Co-diversification of an intestinal Mycoplasma and its salmonid host

Understanding the evolutionary relationships between a host and its intestinal resident bacteria can transform how we understand adaptive phenotypic traits. The interplay between hosts and their resident bacteria inevitably affects the intestinal environment and, thereby, the living conditions of both the host and the microbiota. Thereby this co-existence likely influences the fitness of both bacteria and host. Whether this co-existence leads to evolutionary co-diversification in animals is largely unexplored, mainly due to the complexity of the environment and microbial communities and the often low host selection. We present the gut metagenome from wild Atlantic salmon (Salmo salar), a new wild organism model with an intestinal microbiota of low complexity and a well-described population structure, making it well-suited for investigating co-evolution. Our data reveal a strong host selection of a core gut microbiota dominated by a single Mycoplasma species. We found a clear co-diversification between the population structure of Atlantic salmon and nucleotide variability of the intestinal Mycoplasma populations conforming to expectations from co-evolution between host and resident bacteria. Our results show that the stable microbiota of Atlantic salmon has evolved with its salmonid host populations while potentially providing adaptive traits to the salmon host populations, including defence mechanisms, biosynthesis of essential amino acids, and metabolism of B vitamins. We highlight Atlantic salmon as a novel model for studying co-evolution between vertebrate hosts and their resident bacteria.publishedVersio

Genome-resolved metagenomics suggests a mutualistic relationship between Mycoplasma and salmonid hosts

Salmonids are important sources of protein for a large proportion of the human population. Mycoplasma species are a major constituent of the gut microbiota of salmonids, often representing the majority of microbiota. Despite the frequent reported dominance of salmonid-related Mycoplasma species, little is known about the phylogenomic placement, functions and potential evolutionary relationships with their salmonid hosts. In this study, we utilise 2.9 billion metagenomic reads generated from 12 samples from three different salmonid host species to I) characterise and curate the first metagenome-assembled genomes (MAGs) of Mycoplasma dominating the intestines of three different salmonid species, II) establish the phylogeny of these salmonid candidate Mycoplasma species, III) perform a comprehensive pangenomic analysis of Mycoplasma, IV) decipher the putative functionalities of the salmonid MAGs and reveal specific functions expected to benefit the host. Our data provide a basis for future studies examining the composition and function of the salmonid microbiota

An assessment of the use of human samples in ancient DNA studies

This thesis addresses gaps that exist in the theory and knowledge of ancient DNA (aDNA). Much of the underlying basis of the field has been neglected in the excitement that followed the first aDNA studies. Therefore the results of many studies have been based on untested assumptions about the nature of post mortem DNA damage, sample preservation, contamination, and the efficacy of sample decontamination techniques. The validity of such results is questionable if the assumptions prove false. Hydrolytic post mortem DNA damage may modify recovered aDNA sequences. This thesis reports new insights into the biochemical basis of, predisposition of certain sequences and nucleotide positions towards, and subsequent effects of, such damage. Parallels of post mortem damage with in vivo mutation also enable insights into DNA sequence evolution. The long-term survival of DNA, and contamination of samples with exogenous DNA are two related problems characteristic to aDNA. The survival of endogenous DNA within bone, teeth and hair samples, the susceptibility of such samples to contamination, and the efficacy of decontamination techniques used to remedy such problems are investigated. The results highlight serious flaws in using bone and teeth as a DNA source. In contrast, the results demonstrate that hair may present a valuable DNA source for future studies. Numerous studies have reported the retrieval of ancient pathogen DNA from human samples. Analyses of the DNA content within teeth extracted from putative victims of the 2nd plague argue that such studies are at great risk from DNA degradation, and contamination arising due to environmental microorganisms. An extrapolation of these results using basic physical and chemical theory is used to evaluate the potential survival of aDNA in ancient Egyptian remains. This suggests that positive results from such samples are unlikely.</p

An assessment of the use of human samples in ancient DNA studies

This thesis addresses gaps that exist in the theory and knowledge of ancient DNA (aDNA). Much of the underlying basis of the field has been neglected in the excitement that followed the first aDNA studies. Therefore the results of many studies have been based on untested assumptions about the nature of post mortem DNA damage, sample preservation, contamination, and the efficacy of sample decontamination techniques. The validity of such results is questionable if the assumptions prove false. Hydrolytic post mortem DNA damage may modify recovered aDNA sequences. This thesis reports new insights into the biochemical basis of, predisposition of certain sequences and nucleotide positions towards, and subsequent effects of, such damage. Parallels of post mortem damage with in vivo mutation also enable insights into DNA sequence evolution. The long-term survival of DNA, and contamination of samples with exogenous DNA are two related problems characteristic to aDNA. The survival of endogenous DNA within bone, teeth and hair samples, the susceptibility of such samples to contamination, and the efficacy of decontamination techniques used to remedy such problems are investigated. The results highlight serious flaws in using bone and teeth as a DNA source. In contrast, the results demonstrate that hair may present a valuable DNA source for future studies. Numerous studies have reported the retrieval of ancient pathogen DNA from human samples. Analyses of the DNA content within teeth extracted from putative victims of the 2nd plague argue that such studies are at great risk from DNA degradation, and contamination arising due to environmental microorganisms. An extrapolation of these results using basic physical and chemical theory is used to evaluate the potential survival of aDNA in ancient Egyptian remains. This suggests that positive results from such samples are unlikely.</p

Releasing the microbes from old bones: the effect of different DNA extraction protocols on microbial community profiling

DNA-based characterisation of microbial communities can enable those interested in bone diagenesis to address questions relating to the complexity and diversity of said microbial communities. We explored whether biases could be introduced due to differences in the DNA extraction methods used. We investigated the effect of four alternative approaches on the extraction of DNA from bone, in order to examine the resulting effect on the bacterial and fungal OTUs recovered using metabarcoding. We found that the different extraction methods resulted in differences in the microbial OTU profiles generated, both when looking at the fungal and the bacterial communities within the bone. Our results emphasise the need for consistency when working with DNA extraction if comparison of results between different research groups are to be valid. Furthermore, it is clear that future efforts will be needed to determine which methods may provide the most accurate representation of the microbial community in bones

Beyond DNA barcoding: The unrealized potential of genome skim data in sample identification

Genetic tools are increasingly used to identify and discriminate between species. One key transition in this process was the recognition of the potential of the ca 658bp fragment of the organelle cytochrome c oxidase I (COI) as a barcode region, which revolutionized animal bioidentification and lead, among others, to the instigation of the Barcode of Life Database (BOLD), containing currently barcodes from >7.9 million specimens. Following this discovery, suggestions for other organellar regions and markers, and the primers with which to amplify them, have been continuously proposed. Most recently, the field has taken the leap from PCR-based generation of DNA references into shotgun sequencing-based “genome skimming” alternatives, with the ultimate goal of assembling organellar reference genomes. Unfortunately, in genome skimming approaches, much of the nuclear genome (as much as 99% of the sequence data) is discarded, which is not only wasteful, but can also limit the power of discrimination at, or below, the species level. Here, we advocate that the full shotgun sequence data can be used to assign an identity (that we term for convenience its “DNA-mark”) for both voucher and query samples, without requiring any computationally intensive pretreatment (e.g. assembly) of reads. We argue that if reference databases are populated with such “DNA-marks,” it will enable future DNA-based taxonomic identification to complement, or even replace PCR of barcodes with genome skimming, and we discuss how such methodology ultimately could enable identification to population, or even individual, leve