Consumption of toxic benthic cyanobacteria by two common demersal fish: Growth, antioxidant and liver histopathology responses.

Benthic freshwater cyanobacteria have the potential to produce toxins. Compared with more extensively studied plankton species, little is known about the impact of harmful benthic cyanobacteria on aquatic organisms. As demersal fish are usually in direct contact with benthic cyanobacteria, it is important to understand their interactive effects. This study investigated the physio-chemical responses of two demersal fish (Xenocypris davidi and Crucian carp) after exposure to benthic Oscillatoria (producing cylindrospermopsin, 2 × 106 cells/mL) for 7 days. Interestingly, benthic Oscillatoria had less adverse effects on X. davidi than C. carp. The two demersal fish effectively ingested Oscillatoria, but Oscillatoria cell sheathes could not be fully digested in C. carp intestines and led to growth inhibition. Oscillatoria consumption induced oxidative stress and triggered alterations in detoxification enzyme activities in the X. davidi liver. Superoxide dismutase (SOD) and glutathione reductase (GR) activities significantly increased in the C. carp liver, but catalase (CAT) and detoxification enzymes glutathione S-transferase (GST) and glutathione (GSH) activities were insignificantly changed. This suggested that C. carp may have a relatively weak detoxification capacity for toxic Oscillatoria. Oscillatoria ingestion led to more pronounced liver pathological changes in C. carp, including swelling, deformation, and loss of cytoskeleton structure. Simultaneously, fish consumption of Oscillatoria increased extracellular cylindrospermopsin concentration. These results provide valuable insights into the ecological risks associated with benthic cyanobacteria in aquatic ecosystems.

Comparative transcriptome analyses of the liver between Xenocypris microlepis and Xenocypris davidi under low copper exposure.

Copper is one of the most ubiquitous environmental pollutants worldwide. Previous studies have focused on the toxicology of high copper exposure, while there has been comparatively less research on the biological effects of low copper exposure. Low concentrations of copper often exist in freshwater ecosystems, and its impact on the fish is unclear. Both Xenocypris microlepis and Xenocypris davidi are bottom-feeding fishes widely distributed in freshwater ecosystems of China, and they are more likely to be contaminated by low concentrations of copper. Low copper exposure may have effects on molecular regulation at the level of gene expression in the two Xenocypris species. To investigate gene expression differences involved in the response to low copper concentrations between X. microlepis and X. davidi, we established the responses to low copper exposure of 0.01 mg/L for 14 days at the transcriptional level, and RNA-Seq was used to perform a comparative transcriptomic analysis of the liver. A total of 74,135 and 60,894 unigenes from X. microlepis and X. davidi were assembled by transcriptome profiling, respectively. Among these, 84 genes of X. microlepis and 165 genes of X. davidi were identified as differentially expressed genes (DEGs). There were 60 and 135 up-regulated, 24 and 30 down-regulated genes in the two species, respectively. Comparative transcriptome analyses identified five differentially co-expressed genes (DCGs) related to low copper exposure from the DEGs of the two Xenocypris species. The five DCGs were related to the fishes' growth, antioxidant system, immune system and heavy metal tolerance. The results could help us to understand the molecular mechanisms of the response to low copper exposure, and the data should provide a valuable transcriptomic resource for the genus Xenocypris.

Isolation and characterization of 20 polymorphic microsatellites loci for Xenocypris davidi based on high-throughput sequencing.

Xenocypris davidi is one of the most economically important freshwater fish in China. However, few molecular markers have been reported for this species, impeding in-depth population genetic, dispersal, and gene flow studies. In the present study, a batch of novel polymorphic microsatellites from the genome of X. davidi were isolated and characterized using high-throughput sequencing. A total of 20 microsatellite markers were isolated. Analysis of 33 individuals revealed an average of 7.35 alleles per locus, ranging from 3 to 18. The observed and expected heterozygosities ranged from 0.3 to 1 and from 0.426 to 0.93, respectively. Only one tested locus significantly deviated from Hardy-Weinberg equilibrium. 18 microsatellite loci were highly polymorphic (PIC > 0.5). These newly isolated microsatellite markers would be useful to study the population genetics and stock management of X. davidi.

Molecular identification and phylogenetic analysis of mitogenome of the Xenocypris davidi from Cao'e River.

In this study, the complete mitochondrial genome sequence of a Xenocypris davidi from Cao'e River was sequenced. The complete mitogenome of X. davidi was 16,630 bp in length, it contains the structure of 22 transfer RNA genes, 13 protein coding genes, 2 ribosomal RNA genes, and 1 non-coding region. The gene arrangement and organization in the mitogenome of X. davidi were in accordance with other Cyprinidae fishes. The results of phylogenetic analysis revealed that the mitochondrial genome sequence could provide useful information for the conservation genetics and evolution study of X. davidi.

The complete mitochondrial genome sequence of Xenocypris davidi (Bleeker).

Xenocypris davidi is a member of Cyprindae and widely distributed in China. To understand the systematic status of this species, we sequenced the whole mitochondrial genome of Xenocypris davidi. The complete mitochondrial genome is 16,630 bp in length including the typical structure of 22 tRNA, 2 rRNA, 13 protein-coding genes and the non-coding region. The major non-coding sequence which is the control region containing 6 CSBs (CSB-1, CSB-2, CSB-3, CSB-D, CSB-E and CSB-F). The second non-coding sequence is the origin of light-strand replication (OL). This region has the potential to fold in a step-loop secondary structure. The mitochondrial genomic sequence will help us to study the conservation genetic and evolution of Xenocypris.