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PubMed1977

Response of microorganisms to an accidental gasoline spillage in an arctic freshwater ecosystem.

The response of microorganisms to an accidental spillage of 55,000 gallons of leaded gasoline into an Arctic freshwater lake was studied. Shifts in microbial populations were detected after the spillage, reflecting the migration pattern of the gasoline, enrichment for hydrocarbon utilizers, and selection for leaded-gasoline-tolerant microorganisms. Ratios of gasoline-tolerant/utilizing heterotrophs to "total" heterotrophs were found to be a sensitive indicator of the degree of hydrocarbon contamination. Respiration rates were elevated in the highly contaminated area, but did not reflect differences between moderately and lightly contaminated areas. Hydrocarbon biodegradation potential experiments showed that indigenous microorganisms could extensively convert hydrocarbons to CO(2). In situ measurement of gasoline degradation showed that, if untreated, sediment samples retained significant amounts of gasoline hydrocarbons including "volatile components" at the time the lake froze for the winter. Nutrient addition and bacterial inoculation resulted in enhanced biodegradative losses, significantly reducing the amount of residual hydrocarbons. Enhanced biodegradation, however, resulted in the appearance of compounds not detected in the gasoline. Since the contaminated lake serves as a drinking water supply, treatment to enhance microbial removal of much of the remaining gasoline still may be advisable.

Alaska↗

PubMed2026

Boreal and subarctic freshwaters harbour a diversity of jumbophages.

Bacteriophages (phages) are major drivers of microbial evolution and ecology, yet their diversity and functional roles remain poorly characterized in many natural environments, such as in freshwater systems. In boreal and subarctic freshwater habitats, where bacteria are typically slow-growing and nutrient-limited, phages are predicted to have a critical role in host regulation and horizontal gene exchange. However, only a few isolates have been obtained from such environments, leaving the genetic and functional diversity of these phages largely unexplored. Here, we present a collection of 40 bacteriophages isolated from boreal lakes and rivers using a set of diverse freshwater bacterial hosts. Despite using conventional isolation methods, eight of the isolates possess genomes larger than 200 kilobases and are classified as jumbophages. All jumbophages exhibited myovirus morphology and comparatively slow infection dynamics. These jumbophages include the first known representatives infecting members of Janthinobacterium and Herbaspirillum. Comparative genomic and phylogenetic analyses show that nearly all genomes are distinct from previously described phages, indicating substantial novelty. Diverse auxiliary metabolic and anti-defence systems were identified, including putative NAD+ salvage and acyl carrier protein modules, along with predicted Anti-Thoeris and Anti-CBASS elements. The Pseudomonas-infecting jumbophage Ahti encoded homologues of all 21 core genes that define the nucleus-forming family Chimalliviridae. Additionally, Ahti displayed compartmentalization of DNA during infection, establishing it as the first freshwater nucleus-forming phage. These findings expand our understanding of the ecological, genomic, and functional diversity of phages in boreal environments and highlight the role of freshwater ecosystems as significant reservoirs of novel viral lineages.

anti-defence systems↗

PubMed2026

Ecotoxicological responses of aquatic macrophytes to 2,4-D: A global synthesis of species sensitivity and ecological risk.

