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In this study, we determined the complete mitochondrial DNA sequence of the hybrid of Culter alburnus (♀) x Megalobrama terminalis (♂) for the first time. The complete mitochondrial genome of the hybrid was sequenced to be 16,622 bp in size following the female parent, C. alburnus. The genome contained 13 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes, and two main non-coding regions (the control region and the origin of light strand replication). Sequence alignment between the mitochondrial genomes of the hybrid and its female parent showed that a total of 35 mutation sites were identified in 14 genes or regions. The genome information presented here may play an important role in further study on the genetic mechanisms of mitochondrial DNA in hybrids.

In this study, we determined the complete mitochondrial DNA sequence of the hybrid of Megalobrama terminalis(♀) × Culter alburnus(♂) for the first time. The complete mitochondrial genome of the hybrid was sequenced to be 16,621 bp in size following the female parent, M. terminalis. The genome contained 13 protein-coding genes, 22 transfer RNA genes, 2 ribosomal RNA genes, and 2 main non-coding regions (the control region and the origin of light strand replication). Sequence alignment between the mitochondrial genomes of the hybrid and its female parent showed that a total of 28 mutation sites were identified in 14 genes or regions. The genome information presented here may play an important role in further study on the genetic mechanisms of mitochondrial DNA in hybrids.

Background Interspecific hybridization is a fundamental evolutionary process, yet it often results in male sterility due to genomic incompatibilities and disrupted epigenetic regulation. While fertility variation among hybrids is increasingly acknowledged, the precise mechanisms linking these molecular disruptions to reproductive outcomes in vertebrates remain underexplored. Results To address this, we investigated testicular chromatin dynamics and gene expression in 19 mature male F 1 hybrids derived from a paternal bluntnose black bream ( Megalobrama amblycephala , BSB) and a maternal topmouth culter ( Culter alburnus , TC). These two species diverged approximately 12.74 million years ago, and their F 1 hybrid offspring display a wide range of fertility. Our integrative multi-omics analysis revealed several insights into the molecular basis of hybrid fertility. Genes inherited from the BSB parent exert a disproportionate influence on reproductive outcomes, indicating an asymmetric parental contribution. Furthermore, chromatin architecture analysis showed that BSB-derived enhancer-promoter networks are characterized by longer interaction distances and greater regulatory strength, suggesting distinct subgenome-specific topologies. Then, our analysis identified the BSB-derived gene LOC125267388 as a critical regulator of fertility. Homologous to a retrotransposon esterase and containing conserved teleost motifs, this gene likely contributes to the epigenetic modulation of hybrid fertility. Conclusions These findings provide insights into the chromatin-mediated mechanisms underpinning reproductive barriers in vertebrate hybrids, contributing to a deeper understanding of evolutionary divergence and hybrid incompatibility.

Host-microbiota interactions play critical roles in host development, immunity, metabolism, and behavior. However, information regarding host-microbiota interactions is limited in fishes due to their complex living environment. In the present study, an allodiploid hybrid fish derived from herbivorous Megalobrama amblycephala (♀) × carnivorous Culter alburnus (♂) was used to investigate the successional changes of the microbial communities and host-microbiota interactions during herbivorous and carnivorous dietary adaptations. The growth level was not significantly different in any developmental stage between the two diet groups of fish. The diversity and composition of the dominant microbial communities showed similar successional patterns in the early developmental stages, but significantly changed during the two dietary adaptations. A large number of bacterial communities coexisted in all developmental stages, whereas the abundance of some genera associated with metabolism, including Acinetobacter , Gemmobacter , Microbacterium , Vibrio , and Aeromonas , was higher in either diet groups of fish. Moreover, the abundance of phylum Firmicutes, Actinobacteria, and Chloroflexi was positively correlated with the host growth level. In addition, Spearman’s correlation analysis revealed that the differentially expressed homologous genes in the intestine associated with cell growth, immunity, and metabolism were related to the dominant gut microbiota. Our results present evidence that host genetics-gut microbiota interactions contribute to dietary adaptation in hybrid fish, which also provides basic data for understanding the diversity of dietary adaptations and evolution in fish.

