Biology subjects

Chan, C. X.

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

At least 19 recordsLinked to original sources

bioRxiv2026

Heatwave winners and losers: cryptic coral holobionts differ in thermal tolerance

Extreme climatic events are reshaping ecosystems worldwide as individual organisms vary markedly in their ability to withstand these disturbances. Deciphering patterns of persistence on local scales is therefore critical for predicting biodiversity trajectories under intensifying climate extremes. In this study, we examined variation in thermal stress responses among individuals of the coral Stylophora pistillata species complex during a heatwave at Heron Island Reef, Australia. More than half of the focal coral colonies died on the reef, and survival of coral fragments maintained under ex situ common thermal stress conditions was significantly correlated with the survival of their source colony. This demonstrates that survival differences result largely from biological factors rather than differential thermal exposure across reef habitats. Under common garden conditions, we observed striking differences in bleaching severity and survival times among three sympatric cryptic taxa and their highly host-specific symbiont community. Within the most locally common taxon, corals from historically warmer and more seasonally variable reef habitats seem more susceptible to bleaching, contrary to expectations. Together, these results reveal how biological differences among cryptic taxa and among individuals can shape coral responses during a heatwave and advance our understanding of coral vulnerability in a rapidly warming world.

ecology↗

bioRxiv2025

A catastrophic marine mortality event caused by a complex algal bloom including the novel brevetoxin producer, Karenia cristata (Dinophyceae)

Harmful algal blooms of Karenia brevis (Dinophyceae) are a global anomaly, occurring in one location worldwide, causing severe marine and acute human impacts via brevetoxins (BTX). During 2025 an unprecedented, currently ongoing, mass marine mortality occurred in South Australia, across an area of [~]20,000 km2, persisting for >6 months, resulting in the deaths of [~]106 marine animals of >550 taxa, with human health impacts. Using custom metabarcoding, long-read sequencing and targeted quantitative PCR, we characterized the microalgal assemblage. Karenia cristata dominated over the sampling area, in an assemblage with four other Karenia species with varied abundances spatially and temporally. High abundances of K. cristata appeared in the austral autumn, and hydrodynamic processes appear to have entrained cells coastward in the semi-enclosed seas. We isolated the species and characterized it using light and electron microscopy, liquid chromatography mass spectrometry and toxicity assays. We show for the first time that the rare and little known K. cristata produces significant BTX with a profile (BTX-2, -3, -B5), differing from K. brevis, with toxicological effects. These findings reveal a novel, significant BTX-producing Karenia, which considering its substantial detrimental marine ecosystem impacts, is an emerging international threat with unknown consequences in changing ocean conditions.

ecology↗

bioRxiv2025

Long-distance gene flow and contrasting population structures of reef-building corals and their algal symbionts inform adaptive potential across the Western Pacific

The genetic diversity and connectivity of reef-building coral populations are key to their survival in warming oceans. Yet our understanding of corals demographic resilience and adaptive potential is complicated by cryptic species diversity, wide geographic distributions, and complex coral-algal symbioses. To address these challenges, we investigated genetic connectivity and diversity of the broadcast-spawning coral Acropora spathulata and its associated Symbiodiniaceae across 29 reefs spanning the Great Barrier Reef, the Coral Sea, and New Caledonia, using whole-genome sequencing of 1,088 colonies. We identified four genetically distinct coral populations that diverged between 0.27 and 0.65 million years ago, likely due to geographic isolation across thousands of kilometers. These populations maintained asymmetrical gene flow along major ocean currents despite demographic isolation, and sustained large local effective population sizes ([~]2,900), supported by a high dispersal range of [~]100 km per generation. In contrast, their Symbiodiniaceae partners varied over finer spatial scales, with five distinct Cladocopium taxa distributed along latitudinal and cross-shore gradients, likely driven by local environmental conditions. These results suggest that high dispersal capacity and large local population size promote demographic resilience within reef systems, while environment-specific symbioses and long-distance gene flow across reef-systems support adaptation and evolutionary rescue.

molecular biology↗

bioRxiv2024

Chromosomal inversions harbour excess mutational load in the coral, Acropora kenti, on the Great Barrier Reef