The widespread use of 2,4-dichlorophenoxyacetic acid (2,4-D) has raised concern about its persistence, mobility, and effects on non-target aquatic vegetation in freshwater ecosystems. Here, we provide a global synthesis of the ecotoxicological responses of aquatic macrophytes to 2,4-D based on a PRISMA-guided systematic review of 86 peer-reviewed studies published between 1947 and 2025. A consistent gradient of species-specific sensitivity was observed across macrophyte growth forms. The submerged species Myriophyllum spicatum showed high susceptibility, with EC₅₀ values of 0.04-0.182 mg/L and marked growth inhibition at low concentrations, whereas floating species such as Lemna minor and Pontederia crassipes were more tolerant, requiring higher concentrations (7.08 to >100 and 8.1 mg/L, respectively) to produce comparable effects. Importantly, this sensitivity ranking was consistent across laboratory and field experimental settings. These interspecific differences likely reflect variation in herbicide uptake, translocation, and detoxification capacity associated with growth form. The overlap between EC₅₀ values for M. spicatum and regulatory thresholds for 2,4-D in surface waters suggests that current limits may be insufficient to protect sensitive submerged macrophyte communities. Regarding remediation, L. minor and Salvinia natans emerged as the most promising candidates for phytoremediation, while P. crassipes showed limited capacity to reduce herbicide concentrations in water. Despite advances, no study directly compared oxidative stress biomarkers between submerged and floating species, representing a critical gap in understanding the biochemical basis of the sensitivity gradient. Overall, this synthesis highlights the need to account for taxon-dependent sensitivity when evaluating the ecological risks of 2,4-D and provides a basis for improving regulatory frameworks and management of herbicide contamination in freshwater ecosystems.

2,4-Dichlorophenoxyacetic Acid↗

PubMed1975

Effects of three environmental variables on sulfate uptake by aerobic bacteria.

The effects of various concentrations of sulfate, organic sulfur, and organic carbon on sulfate uptake by aerobic bacteria were studied using pure cultures growing in a defined medium. Cultures of Pseudomonas fluorescens and Corynebacterium striatum took up sulfate faster when young, but sulfate uptake by Serratia marcescens was faster in older cultures. Organic sulfur was found to decrease sulfate uptake, but at concentrations somewhat higher than occurs in most natural freshwater ecosystems. Low levels of sulfate can theoretically directly limit bacterial biomass production but such limitation probably does not occur in natural systems. Evidence is presented which indirectly links the uptake of sulfate and organic carbon, adding credibility to the proposal that sulfate uptake can be used as an indicator of microbial biomass production in freshwater ecosystems.

Aerobiosis↗

PubMed2026

Reproduction on the Rocks: Life History of a Freshwater Macrobioeroding Bivalve.

Macrobioerosion, the excavation and removal of consolidated mineral substrates by macrofauna, is well established in marine systems, where macrobioeroders drive carbonate cycling, sediment production and habitat formation. In freshwater ecosystems, however, it has been documented in only a small number of invertebrate taxa and remains a poorly resolved ecological process. Among these, the teredinid shipworm Lithoredo abatanica represents a remarkable departure from the wood-boring ecology of its family, having evolved to excavate and ingest limestone in fresh water. Despite this remarkable ecological transition, its reproductive biology and life history remain unknown. Here, we investigate the reproductive mode and life history strategy of this species using population size structure, in situ observations of siphonal morphology, and sperm morphometrics. We show that L. abatanica reaches exceptional dimensions, with measured body lengths exceeding 100 mm, one intact empty burrow exceeding 200 mm and in-water observations indicating burrows possibly exceeding 500 mm, establishing it as the largest known freshwater macrobioeroder. Its large size and dense aggregations indicate considerable capacity for local rock breakdown and habitat modification within the Abatan River system (Bohol, Philippines). Recently settled juveniles (< 10 mm) alongside reproductively mature individuals indicate ongoing recruitment. The morphology of the siphons and calcareous tube appears to preclude direct sperm transfer via pseudocopulation, while sperm morphometrics are consistent with external fertilisation. Together, these findings indicate that L. abatanica reproduces via broadcast spawning with external fertilisation and likely possesses a planktotrophic larval phase. This raises a fundamental question: how does a broadcast-spawning species with planktotrophic larvae maintain populations up to 15 km upstream in a flowing freshwater river subject to persistent downstream advection? By resolving the life history of the largest known freshwater macrobioeroder, this study provides critical insight into the persistence, dispersal and ecological role of a globally unique riverine ecosystem engineer.

broadcast spawning↗

PubMed2025

Tracking microplastic contamination across seasons in a freshwater reservoir: Evidence from surface water, sediments, and fishes.

