Biology subjects

Chan, C. X.

Publications and source records attributed to Chan, C. X..

26 records · Page 2Linked to original sources

bioRxiv2019

Genomes of Symbiodiniaceae reveal extensive sequence divergence but conserved functions at family and genus levels

Dinoflagellates of the family Symbiodiniaceae (Order Suessiales) are predominantly symbiotic, and many are known for their association with corals. The genetic and functional diversity among Symbiodiniaceae is well acknowledged, but the genome-wide sequence divergence among these lineages remains little known. Here, we present de novo genome assemblies of five isolates from the basal genus Symbiodinium, encompassing distinct ecological niches. Incorporating existing data from Symbiodiniaceae and other Suessiales (15 genome datasets in total), we investigated genome features that are common or unique to these Symbiodiniaceae, to genus Symbiodinium, and to the individual species S. microadriaticum and S. tridacnidorum. Our whole-genome comparisons reveal extensive sequence divergence, with no sequence regions common to all 15. Based on similarity of k-mers from whole-genome sequences, the distances among Symbiodinium isolates are similar to those between isolates of distinct genera. We observed extensive structural rearrangements among symbiodiniacean genomes; those from two distinct Symbiodinium species share the most (853) syntenic gene blocks. Functions enriched in genes core to Symbiodiniaceae are also enriched in those core to Symbiodinium. Gene functions related to symbiosis and stress response exhibit similar relative abundance in all analysed genomes. Our results suggest that structural rearrangements contribute to genome sequence divergence in Symbiodiniaceae even within a same species, but the gene functions have remained largely conserved in Suessiales. This is the first comprehensive comparison of Symbiodiniaceae based on whole-genome sequence data, including comparisons at the intra-genus and intra-species levels.

genomics↗

bioRxiv2019

Structural rearrangements drive extensive genome divergence between symbiotic and free-living Symbiodinium

Symbiodiniaceae are predominantly symbiotic dinoflagellates critical to corals and other reef organisms. Symbiodinium is a basal symbiodiniacean lineage and includes symbiotic and free-living taxa. However, the molecular mechanisms underpinning these distinct lifestyles remain little known. Here, we present high-quality de novo genome assemblies for the symbiotic Symbiodinium tridacnidorum CCMP2592 (genome size 1.3 Gbp) and the free-living Symbiodinium natans CCMP2548 (genome size 0.74 Gbp). These genomes display extensive sequence divergence, sharing only ~1.5% conserved regions ([≥]90% identity). We predicted 45,474 and 35,270 genes for S. tridacnidorum and S. natans, respectively; of the 58,541 homologous gene families, 28.5% are common to both genomes. We recovered a greater extent of gene duplication and higher abundance of repeats, transposable elements and pseudogenes in the genome of S. tridacnidorum than in that of S. natans. These findings demonstrate that genome structural rearrangements are pertinent to distinct lifestyles in Symbiodinium, and may contribute to the vast genetic diversity within the genus, and more broadly in Symbiodiniaceae. Moreover, the results from our whole-genome comparisons against a free-living outgroup support the notion that the symbiotic lifestyle is a derived trait in, and that the free-living lifestyle is ancestral to, Symbiodinium.

genomics↗

bioRxiv2019

Polarella glacialis genomes encode tandem repeats of single-exon genes with functions critical to adaptation of dinoflagellates

Dinoflagellates are taxonomically diverse, ecologically important phytoplankton in marine and freshwater environments. Here, we present two draft diploid genome assemblies of the free-living dinoflagellate Polarella glacialis, isolated from the Arctic and Antarctica. For each genome, guided using full-length transcriptome data, we predicted >50,000 high-quality genes. About 68% of the genome is repetitive sequence; long terminal repeats likely contribute to intra-species structural divergence and distinct genome sizes (3.0 and 2.7 Gbp). Of all genes, [~]40% are encoded unidirectionally, [~]25% comprised of single exons. Multi-genome comparison unveiled genes specific to P. glacialis and a common, putatively bacterial, origin of ice-binding domains in cold-adapted dinoflagellates. Our results elucidate how selection acts within the context of a complex genome structure to facilitate local adaptation. Since most dinoflagellate genes are constitutively expressed, Polarella glacialis has enhanced transcriptional responses via unidirectional, tandem duplication of single-exon genes that encode functions critical to survival in cold, low-light environments.

genomics↗

bioRxiv2019

Evidence that inconsistent gene prediction can mislead analysis of algal genomes

Comparative algal genomics often relies on predicted gene models from de novo assembled genomes. However, the artifacts introduced by different gene-prediction approaches, and their impact on comparative genomic analysis, remain poorly understood. Here, using available genome data from six dinoflagellate species in Symbiodiniaceae, we identified potential methodological biases in the published gene models that were predicted using different approaches. We developed and applied a comprehensive customized workflow to predict genes from these genomes. The observed variation among predicted gene models resulting from our workflow agreed with current understanding of phylogenetic relationships among these taxa, whereas those published earlier were largely biased by the distinct approaches used in each instance. Importantly, these biases mislead the inference of homologous gene families and synteny among genomes, thus impacting biological interpretation of these data. Our results demonstrate that a consistent gene-prediction approach is critical for comparative genomics, particularly for non-model algal genomes.

