The Chinese concave‐eared frog ( Odorrana tormota ) is a rare and threatened species with remarkable sexual dimorphism. Intestinal microbes are understood to play important roles in animal physiology, growth, ecology, and evolution. However, little is known about the intestinal microbes in female and male frogs, as well as the contributing effect by gut infesting nematodes to the co‐habiting bacteria and their function in degradation food rich in chitin. Here, this study analyzed the microbiota of the intestinal tract of both female and male, healthy as well as nematode‐infested concave‐eared frogs using high throughput 16S rRNA sequencing and metagenomic techniques. The results showed that the bacterial composition of the microbiota at the phylum level was dominated by Firmicutes, Verrucomicrobia, Bacteroidetes, and Proteobacteria. The study also revealed that the community composition below the class level could be represent sex differences, particularly with regard to Enterobacteriales, Enterobacteriaceae, Peptostreptococcaceae, and Rikenellaceae, among others. Carbohydrate‐active enzyme‐encoding genes and modules were identified in related gut bacteria by metagenomic analysis, with Bacteroidia, Clostridia, and gammaproteobacteria predicted to be the main classes of chitin‐decomposing bacteria in the frog intestine. In addition, the abundance of some bacteria significantly increased or decreased in nematode‐infected hosts compared with healthy individuals, including Verrucomicrobia, Verrucomicrobiae, Negativicutes, Actinobacteria, and Bacilli, among others. This indicates that nematode infection may affect the richness and composition of some gut bacteria.

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The Chinese concave-eared torrent frog (Odorrana tormota) was the first non-mammalian vertebrate demonstrated to both produce and perceive ultrasonic frequencies. Due to habitat degradation caused by human activity, currently, the species is at the edge of extinction. In this study, we demonstrated that the species harbors at least four expressed major histocompatibility complex (MHC) class II B loci. Four distinct intron 2 sequences and three intron 1 sequences were isolated from genomic DNA. Additionally, eight alleles were recovered from three cDNA samples, of which five could be assigned to three individual loci while the locus identity of the remaining three alleles could not be determined. Based on the intron sequences, locus-specific primers were designed for Odto-A, Odto-B, and Odto-C. Of these loci, Odto-B was monomorphic in Huangshan population (n = 32), and Odto-C displayed low allelic polymorphism with only three similar sequences obtained from partial individuals. In contrast, Odto-A showed high allelic diversity with 20 unique alleles recovered from the population. No inter- or intra-allele recombination was detected. However, 10 positive selected amino acid residues and an excess of non-synonymous substitutions were detected in the putative antigen binding sites (ABS), these results combined with trans-species evolution suggested some form of balancing selection that maintained higher allelic polymorphism for the locus. Our findings demonstrate that despite the effects of a severely shrinking population, O. tormota exhibits high polymorphism compared to other amphibians, both in terms of the allele number and diversity across the MHC class II B loci.

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The responses of denitrifiers and denitrification ability to dissolved oxygen (DO) concent in different layers of surface lake sediments are still poorly understood. Here, the optimal denitrification condition was constructed based on response surface methodology (RSM) to analyze the denitrification characteristics of surface sediments. The aerobic zone (AEZ), hypoxic zone (HYZ), up-anoxic zone (ANZ-1) and sub-anoxic zone (ANZ-2) were partitioned based on the oxygen contents, and sediments were collected using a customized-designed sub-millimeter scale sampling device. Integrated real-time quantitative PCR, Illumina Miseq-based sequencing and denitrifying enzyme activities analysis revealed that denitrification characteristics varied among different DO layers. Among the four layers, the DNA abundance and RNA expression levels of norB , nirS and nosZ were the highest at the aerobic layer, hypoxic layer and up-axoic layer, respectively. The hypoxia and up-anaerobic layer were the active nitrogen removal layers, since these two layers displayed the highest DNA abundance, RNA expression level and enzyme activities of denitrification functional genes. The abundance of major denitrifying bacteria showed significant differences among layers, with Azoarcus , Pseudogulbenkiania and Rhizobium identified as the main nirS , nirK and nosZ -based denitrifiers. Pearson’s correlation revealed that the response of denitrifiers to environmental factors differed greatly among DO layers. Furthermore, napA showed higher DNA abundance and RNA expression level in the aerobic and hypoxic layers than anaerobic layers, indicating that aerobic denitrifiers might play important roles at these layers. Electronic supplementary material The online version of this article (10.1186/s13568-019-0855-9) contains supplementary material, which is available to authorized users.

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