The future survival of coral reefs in the Anthropocene depends on the capacity of corals to adapt as oceans warm and extreme weather events become more frequent. Targeted interventions designed to assist evolutionary processes in corals require a comprehensive understanding of the distribution and structure of standing variation, however, efforts to map genomic variation in corals have so far focussed almost exclusively on SNPs, overlooking structural variants that have been shown to drive adaptive processes in other taxa. Here we show that the reef-building coral, Acropora kenti (syn. tenuis) harbors at least five large, highly polymorphic structural variants, all of which exhibit signatures of strongly suppressed recombination in heterokaryotypes, a feature commonly associated with chromosomal inversions. Based on their high minor allele frequency, uniform distribution across habitats, and elevated genetic load, we propose that these inversions in A. kenti are likely to be under balancing selection. An excess of SNPs with high impact on protein coding genes within these loci elevates their importance both as potential targets for adaptive selection and as contributors to genetic decline if coral populations become fragmented or inbred in future.

evolutionary biology↗

bioRxiv2023

Genome-wide transcriptome analysis reveals the diversity and function of long non-coding RNAs in dinoflagellates

Dinoflagellates are a diverse group of phytoplankton, ranging from harmful bloom-forming microalgae to photosymbionts that are critical for sustaining coral reefs. Genome and transcriptome data from dinoflagellates are revealing extensive genomic divergence and lineage-specific innovation of gene functions. However, most studies thus far have focused on protein-coding genes; long non-coding RNAs (lncRNAs), known to regulate gene expression in eukaryotes, are largely unexplored. Here, using both genome and transcriptome data, we identified a combined total of 48,039 polyadenylated lncRNAs in the genomes of three dinoflagellate species: the coral symbionts of Cladocopium proliferum and Durusdinium trenchii, and the bloom-forming Prorocentrum cordatum. These putative lncRNAs are shorter, and have fewer introns and lower G+C-content when compared to protein-coding sequences. Although 37,768 (78.6%) lncRNAs shared no significant similarity with one another, we classified all lncRNAs based on conserved sequence motifs (k-mers) into distinct clusters following properties of potential protein-binding and/or subcellular localisation. Interestingly, 3708 (7.7%) lncRNAs were differentially expressed in response to heat stress, lifestyle, and/or growth phases, and they shared co-expression patterns with protein-coding genes. Based on inferred triplex interactions between lncRNA and upstream (putative promoter) regions of protein-coding genes, we identified a combined 19,460 putative gene targets for 3,721 lncRNAs; 907 genes exhibit differential expression under heat stress. These results reveal for the first time the functional diversity of lncRNAs in dinoflagellates, and demonstrate how lncRNAs, often overlooked in transcriptome data, could regulate gene expression as a molecular response to heat stress in these ecologically important organisms.

genomics↗

bioRxiv2023

In vivo structure probing of RNA in Archaea: Novel insights into the ribosome structure of Methanosarcina acetivorans

Structure probing combined with next-generation sequencing (NGS) has provided novel insights into RNA structure-function relationships. To date such studies have focused largely on bacteria and eukaryotes, with little attention given to the third domain of life, archaea. Furthermore, functional RNAs have not been extensively studied in archaea, leaving open questions about RNA structure and function within this domain of life. With archaeal species being diverse and having many similarities to both bacteria and eukaryotes, the archaea domain has the potential to be an evolutionary bridge. In this study, we introduce a method for probing RNA structure in vivo in the archaea domain of life. We investigated the structure of ribosomal RNA (rRNA) from Methanosarcina acetivorans, a well-studied anaerobic archaeal species, grown with either methanol or acetate. After probing the RNA in vivo with dimethyl sulfate (DMS), Structure-seq2 libraries were generated, sequenced, and analyzed. We mapped the reactivity of DMS onto the secondary structure of the ribosome, which we determined independently with comparative analysis, and confirmed the accuracy of DMS probing in M. acetivorans. Accessibility of the rRNA to DMS in the two carbon sources was found to be quite similar, although some differences were found. Overall, this study establishes the Structure-seq2 pipeline in the archaea domain of life and informs about ribosomal structure within M. acetivorans.

biochemistry↗

bioRxiv2023

Intraspecies genomic divergence of coral algal symbionts shaped by gene duplication

