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Alternative splicing is an important way of regulating gene functions in eukaryotes. Several key genes involved in sex determination and gonadal differentiation, such as nr5a1 and ddx4, have sex-biased transcripts between males and females, suggesting a potential regulatory role of alternative splicing in gonads. Currently, the sex-specific alternative splicing events and genes have not been comprehensively studied at the genome-wide level in zebrafish. In this study, through global splicing analysis on three independent sets of RNA-seq data from matched zebrafish testes and ovaries, we identified 120 differentially spliced genes shared by the three datasets, most of which haven't been reported before. Functional enrichment analysis showed that the GO terms of mRNA processing, mRNA metabolism and microtubule-based process were strongly enriched. The testis- and ovary-biased alternative splicing genes were identified, and part of them (tp53bp1, tpx2, mapre1a, kif2c, and ncoa5) were further validated by RT-PCR. Sequence characteristics analysis suggested that the lengths, GC contents, and splice site strengths of the alternative exons or introns may have different influences in different types of alternative splicing events. Interestingly, we identified an unexpected high proportion (over 70%) of non-frameshift exon-skipping events, suggesting that in these cases the two protein isoforms derived from alternative splicing may both have functions. Furthermore, as a representative example, we found that the alternative splicing of ncoa5 causes the loss of a conserved RRM domain in the short transcript predominantly produced in testes. Our study discovers novel sex-specific alternative splicing events and genes with high reliabilities in zebrafish testes and ovaries, which would provide attractive targets for follow-up studies to reveal the biological significances of alternative splicing events and genes in sex determination and gonadal differentiation. [ABSTRACT FROM AUTHOR]

Columnaris disease, induced by Flavobacterium columnare , seriously affects the health of freshwater fish species and damages the mucosal tissues, such as the fins, skin, and gills. Teleosts represent the first bony vertebrate to contain both innate and adaptive immune responses against pathogens. So far, three immunoglobulin isotypes (IgM, IgD, and IgT/IgZ) have been identified in teleost fish, and IgT in mucosal tissues of teleost fish was reported to perform a similar function to IgA in mammals during parasitic infection. However, very limited information is known about the function of IgT in gill mucosal tissues during bacterial infection. In the present study, rainbow trout (Oncorhynchus mykiss) was infected with F. columnare (Fc) via immersion. After Fc infection, the gill structure of rainbow trout showed serious hyperplasia symptoms on the secondary lamellae at 12 h post infection (hpi). Moreover, the mRNA expression levels of NOS2 and cathelicidin-1 were significantly upregulated immediately at 12 hpi and showed high expression throughout the experiment. IgT and IgM showed much higher mRNA expression levels at 28 days post infection (dpi) and 75 dpi, while IgD only showed high mRNA expression levels at 28 dpi. Importantly, the accumulation of IgT+ B cells and strong bacteria-specific IgT responses were detected in the gill lamellae of both infected fish (28 dpi) and survivor fish (75 dpi). Overall, our results suggest that IgT and IgT+ B cells play a central role in the adaptive immune responses of fish gill mucosa against bacterial infection. • Immune functions of IgT, IgM and IgD in gill of trout were illustrated after challenged with bacterial pathogens. • Strong immune responses elicited in trout gill after Flavobacterium columnare infection. • IgT and IgM played an important role against bacteria in fish gill mucosal and systemic immunity, respectively. [ABSTRACT FROM AUTHOR]

Swamp Eels (M. albus) is one of the high-quality freshwater fish with fairly high nutritive value in China. Dissecting the intrinsic variations of digestive tract microbiota in different sizes of M. albus is beneficial for the development of its yield. Here, we generated the sibling generation of M. albus , screened the large-sized and small-sized individuals, and collected the digestive tract including the mouth and gut for investigating the effect of bacterial communities on individual growth pattern. After 16S rRNA sequencing and diversity analyzing, we found the microbial diversity and richness were significant differences in the mouth, whereas almost no difference in the gut between small and large M. albus. Despite similar bacterial composition, a significant proportion difference of bacterial communities was observed between small and large individuals both in mouth and in gut. Specifically, a significant abundance of Burkholderiales were distributed in the small M. albus while Clostridiales had remarkably higher proportions in larger population. In addition, almost all Fusobacteria detected in the gut of large group belonged to the genus Cetobacterium. KEGG analysis further predicted the unique proportions of bacterial communities may be related to individual metabolism such as nucleotide, carbohydrate, glycan biosynthesis, and lipid metabolism. Interestingly, a potential compensatory relationship of bacterial community may exist between the mouth and gut in M. albus. Taken together, these results provide a novel insight and resources for investigating the underlying relationship between the bacterial microbiota and the growth differences in the farmed population, and provide a theoretical guidance for the exploitation of probiotic products in aquaculture. • Bacterial abundance and community diversity of mouth are present at higher levels in larger individuals of sibling M. albus. • The difference of growth traits in sibling M. albus can be attributed to alterations in the composition and functional potentials of the digestive tract bacterial microbiota. • A potential compensatory relationship of bacterial community may exist between the mouth and gut in M. albus. [ABSTRACT FROM AUTHOR]