Microplastics (MPs) are prevalent contaminants in aquatic environments, posing substantial ecological and health risks. These particles migrate within the different layers of aquatic bodies with time and affect the respective biota. Thus, to get an in-depth understanding of the particles, this current study investigated the seasonal distribution, morphological and chemical characteristics, along with potential ecological and human health impacts in the samples including surface water, sediment, and fish from a drinking water supplying reservoir in eastern India. Across three different seasons, pre-monsoon, monsoon, and post-monsoon samples were collected using optimized methods. Results revealed distinct seasonal trends: MP abundance in surface water peaked during the monsoon (mean: 1.15 MPs/L), while sediment showed the highest concentrations in the pre-monsoon (mean: 596 MPs/kg), indicating temporal accumulation dynamics influenced by runoff, hydrodynamics, and sedimentation. Fish gut analysis confirmed ingestion of MPs across five species, with concentrations ranging from 26 to 100 MPs/kg, depending on feeding habits. The most dominant MP type were fragments, followed by fibers, films, and beads. Polymer analysis via µFTIR identified polyethylene, polypropylene, and polyvinyl chloride as prevalent, with hazard assessments (i.e., Polymer Hazard Index (PHI)) indicating medium to very high ecological risks. Heavy metal association was more dominant in the MPs isolated from sediments than the waterborne MPs. Pollution Load Index (PLI) values were > 1 in most seasons, confirming contamination. Health risk analysis suggested potential exposure through both drinking water and fish consumption. This study emphasizes the need for seasonal monitoring, improved waste management, and mitigation strategies to address MP pollution in freshwater ecosystems.

Microplastics↗

PubMed2026

Hypoosmolarity inhibits ammonia oxidation by terrestrial and freshwater Nitrosopumilaceae members.

Salinity strongly influences the physiology and distribution of nitrifying microorganisms, yet the effects of low salinity remain understudied. This study investigates the impact of hypoosmolarity on different groups of ammonia oxidizers in soil and freshwater reservoirs, as well as in pure culture isolates. In soil microcosms amended with ammonium, at low salinity levels (~120 μS/cm), comparable to values commonly found in pristine terrestrial and freshwater environments, the abundance of ammonia-oxidizing bacteria (AOB), dominated by Nitrosomonas oligotropha, significantly increased. In contrast, the growth of ammonia-oxidizing archaea (AOA), dominated by "Candidatus Nitrosotenuis" of the Nitrosopumilaceae family, was stimulated by high salinity (~760 μS/cm). In ammonium-fed freshwater microcosms, the abundance of AOB, dominated by N. oligotropha, significantly increased under both low (~170 μS/cm) and high salinity (~850 μS/cm) conditions. In the presence of allylthiourea (50 μM), used to inhibit bacterial ammonia oxidation, AOA were sensitive to low salinity in both soil and freshwater microcosms. Consistently, culture-dependent studies revealed marked growth inhibition of terrestrial AOA, especially members of Nitrosopumilaceae, under hypoosmolarity, unlike AOB and complete ammonia oxidizer (comammox) strains. Genomic analyses, along with transcriptomic studies, suggested that the sensitivity of AOA to hypoosmolarity stress was possibly due to a lack of osmoregulatory transport systems and their S-layer cell wall structure. Overall, this study indicates hypoosmolarity as an important factor shaping the ecological niches and distribution of ammonia oxidizers, as well as nitrification activities, in terrestrial and freshwater environments that are increasingly affected by intensified water cycles due to global change.

Ammonia↗

PubMed2025

Picocyanobacteria in the Chesapeake Bay: isolation, diversity, and adaptation.

