Signatures of historical selection on MHC reveal different selection patterns in the moor frog (Rana arvalis)

Batrachochytrium dendrobatidis strain affects transcriptomic response in liver but not skin in latitudinal populations of the common toad (Bufo bufo)

Batrachochytrium dendrobatidis (Bd) is a fungal pathogen that has decimated amphibian populations worldwide for several decades. We examined the changes in gene expression in response to Bd infection in two populations of the common toad, Bufo bufo, in a laboratory experiment. We collected B. bufo eggs in southern and northern Sweden, and infected the laboratory-raised metamorphs with two strains of the global panzoonotic lineage Bd-GPL. Differential expression analysis showed significant differences between infected and control individuals in both liver and skin. The skin samples showed no discernible differences in gene expression between the two strains used, while liver samples were differentiated by strain, with one of the strains eliciting no immune response from infected toads. Immune system genes were overexpressed in skin samples from surviving infected individuals, while in liver samples the pattern was more diffuse. Splitting samples by population revealed a stronger immune response in northern individuals. Differences in transcriptional regulation between populations are particularly relevant to study in Swedish amphibians, which may have experienced varying exposure to Bd. Earlier exposure to this pathogen and subsequent adaptation or selection pressure may contribute to the survival of some populations over others, while standing genetic diversity in different populations may also affect the infection outcome.

Association between the skin microbiome and MHC class II diversity in an amphibian

Microbiomes play an important role in determining the ecology and behaviour of their hosts. However, questions remain pertaining to how host genetics shape microbiomes, and how microbiome composition influences host fitness. We explored the effects of geography, evolutionary history and host genetics on the skin microbiome diversity and structure in a widespread amphibian. More specifically, we examined the association between bacterial diversity and composition and the major histocompatibility complex class II exon 2 diversity in 12 moor frog (Rana arvalis) populations belonging to two geographical clusters that show signatures of past and ongoing differential selection. We found that while bacterial alpha diversity did not differ between the two clusters, MHC alleles/supertypes and genetic diversity varied considerably depending on geography and evolutionary history. Bacterial alpha diversity was positively correlated with expected MHC heterozygosity and negatively with MHC nucleotide diversity. Furthermore, bacterial community composition showed significant variation between the two geographical clusters and between specific MHC alleles/supertypes. Our findings emphasize the importance of historical demographic events on hologenomic variation and provide new insights into how immunogenetic host variability and microbial diversity may jointly influence host fitness with consequences for disease susceptibility and population persistence.

Evidence for selective sweeps in the MHC gene repertoire of various cattle breeds

Major Histocompatibility Complex (MHC) genes are among the immune genes that have been extensively studied in vertebrates and are necessary for adaptive immunity. In the immunological response to infectious diseases, they play several significant roles. This research paper provides the selection signatures in the MHC region of the bovine genome as well as how certain genes related to innate immunity are undergoing a positive selective sweep. Here, we investigated signatures of historical selection on MHC genes in 15 different cattle populations and a total of 427 individuals. To identify the selection signatures, we have used three separate summary statistics. The findings show potential selection signatures in cattle from whom we isolated genes involved in the MHC. The most significant regions related to the bovine MHC are BOLA, non-classical MHC class I antigen (BOLA-NC1), Microneme protein 1 (MIC1) , Cluster of Differentiation 244 (CD244), Gap Junction Alpha-5 Protein (GJA5). It will be possible to gain new insight into immune system evolution by understanding the distinctive characteristics of MHC in cattle.

Amphibians and reptiles in North Sweden: distribution, habitat affinities, and abundance (Classes: Amphibia and Reptilia)

