbow trout toxicity data

Article Title: An interspecies correlation estimation (ICE) model to predict acute toxicity of organic chemicals to the African clawed frog (Xenopus laevis) using rainbow trout (Oncorhynchus mykiss) data

Article References: Weltje, L., & Savaliya, D. (2026). An interspecies correlation estimation (ICE) model to predict acute toxicity of organic chemicals to the African clawed frog (Xenopus laevis) using rainbow trout (Oncorhynchus mykiss) data. Ecotoxicology, 35(8), Article 170. https://doi.org/10.1007/s10646-026-03165-x

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03165-x

Keywords: Xenopus laevis, rainbow trout, ICE model, Web-ICE, acute toxicity, LC50, ecotoxicology, amphibians, animal testing reduction, organic chemicals, risk assessment, regulatory toxicology

Afghanistan’s aquaculture sector has grown quietly but steadily despite decades of conflict. National fish production rose from just 870 tons in 1980 to 11,107 tons by 2021, even though the country still records one of the lowest per capita fish consumption rates in the world, at just over 2 kilograms per person. Yet until now, no study had systematically assessed water quality parameters or documented the growth performance of rainbow trout under Afghan production conditions. That gap matters, because water quality is the single most powerful lever over fish health, growth, and farm economics. Poor water chemistry drives stress, disease outbreaks, and slower growth, translating directly into economic losses for farmers operating on thin margins.

The research team, led by scientists at Kabul University with a collaborator at Universiti Putra Malaysia, sampled two sites monthly between April and December: Site 1, the reservoir inflow that supplies the farm with water, and Site 2, the culture pond holding the fish. They measured temperature, dissolved oxygen, pH, total dissolved solids, ammonia, nitrite, nitrate, phosphate, sulfate, iron, and zinc, both in the field with calibrated meters and in the laboratory of the Ministry of Energy and Water using spectrophotometric methods following APHA protocols. Analytical blanks and duplicate analyses were run with each batch to guard against contamination, and the laboratory procedures met the quality standards required for water quality index calculations.

The spatial differences between inflow and pond were striking. Water in the culture pond was consistently cooler, with a mean temperature reduction of 3.02 degrees Celsius relative to the inflow, and the gap was widest in June, when the pond ran 5.8 degrees colder. Total dissolved solids were also significantly lower in the pond. More concerning were the substances that accumulated where fish were held. Ammonia concentrations averaged 110.8 percent higher at the pond site, peaking at a 275 percent increase in June, while zinc rose by 38.4 percent and iron by 31.2 percent on average. Phosphate, by contrast, fell by 61.5 percent in the pond, and nitrate and nitrite showed moderate increases. These patterns are consistent with the excretion and feed inputs typical of cultured systems, though the authors caution that the observational design means the correlations describe association rather than proven causation.

Dissolved oxygen emerged as the study’s most critical constraint. In April, oxygen levels fell to between 3.24 and 3.30 milligrams per liter at both sites, far below the threshold of at least 6 milligrams per liter generally recommended for trout culture. Cold-water salmonids like rainbow trout are notoriously sensitive to hypoxia, and such low concentrations are known to impair growth efficiency and physiological resilience. The oxygen deficit was most pronounced in spring, coinciding with the largest temperature differences between sites, and dissolved oxygen remained on average 15.1 percent lower in the pond than in the inflow throughout the monitoring period. The researchers suggest that biological activity and associated microbial processes in the pond likely contribute to the elevated oxygen demand, although confirming this would require controlled experiments.

One of the study’s most technically interesting findings came from comparing correlation structures between the two sites. At the inflow, nitrate and sulfate were strongly positively correlated, total dissolved solids were strongly negatively correlated with phosphate, and ammonia was strongly negatively correlated with zinc. In the culture pond, the web of relationships changed dramatically, with 18 statistically significant correlations, including 11 at the strictest threshold. The relationship between total dissolved solids and nitrite flipped from positive at the inflow to strongly negative in the pond, and the dissolved oxygen to iron correlation reversed from moderately positive to strongly negative. The authors interpret these reversals as consistent with changes in redox-related conditions under higher organic loading and oxygen consumption, a phenomenon they describe as nutrient-metal coupling, while emphasizing that correlation analysis alone cannot establish mechanistic control.

The fish themselves told a story of robust growth under imperfect conditions. Across 60 specimens sampled monthly and analyzed at Kabul University’s physiology laboratory, mean body weight climbed from 70.31 grams in April to 212.45 grams in December, a 202.2 percent increase, while body length rose 44.1 percent. The length-weight relationship revealed positive allometric growth, with the fitted power model yielding an exponent of 3.055, just above the isometric threshold of 3, meaning the fish gained weight disproportionately fast relative to length. Growth was strongest in the later months of the season, when thermal conditions were more favorable and feeding regimes likely optimized.

Organ-level indices added physiological nuance to the growth data. The hepatosomatic index, which reflects liver mass relative to body weight, peaked at 3.09 percent in May after a sixfold jump from April, suggesting heightened metabolic activity or energy storage during a period when oxygen was critically low. Liver weight itself peaked in June at 4.59 grams, more than a fifteenfold increase over April. The gonadosomatic index, by contrast, remained modest throughout the year, peaking at just 0.20 percent in May, and showed no significant relationship with the hepatosomatic index across individuals. This dissociation indicates that reproductive investment was decoupled from short-term hepatic energy reserves under current farming practices, a pattern the authors attribute to the fish’s reproductive cycle dynamics rather than direct water quality effects.

The study’s authors are careful about its limits. The work was conducted at a single farm, replication at the farm level was not feasible, and sampling from April to December did not capture the full annual cycle. Variables such as feed input rates, stocking density, sediment biogeochemistry, and microbial community structure were not directly quantified and may have shaped the observed patterns. Still, the practical implications are clear. April and May posed the highest risk to the trout because hypoxia coincided with rising temperatures, while autumn and early winter brought cooler but increasingly nutrient-rich water, signaling seasonal nutrient accumulation rather than immediate metabolic stress. The researchers recommend enhanced aeration to combat hypoxia, improved waste removal to mitigate ammonia and nutrient loading, careful monitoring of stocking densities, real-time water quality monitoring, and dietary supplements to bolster fish resilience.

For Afghanistan, where aquaculture is increasingly seen as a pillar of food security and economic development, the study provides something the sector has never had: localized, quantitative evidence linking environmental conditions to biological performance in a semi-intensive farming system. The message for farmers and policymakers alike is that trout production at Qargha is sustainable only with proactive water quality management. As global demand for farmed protein pushes aquaculture toward a projected 140 million tons of production by 2050, lessons from a fish farm beside a Kabul reservoir may prove unexpectedly relevant to the many semi-intensive systems worldwide where fish, water, and economics are locked in the same delicate balance.

Subject of Research: Water quality and rainbow trout growth performance at a semi-intensive aquaculture farm in Afghanistan

Article Title: Comparative assessment of water quality and rainbow trout performance between inflow and culture pond sites at Qargha Fish Farm, Afghanistan

Article References: Comparative assessment of water quality and rainbow trout performance between inflow and culture pond sites at Qargha Fish Farm, Afghanistan. (n.d.). https://doi.org/10.1186/s44399-026-00036-y

Image Credits: AI Generated

DOI: 10.1186/s44399-026-00036-y

Keywords: aquaculture, rainbow trout, water quality, Afghanistan, dissolved oxygen, ammonia, Qargha Fish Farm, hepatosomatic index, fish growth, hypoxia, nutrient loading, semi-intensive farming