Tiny unicellular cyanobacteria or picocyanobacteria (0.5-3 µm) are important due to their ecological significance. Chesapeake Bay is a temperate estuary that contains abundant and diverse picocyanobacteria. Studies of Chesapeake Bay picocyanobacteria in the past 20 years led to the finding of new members of subcluster 5.2 Synechococcus. They laid the foundation for revealing the ecophysiology, biogeography, genomics, and molecular evolution of picocyanobacterial in the Chesapeake Bay and other coastal estuaries. The Bay picocyanobacteria are known to better tolerate the changes in temperature, salinity, and heavy metals compared to their coastal and open-ocean counterparts. Many picocyanobacteria isolated from the Bay contain rich toxin-antitoxin (TA) genes, suggesting that the TA system may provide them with a genetic advance to cope with variable estuarine environments. Distinct winter and summer picocyanobacteria are present in the Bay, suggesting a dynamic seasonal shift of the picocyanobacterial community in the temperate estuary. While the Bay contains subcluster 5.2 Synechococcus, it also contains freshwater Synechococcus, Cyanobium, and marine Synechococcus due to river influx and the ocean's tidal influence. Some Chesapeake Bay picocyanobacterial clades were found in the Bering Sea and Chukchi Sea, showing a link between the Bay and polar picocyanobacteria. Genomic sequences of estuarine picocyanobacteria provide new insight into the taxonomy and evolution of freshwater, estuarine, and marine unicellular cyanobacteria. Estuaries connect freshwater and marine ecosystems. This overview attempts to extend what we learned from Chesapeake Bay picocyanobacteria to picocyanobacteria in freshwater and marine waters.

Chesapeake Bay↗

PubMed1977

Yersinia enterocolitica and Yersinia like microbes isolated from mammals and water in Norway and Denmark.

Data pertaining to 149 strains belonging to genus Yersinia are summarized in this paper. Yersinia enterocolitica (Y.e.) was isolated from the faeces of 31 of 305 small rodents and from 5 of 31 shrews (Soricidae) trapped at five localities in Norway and one locality in Denmark. Isolations were obtained from 9 of 29 water samples collected within the trapping areas. Three of 25 red foxes (Vulpes vulpes) from one locality in Norway harbored Y.e. in their faeces. Y.e. serotype 16 was isolated from one zoologist suffering from diarrhoea. A total of 85 strains from small rodents, shrews, water and foxes showed biochemical properties intermediate to Y.e. and Y. pseudotuberculosis. Another three strains were classified as Y. pseudotuberculosis on biochemical basis. They were obtained from small rodents in Denmark. Serological examination of 59 small rodents naturally infected with Y.e. and related microbes, revealed two cases of low antibody titres (80) against homologous isolates. Pathological examination of 44 of these animals gave a negative result. Strains antigenically related to the same serotypes were frequently isolated from both terrestrial ecosystems and adjacent freshwater. Strains related to serotype 6 dominated both in red fox and in their small rodent prey. No evidence was found for a dynamical significance of Y.e. in populations of small rodents. The results indicate that Y.e. and Yersinia like microbes have a broad occurrence in terrestial ecosystems.

Animals↗

PubMed2025

Microbial metagenomes from Lake Soyang, the largest freshwater reservoir in South Korea.

Lake ecosystems play a fundamental role in the global biogeochemical cycling of essential elements such as carbon, nitrogen, and phosphorus. Microorganisms within these ecosystems mediate key processes that regulate these cycles. Metagenomic analyses provide valuable insights into the taxonomic and functional diversity of microbial communities in various environments, including freshwater habitats. Here, we present a comprehensive metagenomic dataset derived from Lake Soyang, the largest freshwater reservoir in South Korea. A total of 28 metagenomes were generated from water samples collected across two distinct sampling periods: the first set (n = 8) was obtained between April 2014 and January 2015 from two depths (1 m and 50 m) in four different seasons, while the second set (n = 20) was collected between January 2019 and November 2019 from five depths (1, 10, 20, 40, and 90 m) over four seasons. Metagenomic sequencing yielded 9.3-21.8 Gbp per sample. This dataset provides a valuable resource for future studies exploring the ecophysiological characteristics of microbial communities in pelagic freshwater environments.