North Sweden ('Norrland' in Swedish) covers 243 000 km2 and lies mainly in the boreal biome. The herpetofauna comprises five amphibian and four reptile species: Lissotriton vulgaris, Triturus cristatus, Bufo bufo, Rana temporaria, Rana arvalis, Zootoca vivipara, Anguis fragilis, Natrix natrix, and Vipera berus. Successful conservation and management of amphibians and reptiles depend on accurate information about distribution, habitat affinities, and abundance. Such knowledge is also essential as a benchmark to assess changes in distribution and abundance that may come about as a result of climate change and human habitat alteration. This paper aims to present accurate distribution maps, describe habitat affinities, and provide abundance estimates for the herpetofauna of North Sweden for the period 1970-2022. Distribution data are presented by traditional faunistic provinces, as well as by biotic regions and alpine life zones. Separate sections address post-glacial colonization and a herpetological perspective on anthropogenic changes in relation to species´ present status. Bufo bufo, Rana temporaria, Rana arvalis, Zootoca vivipara, and Vipera berus are widely distributed throughout the boreal sub-regions. Rana temporaria, Zootoca vivipara, and Vipera berus also extend into the alpine region. Triturus cristatus, Anguis fragilis, and Natrix natrix occur mainly in the coastal parts of the Southern Boreal region. There are no signs of recent changes in distribution range, but Lissotriton vulgaris, Triturus cristatus, and Rana arvalis have been largely overlooked in the past and have a much wider occurrence than previously recognized. Most species are found in habitats usually not described in all-European field guides. Nearly all anurans hibernate in water. Abundance estimates suggest that some species are more common in the boreal than thought, supporting the notion that a large share of their total European population occurs there. Although local extinctions and declines are known, there are no signs of widespread population decline for any species during the study period.

Understanding the evolution of immune genes in jawed vertebrates

Driven by co-evolution with pathogens, host immunity continuously adapts to optimize defence against pathogens within a given environment. Recent advances in genetics, genomics and transcriptomics have enabled a more detailed investigation into how immunogenetic variation shapes the diversity of immune responses seen across domestic and wild animal species. However, a deeper understanding of the diverse molecular mechanisms that shape immunity within and among species is still needed to gain insight into-and generate evolutionary hypotheses on-the ultimate drivers of immunological differences. Here, we discuss current advances in our understanding of molecular evolution underpinning jawed vertebrate immunity. First, we introduce the immunome concept, a framework for characterizing genes involved in immune defence from a comparative perspective, then we outline how immune genes of interest can be identified. Second, we focus on how different selection modes are observed acting across groups of immune genes and propose hypotheses to explain these differences. We then provide an overview of the approaches used so far to study the evolutionary heterogeneity of immune genes on macro and microevolutionary scales. Finally, we discuss some of the current evidence as to how specific pathogens affect the evolution of different groups of immune genes. This review results from the collective discussion on the current key challenges in evolutionary immunology conducted at the ESEB 2021 Online Satellite Symposium: Molecular evolution of the vertebrate immune system, from the lab to natural populations.

Salamanders reveal novel trajectories of amphibian MHC evolution

Genes of the major histocompatibility complex (MHC) code for immune proteins that are crucial for pathogen recognition in vertebrates. MHC research in nonmodel taxa has long been hampered by its genomic complexity that makes the locus-specific genotyping challenging. The recent progress in sequencing and genotyping methodologies allows an extensive phylogenetic coverage in studies of MHC evolution. Here, we analyzed the peptide-binding region of MHC class I (MHC-I) in 30 species of salamanders from six families representative of Urodela phylogeny. This extensive dataset revealed an extreme diversity of MHC-I in salamanders, both in terms of sequence diversity (about 3000 variants) and architecture (2-22 gene copies per species). The signal of positive selection was moderate and consistent between both peptide-binding domains, but varied greatly between genera. Positions of positively selected sites mostly coincided with human peptide-binding sites, suggesting similar structural properties of MHC-I molecules across distant vertebrate lineages. Finally, we provided evidence for the common intraexonic recombination at MHC-I and for the role of life history traits in the processes of MHC-I expansion/contraction. Our study revealed novel evolutionary trajectories of amphibian MHC and it contributes to the understanding of the mechanisms that generated extraordinary MHC diversity throughout vertebrate evolution.