Dinoflagellates of Order Suessiales include the diverse Family Symbiodiniaceae known for their role as essential coral reef symbionts, and the cold-adapted Polarella glacialis. These taxa inhabit a broad range of ecological niches and exhibit extensive genomic divergence, although their genomes are in the smaller size ranges (haploid size < 3 Gbp) compared to most other dinoflagellates. Different isolates of a species are known to form symbiosis with distinct hosts and exhibit different regimes of gene expression, but intraspecies whole-genome divergence remains little known. Focusing on three Symbiodiniaceae species (the free-living Effrenium voratum, and the symbiotic Symbiodinium microadriaticum and Durusdinium trenchii) and the free-living outgroup P. glacialis, all for which whole-genome data from multiple isolates are available, we assessed intraspecies genomic divergence at sequence and structural levels. Our analysis based on alignment and alignment-free methods revealed greater extent of intraspecies sequence divergence in symbiodiniacean species than in P. glacialis. Our results also reveal the implications of gene duplication in generating functional innovation and diversification of Symbiodiniaceae, particularly in D. trenchii for which whole-genome duplication was involved. Interestingly, tandem duplication of single-exon genes was found to be more prevalent in genomes of free-living species than in those of symbiotic species. These results in combination demonstrate the remarkable intraspecies genomic divergence in dinoflagellates under the constraint of reduced genome sizes, shaped by genetic duplications and symbiogenesis events during diversification of Symbiodiniaceae.

genomics↗

bioRxiv2023

Massive genome reduction occurred prior to the origin of coral algal symbionts

Dinoflagellates in the Family Symbiodiniaceae (Order Suessiales) are diverse, predominantly symbiotic lineages that associate with taxa such as corals and jellyfish. Their ancestor is believed to have been free-living, and the establishment of symbiosis (i.e., symbiogenesis) is hypothesised to have occurred multiple times during Symbiodiniaceae evolution. Among Symbiodiniaceae taxa, the genus Effrenium is an early diverging, free-living lineage that is phylogenetically positioned between two robustly supported groups of genera within which symbiotic taxa have emerged. The lack of symbiogenesis in Effrenium suggests that the ancestral features of Symbiodiniaceae may have been retained in this lineage. Here we present de novo assembled genomes and associated transcriptome data from three isolates of Effrenium voratum. We compared the Effrenium genomes (1.2-1.9 Gbp in size) and gene features with those of 16 Symbiodiniaceae taxa and other outgroup dinoflagellates. Surprisingly, we find that genome reduction, which is often associated with a symbiotic lifestyle, predates the origin of Symbiodiniaceae. We postulate that adaptation to an extreme habitat (e.g., as in Polarella glacialis) or life in oligotrophic conditions resulted in the Suessiales ancestor having a haploid genome size < 2Gbp, which was retained (or reduced) among all extant algae in this lineage. Nonetheless, our data reveal that the free-living lifestyle distinguishes Effrenium from symbiotic Symbiodiniaceae vis-a-vis their longer introns, more-extensive mRNA editing, fewer ([~]30%) lineage-specific gene families, and lower ([~]10%) level of pseudogenisation. These results demonstrate how genome reduction and the adaptation to symbiotic versus free-living lifestyles intersect, and have driven the diversification and genome evolution of Symbiodiniaceae.

genomics↗

bioRxiv2022

Evolution of resilience against heat stress in a red-tide dinoflagellate

"Red tides" are harmful algal blooms (HABs) caused by dinoflagellate microalgae that accumulate toxins lethal to other organisms, including humans via consumption of contaminated seafood. Increasingly frequent, HABs are driven by a combination of environmental factors including nutrient enrichment, particularly in warm waters. Here, we present the de novo assembled genome (~4.75 Gbp), transcriptome, proteome, and metabolome from Prorocentrum cordatum, a globally abundant, bloom-forming dinoflagellate. Using axenic algal cultures, we studied the molecular mechanisms that underpin response to temperature stress, which is relevant to current ocean warming trends. We discovered a complementary interplay between RNA editing and exon usage that regulates the expression and functional diversity of biomolecules, reflected by reduction in photosynthesis, central metabolism, and protein synthesis. Our multi-omics analyses uncover the molecular response to heat stress in an important HAB species, which is driven by complex gene structures in a large, high-G+C genome, combined with multi-level transcriptional regulation.

genomics↗

bioRxiv2022

Improved Cladocopium goreaui genome assembly reveals features of a facultative coral symbiont and the complex evolutionary history of dinoflagellate genes