genomics↗

bioRxiv2019

Transcriptomic insights into the establishment of coral-algal symbioses from the symbiont perspective

Despite the ecological significance of the mutualistic relationship between Symbiodiniaceae and reef-building corals, the molecular machinery underpinning the establishment of this relationship is not well understood. This is especially true of the symbiont side, as previous attempts to understand the interaction between coral larvae and Symbiodiniaceae have focused nearly exclusively on the host. In the current study, Acropora tenuis planula larvae were exposed to a compatible strain of Symbiodiniaceae (Cladocopium) and the transcriptomic landscape of the symbiont profiled at 3, 12, 48 and 72 h post-exposure using RNA-Seq. The transcriptomic response of Cladocopium to the symbiotic state was complex, the most obvious feature being an extensive and generalised downregulation of gene expression. Included in this \"symbiosis-derived transcriptional repression\" were a range of stress response and immune-related genes. In contrast, genes implicated in metabolism were upregulated in the symbiotic state. Consistent with previous ecological studies, this transcriptomic response of Cladocopium implied that active translocation of metabolites to the host occurred, and thus that the mutualistic relationship can be established at the larval stage. This study provides novel insights into the transcriptomic remodelling that occurs in Symbiodiniaceae, with important implications for understanding the establishment of symbiosis between corals and their dinoflagellate partners.

genomics↗

bioRxiv2019

Benchmarking of alignment-free sequence comparison methods

Alignment-free (AF) sequence comparison is attracting persistent interest driven by data-intensive applications. Hence, many AF procedures have been proposed in recent years, but a lack of a clearly defined benchmarking consensus hampers their performance assessment. Here, we present a community resource (http://afproject.org) to establish standards for comparing alignment-free approaches across different areas of sequence-based research. We characterize 74 AF methods available in 24 software tools for five research applications, namely, protein sequence classification, gene tree inference, regulatory element detection, genome-based phylogenetic inference and reconstruction of species trees under horizontal gene transfer and recombination events. The interactive web service allows researchers to explore the performance of alignment-free tools relevant to their data types and analytical goals. It also allows method developers to assess their own algorithms and compare them with current state-of-the-art tools, accelerating the development of new, more accurate AF solutions.

bioinformatics↗

bioRxiv2017

Symbiodinium genomes reveal adaptive evolution of functions related to symbiosis

Symbiosis between dinoflagellates of the genus Symbiodinium and reef-building corals forms the trophic foundation of the worlds coral reef ecosystems. Here we present the first draft genome of Symbiodinium goreaui (Clade C, type C1: 1.03 Gbp), one of the most ubiquitous endosymbionts associated with corals, and an improved draft genome of Symbiodinium kawagutii (Clade F, strain CS-156: 1.05 Gbp), previously sequenced as strain CCMP2468, to further elucidate genomic signatures of this symbiosis. Comparative analysis of four available Symbiodinium genomes against other dinoflagellate genomes led to the identification of 2460 nuclear gene families that show evidence of positive selection, including genes involved in photosynthesis, transmembrane ion transport, synthesis and modification of amino acids and glycoproteins, and stress response. Further, we identified extensive sets of genes for meiosis and response to light stress. These draft genomes provide a foundational resource for advancing our understanding Symbiodinium biology and the coral-algal symbiosis.

genomics↗

bioRxiv2017

K-mer Similarity, Networks Of Microbial Genomes And Taxonomic Rank

Alignment-free (AF) methods have recently been adopted to infer phylogenetic trees. However, the evolutionary relationships among microbes, impacted by common phenomena such as lateral genetic transfer and rearrangement, cannot be adequately captured in a strictly tree-like structure. Bacterial and archaeal genomes consist of highly conserved regions, e.g. ribosomal RNA genes (commonly used as phylogenetic markers), more-variable regions and extrachromosomal elements, i.e. plasmids (that contain genes critical under a selective condition e.g. antibiotic resistance). The impact of these elements on genome-scale inference of microbial phylogeny remains little known. Here, using an AF approach, we inferred phylogenomic networks of microbial life based on 2785 completely sequenced bacterial and archaeal genomes, and systematically assessed the impact of ribosomal RNA genes and plasmid sequences in this network. Our results indicate that k-mer similarity can correlate with taxonomic rank of microbes. Using a relational database approach, we linked the implicated k-mers to annotated genomic regions (thus functions), and defined core functions in specific phyletic groups and genera. We found that, in most phyla, highly conserved functions are often related to Amino acid metabolism and transport, and Energy production and conversion. Our findings indicate that AF phylogenomics can be used to infer reticulate relationships in a scalable manner and provide new perspective into microbial biology and evolution.

evolutionary biology↗