Major Histocompatibility Complex Variation and Haplotype Associated Survival in Response to Experimental Infection of Two Bd-GPL Strains Along a Latitudinal Gradient

While both innate and adaptive immune system mechanisms have been implicated in resistance against the chytrid fungus Batrachochytrium dendrobatidis (Bd), studies on the role of specific MHC haplotypes on Bd infection are rare. Here, we studied variation in MHC Class IIB loci in the common toad Bufo bufo along a latitudinal gradient across Sweden. In general, Swedish toad populations had few MHC Class IIB haplotypes and MHC diversity declined from south (13 haplotypes) to the north (four haplotypes). The low diversity may compromise the ability of northern populations to fight emerging disease, such as Bd. In a laboratory experiment, we infected newly metamorphosed toads with two strains of the Global Pandemic Lineage of the fungus (Bd-GPL) and compared survival with sham controls. Bd-infected toads had lower survival compared to controls. Moreover, survival was dependent on the Bd-strain and northern toads had lower Bd-mediated survival than southern individuals. MHC diversity was lower in northern toads. All northern experimental animals were monomorphic for a single MHC haplotype, whereas we found seven different haplotypes in southern experimental animals. In southern toads, survival was dependent on both Bd-strain and MHC haplotype suggesting differential infection dynamics depending on both Bd-strain and host immune system characteristics.

Comparative Analysis of SLA-1, SLA-2, and DQB1 Genetic Diversity in Locally-Adapted Kenyan Pigs and Their Wild Relatives, Warthogs

Swine leukocyte antigen (SLA) plays a central role in controlling the immune response by discriminating self and foreign antigens and initiating an immune response. Studies on SLA polymorphism have demonstrated associations between SLA allelic variants, immune response, and disease resistance. The SLA polymorphism is due to host-pathogen co-evolution resulting in improved adaptation to diverse environments making SLA a crucial genomic region for comparative diversity studies. Although locally-adapted African pigs have small body sizes, they possess increased resilience under harsh environmental conditions and robust immune systems with reported tolerance to some diseases, including African swine fever. However, data on the SLA diversity in these pigs are not available. We characterized the SLA of unrelated locally-adapted domestic pigs from Homa Bay, Kenya, alongside exotic pigs and warthogs. We undertook SLA comparative diversity of the functionally expressed SLA class I (SLA-1, SLA-2) and II (DQB1) repertoires in these three suids using the reverse transcription polymerase chain reaction (RT-PCR) sequence-based typing (SBT) method. Our data revealed higher genetic diversity in the locally-adapted pigs and warthogs compared to the exotic pigs. The nucleotide substitution rates were higher in the peptide-binding regions of the SLA-1, SLA-2, and DQB1 loci, indicative of adaptive evolution. We obtained high allele frequencies in the three SLA loci, including some breed-specific private alleles, which could guide breeders to increase their frequency through selection if confirmed to be associated with enhanced resilience. Our study contributes to the growing body of knowledge on genetic diversity in free-ranging animal populations in their natural environment, availing the first DQB1 gene data from locally-adapted Kenyan pigs.

Hybridization with mountain hares increases the functional allelic repertoire in brown hares

Brown hares (Lepus europaeus Pallas) are able to hybridize with mountain hares (L. timidus Linnaeus) and produce fertile offspring, which results in cross-species gene flow. However, not much is known about the functional significance of this genetic introgression. Using targeted sequencing of candidate loci combined with mtDNA genotyping, we found the ancestral genetic diversity in the Finnish brown hare to be small, likely due to founder effect and range expansion, while gene flow from mountain hares constitutes an important source of functional genetic variability. Some of this variability, such as the alleles of the mountain hare thermogenin (uncoupling protein 1, UCP1), might have adaptive advantage for brown hares, whereas immunity-related MHC alleles are reciprocally exchanged and maintained via balancing selection. Our study offers a rare example where an expanding species can increase its allelic variability through hybridization with a congeneric native species, offering a route to shortcut evolutionary adaptation to the local environmental conditions.

Population demographic history and adaptability of the vulnerable Lolokou Sucker Frog

Amphibians are experiencing worldwide declines due to increasing anthropogenetic disturbances. However, the genetic variability and hence adaptability are still unknown for most frogs. We integrated the mitochondrial (ND2 gene), nuclear (TYR gene) and major histocompatibility complex (MHC) loci, to clarify the demographic patterns and immune-gene diversity of the Lolokou Sucker Frog (Amolops loloensis). Demographic analysis of the ND2 and TYR genes suggested that the Lolokou Sucker Frog experienced a population expansion within the last 10,000 years. High MHC diversity was detected, which has likely resulted from positive selection, indicating the current diversity bodes well for the species' adaptive potential to pathogenic challenges. These findings broaden our knowledge on the population history and evolution adaptation of the reclusive torrent frog, and conservation implications are provided.

