Species specificity of O-linked carbohydrate chains of the oviducal mucins in amphibians: structural analysis of neutral oligosaccharide alditols released by reductive beta-elimination from the egg-jelly coats of Rana clamitans

De novo oviduct transcriptome of the moor frog Rana arvalis: a quest for maternal effect candidate genes

Maternal effects can substantially affect ecological and evolutionary processes in natural populations. However, as they often are environmentally induced, establishing their genetic basis is challenging. One important, but largely neglected, source of maternal effects are egg coats (i.e., the maternally derived extracellular matrix that surrounds the embryo). In the moor frog, the gelatinous egg coats (i.e., egg jelly) are produced in the mother’s oviduct and consist primarily of highly glycosylated mucin type O-glycans. These O-glycans affect jelly water balance and, subsequently, contribute to adaptive divergence in embryonic acid tolerance. To identify candidate genes for maternal effects, we conducted RNAseq transcriptomics on oviduct samples from seven R. arvalis females, representing the full range of within and among population variation in embryonic acid stress tolerance across our study populations. De novo sequencing of these oviduct transcriptomes detected 124,071 unigenes and functional annotation analyses identified a total of 57,839 unigenes, of which several identified genes likely code for variation in egg jelly coats. These belonged to two main groups: mucin type core protein genes and five different types of glycosylation genes. We further predict 26,711 gene-linked microsatellite (simple sequence repeats) and 231,274 single nucleotide polymorphisms. Our study provides the first set of genomic resources for R. arvalis, an emerging model system for the study of ecology and evolution in natural populations, and gives insight into the genetic architecture of egg coat mediated maternal effects.

Interacting symbionts and immunity in the amphibian skin mucosome predict disease risk and probiotic effectiveness

Pathogenesis is strongly dependent on microbial context, but development of probiotic therapies has neglected the impact of ecological interactions. Dynamics among microbial communities, host immune responses, and environmental conditions may alter the effect of probiotics in human and veterinary medicine, agriculture and aquaculture, and the proposed treatment of emerging wildlife and zoonotic diseases such as those occurring on amphibians or vectored by mosquitoes. Here we use a holistic measure of amphibian mucosal defenses to test the effects of probiotic treatments and to assess disease risk under different ecological contexts. We developed a non-invasive assay for antifungal function of the skin mucosal ecosystem (mucosome function) integrating host immune factors and the microbial community as an alternative to pathogen exposure experiments. From approximately 8500 amphibians sampled across Europe, we compared field infection prevalence with mucosome function against the emerging fungal pathogen Batrachochytrium dendrobatidis. Four species were tested with laboratory exposure experiments, and a highly susceptible species, Alytes obstetricans, was treated with a variety of temperature and microbial conditions to test the effects of probiotic therapies and environmental conditions on mucosome function. We found that antifungal function of the amphibian skin mucosome predicts the prevalence of infection with the fungal pathogen in natural populations, and is linked to survival in laboratory exposure experiments. When altered by probiotic therapy, the mucosome increased antifungal capacity, while previous exposure to the pathogen was suppressive. In culture, antifungal properties of probiotics depended strongly on immunological and environmental context including temperature, competition, and pathogen presence. Functional changes in microbiota with shifts in temperature provide an alternative mechanistic explanation for patterns of disease susceptibility related to climate beyond direct impact on host or pathogen. This nonlethal management tool can be used to optimize and quickly assess the relative benefits of probiotic therapies under different climatic, microbial, or host conditions.

