Mapping epigenetic divergence in the massive radiation of Lake Malawi cichlid fishes.

Epigenetic variation modulates gene expression and can be heritable. However, knowledge of the contribution of epigenetic divergence to adaptive diversification in nature remains limited. The massive evolutionary radiation of Lake Malawi cichlid fishes displaying extensive phenotypic diversity despite extremely low sequence divergence is an excellent system to study the epigenomic contribution to adaptation. Here, we present a comparative genome-wide methylome and transcriptome study, focussing on liver and muscle tissues in phenotypically divergent cichlid species. In both tissues we find substantial methylome divergence among species. Differentially methylated regions (DMR), enriched in evolutionary young transposons, are associated with transcription changes of ecologically-relevant genes related to energy expenditure and lipid metabolism, pointing to a link between dietary ecology and methylome divergence. Unexpectedly, half of all species-specific DMRs are shared across tissues and are enriched in developmental genes, likely reflecting distinct epigenetic developmental programmes. Our study reveals substantial methylome divergence in closely-related cichlid fishes and represents a resource to study the role of epigenetics in species diversification

Lateral line system diversification during the early stages of ecological speciation in cichlid fish

Background: The mechanosensory lateral line system is an important sensory modality in fishes, informing multiple behaviours related to survival including finding food and navigating in dark environments. Given its ecological importance, we may expect lateral line morphology to be under disruptive selection early in the ecological speciation process. Here we quantify the lateral line system morphology of two ecomorphs of the cichlid fish Astatotilapia calliptera in crater Lake Masoko that have diverged from common ancestry within the past 1,000 years. Results: Based on geometric morphometric analyses of CT scans, we show that the zooplanktivorous benthic ecomorph that dominates the deeper waters of the lake has large cranial lateral line canal pores, relative to those of the nearshore invertebrate-feeding littoral ecomorph found in the shallower waters. In contrast, fluorescence imaging revealed no evidence for divergence between ecomorphs in the number of either superficial or canal neuromasts. We illustrate the magnitude of the variation we observe in Lake Masoko A. calliptera in the context of the neighbouring Lake Malawi mega-radiation that comprises over 700 species. Conclusions: These results provide the first evidence of divergence in this often-overlooked sensory modality in the early stages of ecological speciation, suggesting that it may have a role in the broader adaptive radiation process

Author Correction: Evolutionary divergence of novel open reading frames in cichlids speciation.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.</jats:p

Evolutionary divergence of novel open reading frames in cichlids speciation

Funder: DBT-Cambridge LectureshipFunder: Wellcome Trust Senior Investigator AwardAbstract: Novel open reading frames (nORFs) with coding potential may arise from noncoding DNA. Not much is known about their emergence, functional role, fixation in a population or contribution to adaptive radiation. Cichlids fishes exhibit extensive phenotypic diversification and speciation. Encounters with new environments alone are not sufficient to explain this striking diversity of cichlid radiation because other taxa coexistent with the Cichlidae demonstrate lower species richness. Wagner et al. analyzed cichlid diversification in 46 African lakes and reported that both extrinsic environmental factors and intrinsic lineage-specific traits related to sexual selection have strongly influenced the cichlid radiation, which indicates the existence of unknown molecular mechanisms responsible for rapid phenotypic diversification, such as emergence of novel open reading frames (nORFs). In this study, we integrated transcriptomic and proteomic signatures from two tissues of two cichlids species, identified nORFs and performed evolutionary analysis on these nORF regions. Our results suggest that the time scale of speciation of the two species and evolutionary divergence of these nORF genomic regions are similar and indicate a potential role for these nORFs in speciation of the cichlid fishes

Evolutionary divergence of novel open reading frames in cichlids speciation

Funder: DBT-Cambridge LectureshipFunder: Wellcome Trust Senior Investigator AwardAbstract: Novel open reading frames (nORFs) with coding potential may arise from noncoding DNA. Not much is known about their emergence, functional role, fixation in a population or contribution to adaptive radiation. Cichlids fishes exhibit extensive phenotypic diversification and speciation. Encounters with new environments alone are not sufficient to explain this striking diversity of cichlid radiation because other taxa coexistent with the Cichlidae demonstrate lower species richness. Wagner et al. analyzed cichlid diversification in 46 African lakes and reported that both extrinsic environmental factors and intrinsic lineage-specific traits related to sexual selection have strongly influenced the cichlid radiation, which indicates the existence of unknown molecular mechanisms responsible for rapid phenotypic diversification, such as emergence of novel open reading frames (nORFs). In this study, we integrated transcriptomic and proteomic signatures from two tissues of two cichlids species, identified nORFs and performed evolutionary analysis on these nORF regions. Our results suggest that the time scale of speciation of the two species and evolutionary divergence of these nORF genomic regions are similar and indicate a potential role for these nORFs in speciation of the cichlid fishes

Epigenetic remodelling licences adult cholangiocytes for organoid formation and liver regeneration.