Occurrence of Batrachochytrium dendrobatidis in Sweden: higher infection prevalence in southern species

The chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused worldwide declines in amphibian populations. While Bd is widespread in southern and central Europe, its occurrence and distribution in northernmost Europe is mostly unknown. We surveyed for Bd in breeding anurans in Sweden by sampling 1917 amphibians from 101 localities and 3 regions in Sweden (southern, northern and central). We found that Bd was widespread in southern and central Sweden, occurring in all 9 investigated species and in 45.5% of the 101 localities with an overall prevalence of 13.8%. No infected individuals were found in the 4 northern sites sampled. The records from central Sweden represent the northernmost records of Bd in Europe. While the proportion of sites positive for Bd was similar between the southern and central regions, prevalence was much higher in the southern region. This was because southern species with a distribution mainly restricted to southernmost Sweden had a higher prevalence than widespread generalist species. The nationally red-listed green toad Bufotes variabilis and the fire-bellied toad Bombina bombina had the highest prevalence (61.4 and 48.9%, respectively). Across species, Bd prevalence was strongly positively, correlated with water temperature at the start of egg laying. However, no individuals showing visual signs of chytridiomycosis were found in the field. These results indicate that Bd is widespread and common in southern and central Sweden with southern species, breeding in higher temperatures and with longer breeding periods, having higher prevalence. However, the impact of Bd on amphibian populations in northernmost Europe remains unknown.

Latitudinal diversity gradient and cetaceans from the perspective of MHC genes

Pathogen diversity is a key source of selective pressure on immune system genes, shaping molecular evolution mainly on widely distributed or migratory organisms such as cetaceans. Here, we investigated the effects of latitudinal span migration, different biomes occupation, and pathogen-mediated selection on MHC DQB locus divergence on cetaceans. We applied some evolutionary genetics methods using a dataset of 15 species and 121 sequences, and we found a trend on greater MHC divergence on tropical species when compared with either temperate or migratory species. In addition, oceanic cetaceans exhibit greater MHC divergence. Here, we show that, despite there was a correlation between the diversity of MHC DQB alleles with the distribution of organisms, the pattern of diversity found is not completely explained by pathogenic pressure, suggesting that other factors must be investigated for a better understanding of the processes related to the diversity of MHC in cetaceans.

Antimicrobial peptide and sequence variation along a latitudinal gradient in two anurans

BACKGROUND

While there is evidence of both purifying and balancing selection in immune defense genes, large-scale genetic diversity in antimicrobial peptides (AMPs), an important part of the innate immune system released from dermal glands in the skin, has remained uninvestigated. Here we describe genetic diversity at three AMP loci (Temporin, Brevinin and Palustrin) in two ranid frogs (Rana arvalis and R. temporaria) along a 2000 km latitudinal gradient. We amplified and sequenced part of the Acidic Propiece domain and the hypervariable Mature Peptide domain (~ 150-200 bp) in the three genes using Illumina Miseq and expected to find decreased AMP genetic variation towards the northern distribution limit of the species similarly to studies on MHC genetic patterns.

RESULTS

We found multiple loci for each AMP and relatively high gene diversity, but no clear pattern of geographic genetic structure along the latitudinal gradient. We found evidence of trans-specific polymorphism in the two species, indicating a common evolutionary origin of the alleles. Temporin and Brevinin did not form monophyletic clades suggesting that they belong to the same gene family. By implementing codon evolution models we found evidence of strong positive selection acting on the Mature Peptide. We also found evidence of diversifying selection as indicated by divergent allele frequencies among populations and high Theta k values.

CONCLUSION

Our results suggest that AMPs are an important source of adaptive diversity, minimizing the chance of microorganisms developing resistance to individual peptides.