Dinoflagellates of the family Symbiodiniaceae are crucial photosymbionts in corals and other marine organisms. Of these algae, Cladocopium goreaui is one of the most dominant symbiont species in the Indo-Pacific. Here, we present an improved genome assembly of C. goreaui combining new long-read sequence data with earlier generated short-read data. Incorporating new full-length transcripts to guide gene prediction, the C. goreaui genome (1.2 Gb) exhibits a high extent of completeness (82.4% based on BUSCO protein recovery) and better resolution of repetitive sequence regions; 45,322 gene models were predicted, and 327 putative, topologically associated domains of the chromosomes were identified. Comparison with other Symbiodiniaceae genomes revealed a prevalence of repeats and duplicated genes in C. goreaui, and lineage-specific genes indicating functional innovation. Incorporating 2,841,408 protein sequences from 96 broadly sampled eukaryotes and representative prokaryotes in a phylogenomic approach, we assessed the evolutionary history of C. goreaui genes. Of the 5,246 phylogenetic trees inferred from homologous protein sets containing two or more phyla, 35-36% have putatively originated via horizontal gene transfer (HGT), predominantly (19-23%) via an ancestral Archaeplastida lineage implicated in the endosymbiotic origin of plastids: 10-11% are of green algal origin, including genes encoding photosynthetic functions. Our results demonstrate the utility of long-read sequence data in resolving structural features of a dinoflagellate genome and highlight how genetic transfer has shaped genome evolution of a facultative symbiont, and more broadly of dinoflagellates.

genomics↗

bioRxiv2022

Whole-genome duplication in an algal symbiont serendipitously confers thermal tolerance to corals

The algal endosymbiont Durusdinium trenchii enhances the resilience of coral reefs under thermal stress1,2. As an endosymbiont, D. trenchii is generally expected to have a reduced genome compared to its free-living relatives, due in part to the lack of selective pressure for maintaining redundant gene functions in a stable intracellular environment within the host3. However, D. trenchii can live freely or in endosymbiosis, and the analysis of genetic markers4 suggests that this species has undergone whole-genome duplication (WGD). Here we present genome assemblies for two D. trenchii isolates, confirm WGD in these taxa, and examine how selection has shaped the duplicated genome regions. We assess how the competing free-living versus endosymbiotic lifestyles of D. trenchii have contributed to the retention and divergence of duplicated genes, and how these processes have enhanced thermotolerance of corals hosting these symbionts. We find that lifestyle is the driver of post-WGD evolution in D. trenchii, with the free-living phase being most important, followed by endosymbiosis. Adaptations to both lifestyles collectively result in increased cellular fitness for D. trenchii, which provides enhanced thermal stress protection to the host coral. Beyond corals, this polyploid alga is a valuable model for understanding how genome-wide selective forces act to balance the often, divergent constraints imposed by competing lifestyles.

genomics↗

bioRxiv2022

Evolutionary responses of a reef-building coral to climate change at the end of the last glacial maximum

Climate change threatens the survival of coral reefs on a global scale, primarily through mass bleaching and mortality as a result of marine heatwaves. While these short-term effects are clear, predicting the fate of coral reefs over the coming century is a major challenge. One way to understand the longer-term effects of rapid climate change is to examine the response of coral populations to past climate shifts. Coastal and shallow-water marine ecosystems such as coral reefs have been reshaped many times by sea-level changes during the Pleistocene, yet, few studies have directly linked this with its consequences on population demographics, dispersal, and adaptation. Here we use powerful analytical techniques, afforded by haplotype phased whole-genomes, to establish such links for the reef-building coral, Acropora digitifera. We show that three genetically distinct populations are present in northwestern Australia, and that their rapid divergence since the last glacial maximum (LGM) can be explained by a combination of founder-effects and restricted gene flow. Signatures of selective sweeps, too strong to be explained by demographic history, are present in all three populations and overlap with genes that show different patterns of functional enrichment between inshore and offshore habitats. In contrast to rapid divergence in the host, we find that photosymbiont communities are largely undifferentiated between corals from all three locations, spanning almost 1000 km, indicating that selection on host genes and not acquisition of novel symbionts, has been the primary driver of adaptation for this species in northwestern Australia.

evolutionary biology↗

bioRxiv2022

Transcriptome of the coralline alga Calliarthron tuberculosum (Corallinales, Rhodophyta) reveals convergent evolution of a partial lignin biosynthesis pathway