Aim Extreme droughts, intensified by climate change, threaten floodplain ecosystems. However, the mechanisms underlying biotic responses remain insufficiently understood. We assessed drought‐induced changes in the taxonomic and functional structure of fish communities, identified key environmental and biotic drivers during the drought period, and evaluated post‐drought community recovery trajectories. Location Poyang Lake, Yangtze River Basin, China. Methods Fish assemblages were surveyed across four floodplain lakes of Poyang Lake from 2022 to 2024 during the same seasonal window to isolate drought effects. Community composition and traits were analysed in multivariate trait space to assess changes in functional identity and the abundance of key species, and diversity indices at multiple scales were calculated. Key environmental drivers were identified using ordination and trait‐environment association analyses. Results Extreme drought induced substantial shifts in fish community structure and functional composition, favouring small‐bodied, benthopelagic omnivores and causing a 94% decline in the abundance of the apex predator Culter alburnus. Simultaneously, taxonomic and functional α‐diversity declined significantly, while β‐diversity increased, driven primarily by species turnover. The mechanisms underlying community assembly also shifted, with physicochemical factors predominant before the drought, but macrophyte growth form and macrobenthic biomass becoming the dominant drivers during drought conditions. Although hydrological connectivity was restored in 2024, community recovery remained incomplete, characterised by only partial functional trait rebound and persistent suppression of taxonomic richness, particularly among habitat specialists. Main Conclusions Extreme drought functioned as a basin‐scale filter, selectively favouring fish assemblages with drought‐tolerant trait combinations while leading to the decline or local extinction of intolerant taxa. The resulting habitat contraction and fragmentation reduced the local capacity to support biodiversity and promoted niche partitioning along gradients of habitat structure and food availability. Even after the restoration of hydrological connectivity, drought‐induced habitat legacies and priority effects constrained specialist recovery, impeding the re‐establishment of pre‐drought community composition.

This study aims to investigate the influence of the mixture (CGO/EWP) of carrageenan oligosaccharide (CGO) and egg white protein (EWP) (CGO/EWP, CGO: EWP = 1:1, m/m) on the functional, structural, and gelling properties of Culter alburnus myofibrillar protein (MP) during repeated freezing–thawing cycles by treating MP samples separately with EWP, CGO, or CGO/EWP based on the wet weight (1%, m/m), using samples without any cryoprotectant as the blank group. After the second repeated freezing–thawing cycle, the sulfhydryl group content was found to be significantly (p < 0.05) higher in the CGO/EWP (30.57 nmol/mg) and CGO (36.14 nmol/mg) groups than in the EWP group (23.80 nmol/mg), indicating that CGO/EWP and CGO can more effectively delay the oxidative deterioration of functional groups. Additionally, the surface hydrophobicity was shown to be significantly lower in the CGO (25.74) and CGO/EWP (27.46) groups than in the EWP (34.66) and blank (39.32) groups. Moreover, the α-helix content was higher in the CGO (35.2%) and CGO/EWP (32.3%) groups than in the EWP (29.2%) and blank (25.0%) groups. These data indicated that CGO and CGO/EWP could more effectively increase the structural stability, thereby inhibiting the exposure of hydrophobic groups and curbing the decline of α-helix content. During the heat-induced gel-forming process, EWP and CGO/EWP could enhance the gel viscoelasticity and strength. After the second freezing–thawing cycle, when compared with the blank group, the CGO/EWP group showed significantly (p < 0.05) higher water-holding capacity (66.30% versus 53.93%) and shorter T22 relaxation time (413.56 versus 474.99 ms). The integrated results indicated that CGO/EWP could more effectively delay the decrease of protein–water molecular interaction forces in the MP gel. This study shed light on the mechanism of CGO/EWP as a cryoprotective mixture in improving the deterioration of MP gelation properties during repeated freezing–thawing cycles.

Microsporidia of the genus Ovipleistophora are generally parasites of fishes and aquatic crustaceans. In the current study, Ovipleistophora diplostomuri and O. ovariae were firstly reported from Culter alburnus and Xenocypris argentea and Parabramis pekinensis, respectively. Both of them exclusively infected fish ovary and were morphologically, ultrastructurally and genetically characterized. Sporogony occurred in direct contact with the host cell cytoplasm and sporophorous vesicles were not observed for the new isolates of these two Ovipleistophora species. Spores of O. ovariae were for the first time observed to be dimorphic. Genetic analysis indicated that the genetic variation in the ITS and LSU sequences was distinct among between-host O. diplostomuri isolates. High sequence variation in ITS sequence suggests that it can be a reliable molecular marker to explore the population genetics of O. diplostomuri. This is the first report of these two Ovipleistophora species in China which extends their host and geographical range.