The amphibian skin-associated microbiome across species, space and life history stages

Skin-associated bacteria of amphibians are increasingly recognized for their role in defence against pathogens, yet we have little understanding of their basic ecology. Here, we use high-throughput 16S rRNA gene sequencing to examine the host and environmental influences on the skin microbiota of the cohabiting amphibian species Anaxyrus boreas, Pseudacris regilla, Taricha torosa and Lithobates catesbeianus from the Central Valley in California. We also studied populations of Rana cascadae over a large geographic range in the Klamath Mountain range of Northern California, and across developmental stages within a single site. Dominant bacterial phylotypes on amphibian skin included taxa from Bacteroidetes, Gammaproteobacteria, Alphaproteobacteria, Firmicutes, Sphingobacteria and Actinobacteria. Amphibian species identity was the strongest predictor of microbial community composition. Secondarily, within a given amphibian species, wetland site explained significant variation. Amphibian-associated microbiota differed systematically from microbial assemblages in their environments. Rana cascadae tadpoles have skin bacterial communities distinct from postmetamorphic conspecifics, indicating a strong developmental shift in the skin microbes following metamorphosis. Establishing patterns observed in the skin microbiota of wild amphibians and environmental factors that underlie them is necessary to understand skin symbiont community assembly, and ultimately, the role skin microbiota play in the extended host phenotype including disease resistance.

Tracing the history of Galalpha1-4Gal on glycoproteins in modern birds

Galalpha1-4Gal is typically found in mammalian glycolipids in small quantities, and recognized by some pathogens, such as uropathogenic Escherichia coli. In contrast, glycoproteins containing Galalpha1-4Gal were rarely found in vertebrates except in a few species of birds and amphibians until recently. However, we had previously reported that pigeon (Columba livia) egg white and serum glycoproteins are rich in N-glycans with Galalpha1-4Gal at non-reducing termini. Our investigation with egg white glycoproteins from 181 avian species also revealed that the distribution of (Galalpha1-4Gal)-containing glycoproteins was not rare among avians, and is correlated with the phylogeny of birds. The differentiated expression was most likely emerged at earlier stage of diversification of modern birds, but some birds might have lost the facility for the expression relatively recently.

Phylogenetic expression of Galalpha1-4Gal on avian glycoproteins: glycan differentiation inscribed in the early history of modern birds

Glycoproteins containing Galalpha1-4Gal (galabiose) had been rarely found in vertebrates, except in a few species of birds and amphibians. We had previously reported that pigeon (Columba livia) egg white and serum glycoproteins are rich in N-glycans with Galalpha1-4Gal at nonreducing termini. To investigate the origin of Galalpha1-4Gal expression in avian evolution, we examined the presence of Galalpha1-4Gal glycoproteins in egg whites from 20 orders, 88 families, 163 genera, and 181 species of birds, as probed by Western blot with Griffonia simplicifolia-I lectin (terminal alpha-Gal/GalNAc-specific) and anti-P(1) mAb (Galalpha1-4Galbeta1-4GlcNAcbeta1-specific). One of the significant observations is the total absence of Galalpha1-4Gal glycoproteins in Struthioniformes (four species), Tinamiformes (three species), Craciformes (two species), Galliformes (14 species), and Anseriformes (10 species), which are phylogenetically separated from other orders at earlier stage of modern bird diversification (100-65 million years ago). The presence or absence of Galalpha1-4Gal glycoproteins in other avian orders varied by the species (104 species positive, and 44 species negative), even though some of them belong to the same order or family. Our results revealed that the expression of Galalpha1-4Gal glycoproteins is not rare among avians, and is correlated with the phylogeny. The expression was most likely differentiated at earlier stage of diversification in modern birds, but some birds might have lost the facility for the expression relatively recently.

Structural analysis of the oligosaccharide alditols released from the jelly coat of Rana dalmatina eggs by reductive beta-elimination

A combination of ion-exchange chromatography and high performance liquid chromatography (HPLC) has been used to separate the reduced oligosaccharides produced by alkaline borohydride degradation of oviducal mucins obtained from the jelly coat of Rana dalmatina. The primary structures of 26 O-glycans were determined by one-dimensional and two-dimensional 1H and 1H13C NMR spectroscopy. As observed for 20 other amphibian species, these carbohydrate chains are highly species-specific. The main typical feature of the species R. dalmatina consists in the presence of the backbone Gal(beta1-3)[Gal(beta1-4)]Gal(beta1-3)GalNAc-ol, previously observed among Ranidae, such as R. temporaria and R. ridibunda. Nevertheless, the nature of carbohydrates present at the periphery of the glycans perfectly differentiates the three species.