The discovery of lignins in the coralline red alga Calliarthron tuberculosum raised new questions about the deep evolution of lignin biosynthesis. Here we present the transcriptome of C. tuberculosum supported with newly generated genomic data to identify gene candidates from the monolignol biosynthetic pathway using a combination of sequence similarity-based methods. We identified candidates in the monolignol biosynthesis pathway for the genes 4CL, CCR, CAD, CCoAOMT, and CSE but did not identify candidates for PAL, CYP450 (F5H, C3H, C4H), HCT, and COMT. In gene tree analysis, we present evidence that these gene candidates evolved independently from their land plant counterparts, suggesting convergent evolution of a complex multistep lignin biosynthetic pathway in this red algal lineage. Additionally, we provide tools to extract metabolic pathways and genes from the newly generated transcriptomic and genomic datasets. Using these methods, we extracted genes related to sucrose metabolism and calcification. Ultimately, this transcriptome will provide a foundation for further genetic and experimental studies of calcifying red algae.

plant biology↗

bioRxiv2021

Nuclear genome of a pedinophyte pinpoints genomic innovation and streamlining in the green algae

The genomic diversity underpinning high ecological and species diversity in the green algae (Chlorophyta) remains little known. Here, we aimed to track genome evolution in the Chlorophyta, focusing on loss and gain of homologous genes, and lineage-specific innovations of the Core Chlorophyta. We generated a high-quality nuclear genome for pedinophyte YPF701, a sister lineage to others in the Core Chlorophyta, and incorporated this genome in a comparative analysis with 25 other genomes from diverse Viridiplantae taxa. The nuclear genome of pedinophyte YPF701 has an intermediate size and gene number between those of most early-diverging prasinophytes and the remainder of the Core Chlorophyta. Our results suggest positive selection for genome streamlining in Pedinophyceae, independent from genome minimisation observed among prasinophyte lineages. Genome expansion was predicted along the branch leading to the UTC clade (classes Ulvophyceae, Trebouxiophyceae and Chlorophyceae) after divergence from their common ancestor with pedinophytes, with genomic novelty implicated in a range of basic biological functions. These results emphasise multiple independent signals of genome minimisation within the Chlorophyta, as well as the genomic novelty arising prior to diversification in the UTC clade, which may underpin the success of this species-rich clade in a diversity of habitats.

evolutionary biology↗

bioRxiv2021

Tightly constrained genome reduction and relaxation of purifying selection during secondary plastid endosymbiosis

Endosymbiosis, the establishment of a former free-living prokaryotic or eukaryotic cell as an organelle inside a host cell, can dramatically alter the genomic architecture of the endosymbiont. Plastids, the light harvesting organelles of photosynthetic eukaryotes, are excellent models to study this phenomenon because plastid origin has occurred multiple times in evolution. Here, we investigate the genomic signature of molecular processes acting through secondary plastid endosymbiosis - the origination of a new plastid from a free-living eukaryotic alga. We used phylogenetic comparative methods to study gene loss and changes in selective regimes on plastid genomes, focusing on the green lineage that has given rise to three independent lineages with secondary plastids (euglenophytes, chlorarachniophytes, Lepidodinium). Our results show an overall increase in gene loss associated with secondary endosymbiosis, but this loss is tightly constrained by retention of genes essential for plastid function. The data show that secondary plastids have experienced temporary relaxation of purifying selection during secondary endosymbiosis. However, this process is tightly constrained as well, with selection relaxed only relative to the background in primary plastids, but purifying selection remaining strong in absolute terms even during the endosymbiosis events. Selection intensity rebounds to pre-endosymbiosis levels following endosymbiosis events, demonstrating the changes in selection efficiency during different phases of secondary plastid origin. Independent endosymbiosis events in the euglenophytes, chlorarachniophytes, and Lepidodinium differ in their degree of relaxation of selection, highlighting the different evolutionary contexts of these events. This study reveals the selection-drift interplay during secondary endosymbiosis, and evolutionary parallels during the process of organelle origination.

evolutionary biology↗

bioRxiv2020

Comparative genomics supports that Brazilian bioethanol Saccharomyces cerevisiae comprise a unified group of domesticated strains related to cachaca spirit yeasts