Emerging evidence suggests that bitter and sweet Taste receptors (TRs) in the airway are important sentinels of innate immunity. TRs are G protein-coupled receptors that trigger downstream signaling cascades in response to activation of specific ligands. Among them, the T1R family consists of three genes: T1R1, T1R2, and T1R3, which function as heterodimers for sweet tastants and umami tastants. While the other TRs family components T2Rs function as bitter tastants. To understand the relationship between TRs and mucosal immunity in teleost, here, we firstly identified and analyzed the molecular characteristics of three TRs (T1R1, T1R3, and T2R4) in rainbow trout (Oncorhynchus mykiss). Secondly, by quantitative real-time PCR (qPCR), we detected the mRNA expression levels of T1R1, T1R3 and T2R4 and found that the three genes could be tested in all detected tissues (pharynx, buccal cavity, tongue, nose, gill, eye, gut, fin, skin) and the expression levels of T1R3 and T2R4 were higher in buccal mucosa (BM) and pharyngeal mucosa (PM) compare to other tissues. It may suggest that T1R3 and T2R4 play important roles in BM and PM. Then, to analyses the changes of expression levels of the three genes in rainbow trout infected with pathogens, we established three infection models Flavobacterium columnare (F. cloumnare), infectious hematopoietic necrosis virus (IHNV) and Ichthyophthirius multifiliis (Ich). Subsequently, by qPCR, we detected the expression profiles of TRs in the gustatory tissues (BM, PM and skin) of rainbow trout after infection with F. cloumnare , IHNV, and Ich, respectively. We found that under three different infection models, the expression of the T1R1, T1R3 and T2R4 showed their own changes in mRNA levels. And the expression levels of the T1R1, T1R3 and T2R4 changed significantly at different time points in response to three infection models, respectively, suggesting that TRs may be associated with mucosal immunity. • Three infection models with bacterial, viral and parasitic pathogens were successfully established. • Significantly expression of taste receptor genes occurred in trout gustatory tissues after infected with pathogens. • Taste receptor genes may be associated with mucosal immune response in teleost. [ABSTRACT FROM AUTHOR]

Organisms are colonized by microorganism communities and play a pivotal role in host function by influencing physiology and development. In mammals, bacterial community may alter gonadal maturation and drive sex-specific differences in gene expression and metabolism. However, bacterial microbiota diversity in the gonads of early vertebrates has not been fully elucidated. Here, we focused on the swamp eel ( Monopterus albus ), which naturally undergoes sex reversal, and systematically analyzed the bacterial microbiota profiles between females and males using 16S rRNA gene sequences. Specifically, the microbial abundance and community diversity of gonads in males were higher than in females. Although Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria were characterized as the dominating phyla in ovary and testis, the relative abundance of Firmicutes was significantly higher in males than females. Detailed analysis of the microbial community revealed that Bacilli were the dominant bacteria in ovaries and Clostridium in testes of M. albus . More importantly, we proposed that differences in the microbial composition and distribution between ovaries and testes may be linked to functional categories in M. albus , especially metabolism. These findings represent a unique resource of bacterial community in gonads to facilitate future research about the mechanism of how microbiota influence sex-specific differences and sex reversal in vertebrates., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Meng, Lin, Liu, Chen, Liu, Xu, Sun and Luo.)

The immunoglobulin (Ig) is a crucial component of adaptive immune system in vertebrates including teleost fish. Here complete cDNA sequence of IgD heavy chain gene from common carp ( Cyprinus carpio ) was cloned and analyzed. The full-length cDNA of IgD heavy chain gene contained an open reading frame (ORF) of 2460 bp encoding 813 amino acids. According to amino acids sequence, multiple alignment and phylogenetic analysis showed that carp Igs are closely related to those of Cyprinidae fish. Transcriptional expression of IgD as well as IgM, IgZ1 and IgZ2 showed similar expression patterns in different organs, this is, high expression level in systemic immune tissues (ie, head kidney, heart and spleen) and low expression in mucosal tissues (ie, gill, skin and gut). Following viral infection with spring viraemia of carp virus (SVCV), obvious pathological changes in skin, gill and gut mucosa and up-regulated expression of antiviral related genes in skin, gill, gut and spleen were observed, indicating that SVCV successfully infected common carp and activated the systemic and mucosal immune system. Interestingly, IgM showed a significant up-regulation only in systemic tissue (spleen), but not in mucosal tissues (gut, gills and skin), while increased expression of IgZ1 and IgZ2 was found in gut. In contrast, the expression of IgD increased significantly in spleen, gills and skin. These strongly suggest that fish Ig isotypes play different roles in mucosal and systemic immunity during viral infection. Common carp (Cyprinus carpio); Igs; Spring viraemia of carp virus (SVCV)., Competing Interests: None., (© 2021 The Author(s).)