Long-read sequencing technologies can generate highly contiguous genome assemblies compared to short-read methods. However, their higher cost often poses a significant barrier. To address this, we explore the utilization of mapping-based genome assembly and reference-guided assembly as cost-effective alternative approaches. We assess the efficacy of these approaches in improving the contiguity of Clarias batrachus and Culter alburnus draft genomes. Our findings demonstrate that employing an iterative mapping strategy leads to a reduction in assembly errors. Specifically, after three iterations, the Mismatches per 100 kbp value for the C. batrachus genome decreased from 2447.20 to 2432.67, reaching a minimum of 2422.67 after two iterations. Additionally, the N50 value for the C. batrachus genome increased from 362,143 to 1,315,126 bp, with a maximum of 1,315,403 bp after two iterations. Furthermore, we achieved Mismatches per 100 kbp values of 3.70 for the reference-guided assembly of C. batrachus and 0.34 for C. alburnus. Correspondingly, the N50 value for the C. batrachus and C. alburnus genomes increased from 362,143 bp and 3,686,385 bp to 2,026,888 bp and 43,735,735 bp, respectively. Finally, we successfully utilized the improved C. batrachus and C. alburnus genomes to compare genome studies using the combined approach of Ragout and Ragtag. Through a comprehensive comparative analysis of mapping-based and reference-guided genome assembly methods, we shed light on the specific contributions of reference-guided assembly in reducing assembly errors and improving assembly continuity and integrity. These advancements establish reference-guided assembly and the utilization of in silico libraries as a promising and suitable approach for comparative genomics studies.

Fish ontogeny, allometric growth patterns, and otolith microstructure are fundamental in aquaculture and essential for understanding the early-life ecology of fish. Culter alburnus is a commercially important fish species and an excellent breeding target for aquaculture. In this study, newly hatched C. alburnus larvae were reared to the juvenile stage in a hatchery. Three days post hatch, the eyes, mouth, and intestine had developed, and the larval yolk sac had been completely absorbed. Fin rays started to differentiate in the flexion stage and were fully developed by the postflexion stage. Pigmentations were well developed in the juveniles. Culter alburnus larvae were characterized by 39–43 myomeres and 23–25 anal fin rays, swim bladder shape, and pigmentation. Head length, height, and musculature height showed positive allometric growth patterns, suggesting that head and trunk growth was prioritized. Positive allometric growth of the tail was simultaneous with the formation of fins, suggesting that swimming mode transformation, diet changes, and habitat shifts occurred after the start of the postflexion stage. Otolith growth increments in both the sagittae and lapilli were deposited daily, with the first increment formed on day 1 on the sagittae and day 4 of yolk sac absorption on the lapilli. Increments were thin and faint during the early developmental stage, gradually increasing to reach the broadest widths and the strongest contrast at the postflexion stage, which may be related to habitat shifts. This information will assist in the breeding and hatchery production of C. alburnus and establish suitable methods for analyzing this species’ spatiotemporal distributions and early-life traits.

Culter alburnus is sensitive to stressors. Arginine is a precursor of nitric oxide, which can effectively relieve the level of oxidative stress and improve the antioxidant and immune capacity of fish. The effect of different arginine levels on topmouth culter (Culter alburnus) fry development performance, liver antioxidant capacity, and immune parameters were investigated in this study. Five diets (1.96%, ARG1, control group; 2.28%, ARG2; 2.52%, ARG3; 2.81%, ARG4; 3.09%, ARG5) were used to feed fry (initial weight 0.31 ± 0.01 g) for 8 weeks. The data showed that the final weight (FW), weight gain rate (WGR), and specific growth rate (SGR) of the ARG3 and ARG4 groups were significantly improved, while the feed conversion ratio (FCR) reduced significantly. Compared with the ARG1 group, all groups remarkably reduced the crude ash content of the whole body. The activity of hepatic superoxide dismutase (SOD) and the content of hepatic glutathione (GSH) were significantly increased in the ARG3 and ARG4 groups, while the malondialdehyde (MDA) content was significantly decreased. Compared with the ARG1 group, arginine levels in ARG2, ARG3, and ARG4 groups up-regulated the expression levels of Nrf2, down-regulated the gene expression level of Keap1 in the liver. And the expression of Nrf2/Keap1 pathway downstream genes Mn-SOD and CAT was up-regulated in ARG2 and ARG3 groups. Furthermore, the expression levels of MyD88 and IL-1β were down-regulated, and the anti-inflammatory gene TGF-β expression levels were up-regulated in the ARG2, ARG3, and ARG4 groups. Additionally, compared to the ARG1 group, there was a significant increase in the relative expression levels of the C3 and C4 genes in the ARG4 group. In conclusion, 2.28–2.81% dietary arginine levels improved the growth performance, promoted antioxidant capacity, and enhance immune response. The optimal level of arginine was determined by the quadratic regression analysis of SGR and FCR to be 2.55% of diet (5.43% of dietary protein) and 2.53% of diet (5.38% of dietary protein), accordingly.