Ethanol production from sugarcane is a key renewable fuel industry in Brazil. Major drivers of this alcoholic fermentation are Saccharomyces cerevisiae strains that originally were contaminants to the system and yet prevail in the industrial process. Here we present newly sequenced genomes (using Illumina short-read and PacBio long-read data) of two monosporic isolates (H3 and H4) of the S. cerevisiae PE-2, a predominant bioethanol strain in Brazil. The assembled genomes of H3 and H4, together with 42 draft genomes of sugarcane-fermenting (fuel ethanol plus cachaca) strains, were compared against those of the reference S288c and diverse S. cerevisiae. All genomes of bioethanol yeasts have amplified SNO2(3)/SNZ2(3) gene clusters for vitamin B1/B6 biosynthesis, and display ubiquitous presence of SAM-dependent methyl transferases, a gene family rare in S. cerevisiae. Widespread amplifications of quinone oxidoreductases YCR102C/YLR460C/YNL134C, and the structural or punctual variations among aquaporins and components of the iron homeostasis system, likely represent adaptations to industrial fermentation. Interesting is the pervasive presence among the bioethanol/cachaca strains of a five-gene cluster (Region B) that is a known phylogenetic signature of European wine yeasts. Combining genomes of H3, H4, and 195 yeast strains, we comprehensively assessed whole-genome phylogeny of these taxa using an alignment-free approach. The 197-genome phylogeny substantiates that bioethanol yeasts are monophyletic and closely related to the cachaca and wine strains. Our results support the hypothesis that biofuel-producing yeasts in Brazil may have been co-opted from a pool of yeasts that were pre-adapted to alcoholic fermentation of sugarcane for the distillation of cachaca spirit, which historically is a much older industry than the large-scale fuel ethanol production.

genomics↗

bioRxiv2020

Genomic adaptations to an endolithic lifestyle in the coral-associated alga Ostreobium

The green alga Ostreobium is an important coral holobiont member, playing key roles in skeletal decalcification and providing photosynthate to bleached corals that have lost their dinoflagellate endosymbionts. Ostreobium lives in the corals skeleton, a low-light environment with variable pH and O{square} availability. We present the Ostreobium nuclear genome and a metatranscriptomic analysis of healthy and bleached corals to improve our understanding of Ostreobiums adaptations to its extreme environment and its roles as a coral holobiont member. The Ostreobium genome has 10,663 predicted protein-coding genes and shows adaptations for life in low and variable light conditions and other stressors in the endolithic environment. This alga presents a rich repertoire of light-harvesting complex proteins but lacks many genes for photoprotection and photoreceptors. It also has a large arsenal of genes for oxidative stress response. An expansion of extracellular peptidases suggests that Ostreobium may supplement its energy needs by feeding on the organic skeletal matrix, and a diverse set of fermentation pathways allow it to live in the anoxic skeleton at night. Ostreobium depends on other holobiont members for vitamin B12, and our metatranscriptomes identify potential bacterial sources. Metatranscriptomes showed Ostreobium becoming a dominant agent of photosynthesis in bleached corals and provided evidence for variable responses among coral samples and different Ostreobium genotypes. Our work provides a comprehensive understanding of the adaptations of Ostreobium to its extreme environment and an important genomic resource to improve our comprehension of coral holobiont resilience, bleaching and recovery.

genomics↗

bioRxiv2019

Close relationships between coral-associated Chromera strains despite differences within the Symbiodiniaceae

Reef-building corals live in a mutualistic relationship with photosynthetic algae (family Symbiodiniaceae) that usually provide most of the energy required by the coral host. This relationship is sensitive to temperature stress; as little as a 1{degrees}C increase often leading to collapse of the association. This sensitivity has led to interest in the potential of more stress tolerant algae to supplement or substitute for the normal Symbiodiniaceae mutualists. In this respect, the apicomplexan-like microalga Chromera is of particular interest due to its greater temperature tolerance. We generated a de novo transcriptome for a Chromera strain isolated from a GBR coral ("GBR Chromera") and compared to those of the reference strain of Chromera ("Sydney Chromera"), and to those of Symbiodiniaceae (Fugacium, Cladocopium and Breviolum), as well as the apicomplexan parasite, Plasmodium falciparum. By contrast with the Symbiodiniaceae, the two Chromera strains had a high level of sequence similarity evident by very low levels of divergence in orthologous genes. Although KEGG categories provide few criteria by which true coral mutualists might be identified, they do supply a molecular rationalization for the ubiquitous association of Cladocopium strains with Indo-Pacific reef corals. The presence of HSP20 genes may underlie the higher thermal tolerance of Chromera.

genomics↗