Republic of Korea↗

PubMed1978

Distribution of 238Pu in tissues of fish from the canal in Miamisburg, Ohio.

The 238Pu concentrations of various tissues were measured for seven species of freshwater fish from an ecosystem containing elevated levels of 238Pu. The highest levels of 238Pu were found in the gastrointestinal tracts and gills, while the lowest levels were found in muscle tissue. A rapid uptake of 238Pu was observed for hatchery bluegills introduced into this system. High plutonium concentrations in the gastrointestinal tracts and gills suggest that these organs are potential uptake sites. The presence of 238Pu in certain tissues (liver, gonads, bone, and muscle) indicates that there is translocation of 238Pu from the uptake sites.

Animals↗

PubMed2025

Intraspecific divergence within Microcystis aeruginosa mediates the dynamics of freshwater harmful algal blooms under climate warming scenarios.

Intraspecific biodiversity can have ecosystem-level consequences and may affect the accuracy of ecological forecasting. For example, rare genetic variants may have traits that prove beneficial under future environmental conditions. The cyanobacterium responsible for most freshwater harmful algal blooms worldwide, Microcystis aeruginosa, occurs in at least three types. While the dominant type occurs in eutrophic environments and is adapted to thrive in nutrient-rich conditions, two additional types have recently been discovered that inhabit oligotrophic and eutrophic environments and have genomic adaptations for survival under nutrient limitation. Here, we show that these oligotrophic types are widespread throughout the Eastern USA. By pairing an experimental warming study with gene expression analyses, we found that the eutrophic type may be most susceptible to climate warming. In comparison, oligotrophic types maintained their growth better and persisted longer under warming. As a mechanistic explanation for these patterns, we found that oligotrophic types responded to warming by widespread elevated expression of heat shock protein genes. Reduction of nutrient loading has been a historically effective mitigation strategy for controlling harmful algal blooms. Our results suggest that climate warming may benefit oligotrophic types of M. aeruginosa, potentially reducing the effectiveness of such mitigation efforts. In-depth study of intraspecific variation may therefore improve forecasting for understanding future whole ecosystem dynamics.

Microcystis↗

PubMed2026

The Multiple Roles of Genetics on Freshwater Macrophyte Functional Traits in the Interplay With the Environment: A Review.

The study of functional trait variation is increasingly used to understand macrophyte adaptation, as traits reflect organismal performance under different ecosystem conditions. Phenotypic expression results from the interplay of genetic and environmental factors: genetics provides the molecular basis for heritable traits and constrains potential phenotypes, while the environment acts as a selective and modulatory force. However, the genetic insight into traits has rarely been addressed in freshwater macrophyte studies. This review examines the different ways in which the DNA of macrophytes interplays with the environment and contributes to the variation in their functional traits, outlining main approaches, gaps, and future challenges. Only 21 studies explicitly combined genetics with functional traits and environment in the last fifteen years. The most common approach was the use of common garden experiments to explore acclimation and adaptation in a few model species. Current studies mainly focus on morphological and growth traits that best describe macrophytes' economic strategies, with limited attention to other trait categories, while the genetic and DNA traits studied are more variable. Across studies, environmental factors generally explained a larger proportion of functional trait variation, highlighting the dominant role of phenotypic plasticity for macrophyte acclimatation, whereas genetic contribution increased under experimentally manipulated conditions. Genome size and epigenetic variation influenced phenotypic plasticity; however, the effect was different and inconsistent on traits and depended on phylogenetic relationships and geographical environment variation. In field studies of natural populations, life history traits and hydrology had a strong effect on the geographic distribution of genetic diversity and the response to selection, as well as on our ability to distinguish selection from genetic drift. Future research should enhance molecular analyses, adopt multifactorial and long-term experimental designs, develop conceptual frameworks to address the relationships between genomics, environment and functional traits and integrate emerging tools to capture macrophyte adaptation better.

adaptation↗

PubMed2026

First Report and Integrated Characterization of Aeromonas veronii Associated with the Protected Fish Diptychus maculatus in Xinjiang, China.