Following severe or chronic liver injury, adult ductal cells (cholangiocytes) contribute to regeneration by restoring both hepatocytes and cholangiocytes. We recently showed that ductal cells clonally expand as self-renewing liver organoids that retain their differentiation capacity into both hepatocytes and ductal cells. However, the molecular mechanisms by which adult ductal-committed cells acquire cellular plasticity, initiate organoids and regenerate the damaged tissue remain largely unknown. Here, we describe that ductal cells undergo a transient, genome-wide, remodelling of their transcriptome and epigenome during organoid initiation and in vivo following tissue damage. TET1-mediated hydroxymethylation licences differentiated ductal cells to initiate organoids and activate the regenerative programme through the transcriptional regulation of stem-cell genes and regenerative pathways including the YAP-Hippo signalling. Our results argue in favour of the remodelling of genomic methylome/hydroxymethylome landscapes as a general mechanism by which differentiated cells exit a committed state in response to tissue damage.RCUK Cancer Research UK ERC H2020 Wellcome Trus

Epigenetic variability and inheritance in East African cichlid fishes

The hundreds of cichlid species forming part of the radiation of East African Lakes show a remarkable diversity of phenotypic and ecological adaptation. Despite this, recent studies highlighted that genetic diversity within the radiation is among the lowest ever observed in vertebrates. Such a high phenotype/genotype diversity ratio makes cichlids a promising system to investigate the role of genetics and, for the first time at a species level, epigenetics in the context of adaptation and convergent evolution. Yet, the molecular mechanisms and, in particular, any epigenetic aspects underlying such phenotypic diversity and speciation success remain largely unknown. Here, I focus on whole-genome DNA methylation (methylome), a heritable and dynamic epigenetic mark that has been reported to be responsible for rapid and transmissible changes in phenotype in plants and mammals. In light with phenotypic plasticity related to diet adaptation, I hypothesise that the liver methylome may affect liver function and thus be related to diet. I thus performed sequencing of liver tissues of different cichlid species presenting distinct eco-morphological and trophic adaptation from both Lake Malawi and crater lake Massoko, Tanzania. The main results reveal striking differences in methylome at conserved underlying DNA sequences – some variation shared in cichlids of both lakes. Furthermore, I observe an enrichment for methylome variation in transposable elements (TE) and promoter regions. Remarkably, most of the variation (ca 80%) common to fishes from both lakes are located in TEs and is, in part, correlated with differential expression levels at some key metabolic and developmental genes in liver. This suggests a possible conserved role of TE-related methylome in the adaptation of liver function. Furthermore, I generated inter-species hybrids to investigate the inheritance of DNAme variation in cichlids. The liver methylome of F1 hybrids, although mostly resembling parental methylomes, exhibited some level of divergence, suggesting unique DNAme patterns in hybrid offspring and possible transgressive segregation. I conclude there might be a conserved crosstalk between the local environment and methylome in different natural populations of cichlids. The results presented in this thesis postulate an important role of natural DNA methylation variation in promoting adaptive phenotypic diversification in divergent habitats during the early stages of speciation.Wellcome Genetics Society Wolfson College, Cambridge Universit

Taming transposable elements in vertebrates: from epigenetic silencing to domestication.

Transposable element (TE)-derived sequences are ubiquitous in most eukaryotic genomes known to date. Because their expression and mobility can lead to genomic instability, several pathways have evolved to control TEs. Nevertheless, TEs represent an important source of genomic novelty and are often co-opted for novel functions that are relevant for phenotypic divergence and adaptation. Here, we review how animals, in particular vertebrates, mitigate TE mobility and expression, alongside known examples of TE domestication. We argue that the next frontier is to understand the determinants and dynamics of TE domestication: how they shift from 'non-self' targets of epigenetic silencing to 'self' genetic elements. New technologies enable avenues of research that may close the gap between epigenetic silencing and domestication of TEs.This work was supported by the following grants to E.A.M.: Wellcome Trust Senior Investigator Award (219475/Z/19/Z) and CRUK awards (C13474 and A27826). M.V.A. is funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 101027241. G.V. thanks Wolfson College at the University of Cambridge, and the Genetics Society, London for financial support. A.L.K.P. is grateful to the Cambridge Commonwealth, European & International Trust for financial support. Finally, the authors also acknowledge core funding to the Gurdon Institute from Wellcome (092096/Z/10/Z, 203144/Z/16/Z) and CRUK (C6946/A24843). For Open Access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission

Genetic and developmental divergence in the neural crest program between cichlid fish species.

Neural crest (NC) is a vertebrate-specific embryonic progenitor cell population at the basis of important vertebrate features such as the craniofacial skeleton and pigmentation patterns. Despite the wide-ranging variation of NC-derived traits across vertebrates, the contribution of NC to species diversification remains underexplored. Here, leveraging the adaptive diversity of African Great Lakes' cichlid species, we combined comparative transcriptomics and population genomics to investigate the evolution of the NC genetic program in the context of their morphological divergence. Our analysis revealed substantial differences in transcriptional landscapes across somitogenesis, an embryonic period coinciding with NC development and migration. This included dozens of genes with described functions in the vertebrate NC gene regulatory network, several of which showed signatures of positive selection. Among candidates showing between-species expression divergence, we focused on teleost-specific paralogs of the NC-specifier sox10 (sox10a and sox10b) as prime candidates to influence NC development. These genes, expressed in NC cells, displayed remarkable spatio-temporal variation in cichlids, suggesting their contribution to inter-specific morphological differences, such as craniofacial structures and pigmentation. Finally, through CRISPR/Cas9 mutagenesis, we demonstrated the functional divergence between cichlid sox10 paralogs, with the acquisition of a novel skeletogenic function by sox10a. When compared to teleost models zebrafish and medaka, our findings reveal that sox10 duplication, although retained in most teleost lineages, had variable functional fates across their phylogeny. Altogether, our study suggests that NC-related processes - particularly those controlled by sox10s - are involved in generating morphological diversification between species and lays the groundwork for further investigations into the mechanisms underpinning vertebrate NC diversification.Wellcome Trust PhD in Developmental Mechanisms Studentship 215223/Z/19/Z. NERC Independent Research Fellowship NE/R01504X/1. EMBO Fellowship ALTF 339-2022