Aeromonas veronii is a widely distributed opportunistic aquatic pathogen associated with diseases in freshwater fish. Despite the ecological and conservation significance of Diptychus maculatus, a protected cold-water fish inhabiting high-altitude ecosystems, information regarding its associated bacterial communities remains limited. This study aimed to isolate and characterize A. veronii recovered from D. maculatus and provide baseline information on its occurrence and phenotypic characteristics. Eight bacterial isolates were recovered from various tissues, including skin, gills, eye, intestine, dorsal fin, body kidney, gonad, and spleen of randomly sampled fish individuals from Xinjiang, China. Phenotypic and biochemical characterization, together with 16S rRNA sequencing, supported their identification as A. veronii, while gyrB analysis of a representative isolate provided additional species-level confirmation. Antimicrobial susceptibility testing revealed a consistent multidrug-resistance phenotype among all isolates. The isolates were susceptible to enrofloxacin, cefotaxime, ceftriaxone, and florfenicol. Intermediate responses were observed for ciprofloxacin, ofloxacin, doxycycline, oxytetracycline, and trimethoprim-sulfamethoxazole, whereas resistance was detected against norfloxacin, neomycin, penicillin, amoxicillin, tetracycline, and erythromycin. In vitro biofilm assays demonstrated weak to moderate biofilm-forming capacity among isolates. These findings provide baseline data for wildlife microbial surveillance and conservation-oriented monitoring of protected fish populations, supporting future investigations into environmental monitoring, genomic characterization, and host-microbe interactions.

Aeromonas veronii↗

PubMed1978

Organochlorine residues in aquatic environments in Iran, 1974.

Organochlorine pesticide residues in various organisms from different aquatic ecosystems in Iran were investigated in spring 1974. DDT levels were high in fish taken from two rivers in southern Iran, whereas low levels were detected in samples obtained from a freshwater lake in the same area. Fish from two of the reservoirs supplying Tehran with potable water contained moderate levels of DDT. The low residue level in pike collected in the Bandar-Pahlavi Mordab in northwest Iran indicates that only a small amount of organochlorine pesticides used in this area enters the pelagic food chain. Sturgeon collected at different places in the Caspian Sea showed similar accumulations of DDT in the muscles and in the eggs. Polychlorinated biphenyls (PCBs) were detected only in samples of sediment from the drainage systems in Tehran.

Aldrin↗

PubMed2026

Unraveling critical role of photosynthetic bacteria in sustaining aquatic microbial community stability and function through large-scale genomic data analyses.

The application of photosynthetic bacteria (PB) in water remediation has demonstrated exceptional advantages in terms of high efficiency and low-carbon benefits. However, the limited understanding of PB across natural aquatic environments has constrained the rational development of this strategy. Here, we analyzed 3198 genomic sequencing samples from seven types of natural aquatic ecosystems to investigate the distribution and functions of 42 PB genera commonly used in water remediation. The results showed that the average abundance of the targeted PB reached 9.83 %, with the highest value of 14.93 % observed in River, while Lake harbored the greatest PB genus diversity. PB genera exhibited high sensitivity to salinity, with Rhodoferax dominating freshwater habitats, whereas Rhodovulum was predominant in marine environments. Notably, co-occurrence network analysis revealed that PB were closely associated with microbial community stability and optimized interspecific interactions. Aquatic microbial communities with high PB abundance were characterized by efficient division-of-labor modules, accompanied by enhanced PB-associated functional potential for carbon fixation, denitrification, and sulfur oxidation. In summary, this study systematically elucidates the regional biogeographical patterns and ecological roles of PB in natural aquatic environments, providing a comprehensive scientific basis and theoretical guidance for the development and practical application of PB-based water remediation technologies.

Bacteria↗

PubMed2026

Widespread marine and freshwater distributions of active sulfoquinovose-degrading bacteria.

Sulfoquinovose (SQ), a sulfonated sugar produced on a gigaton scale each year, contributes to global sulfur cycling, yet the microbes and pathways mediating its turnover in the environment have been inferred largely from genomic potential rather than direct activity. Here, we coupled incubations of environmental samples with 13C-labeled SQ to DNA-stable isotope probing to identify active SQ carbon assimilators across estuary, mangrove, and lake ecosystems. In estuarine communities, Vibrio and Cognatishimia incorporated SQ-derived 13C; Novosphingobium dominated in the mangrove, and Agrobacterium in the lake. Pure-culture experiments, coupled with comparative proteomics and gene knockout validation, demonstrated that Vibrio strains degrade SQ via modified sulfoglycolytic Embden-Meyerhof-Parnas and Entner-Doudoroff pathways to produce the environmentally significant organosulfur 2,3-dihydroxypropanesulfonate. Comparative genomic analyses suggested that closely related genome representatives of Novosphingobium, Cognatishimia, and Agrobacterium encode the sulfolytic SQ monooxygenase pathway. A global survey of aquatic microbial genomes indicated that over 9% harbor SQ degradation clusters, supporting a widespread distribution of bacterial SQ catabolic potential in aquatic environments.

Fresh Water↗

PubMed2026

Comparative analysis of olfactory receptor repertoires reveals evolutionary dynamics and high-altitude adaptation in Schizopygopsis younghusbandi based on the chromosome-level genomes.

The olfactory receptor (OR) gene represent a significant multigene family in vertebrates, forming the core molecular basis of olfactory perception and playing a crucial role in the environmental adaptation of species. High-altitude ecosystems represent extreme habitats characterized by specific abiotic stresses, including low oxygen levels, low temperatures, and intense ultraviolet radiation. These environments also exhibit low aquatic biodiversity and a limited variety of odor molecules, factors that have influenced the adaptive evolution of the sensory systems in endemic species. However, the genetic mechanisms underlying olfactory adaptation in high-altitude freshwater fish remained inadequately understood. In this study, we performed comparative genomics analyses to reveal the evolutionary processes underlying the adaptive and functional evolution of OR genes in S. younghusbandi, a cyprinid fish endemic to the Qinghai-Xizang Plateau. The results indicated that, compared to their low-altitude relatives, S. younghusbandi possessed a significantly smaller number of OR genes, with only 98 genes, which revealed the contraction of the gene family. Phylogenetic analysis revealed that the OR genes of cyprinid fish could be categorized into two major lineages: type I and type II. The η and δ families, which perceive water-soluble odors, in S. younghusbandi underwent significant and specific expansion, while the ε family was completely absent. This pattern reflected adaptive changes in olfactory recognition to accommodate the simplified odor spectrum of high-altitude water bodies. Chromosomal localization analysis demonstrated that OR genes were clustered, and collinearity analysis confirmed the presence of conserved genomic fragments among species. Selection pressure analysis revealed that the Ka/Ks values of all homologous gene pairs were less than 1, indicating that the OR genes of S. younghusbandi underwent strong purifying selection as a group to preserve core olfactory function. A few genes exhibited relaxed selection characteristics, which may have facilitated the fine-tuning of adaptability to high-altitude environments. In conclusion, this study elucidated the evolutionary dynamics and adaptive characteristics of the OR gene in S. younghusbandi, offering a new perspective on the molecular mechanisms underlying olfactory adaptation at high altitudes and enriching the research on sensory evolution in vertebrates.

Schizopygopsis younghusbandi↗