Identification of Genes under Positive Selection Reveals Differences in Evolutionary Adaptation between Brown-Algal Species

Exon shuffling potentiates a diverse repertoire of brown algal NB-ARC-TPR candidate immune receptor proteins via alternative splicing

Like other organisms, brown algae are subject to diseases caused by bacteria, fungi, and viruses. Brown algal immunity mechanisms are not well characterized; however, there is evidence suggesting that pathogen receptors exist in brown algae. One key protein family likely associated with brown algal innate immunity possesses an NB-ARC domain analogous to innate immune proteins in plants and animals. In this study, we conducted an extensive survey of NB-ARC genes in brown algae and obtained insights into the domain organization and evolutionary history of the encoded proteins. Our data show that brown algae possess an ancient NB-ARC-tetratricopeptide repeat (NB-TPR) domain architecture. We identified an N-terminal effector domain, the four-helix bundle, which was not previously found associated with NB-ARC domains. The phylogenetic tree including NB-ARC domains from all kingdoms of life suggests the three clades of brown algal NB-TPRs are likely monophyletic, whereas their TPRs seem to have distinct origins. One group of TPRs exhibit intense exon shuffling, with various alternative splicing and diversifying selection acting on them, suggesting exon shuffling is an important mechanism for evolving ligand-binding specificities. The reconciliation of gene duplication and loss events of the NB-ARC genes reveals that more independent gene gains than losses have occurred during brown algal evolution, and that tandem duplication has played a major role in the expansion of NB-ARC genes. Our results substantially enhance our understanding of the evolutionary history and exon shuffling mechanisms of the candidate innate immune repertoire of brown algae.

Genomic coevolution of papillomavirus and immune system in placental mammals indicates the role of IFN-γ in the emergence of new variants

Papillomaviruses (PVs) are causative agents for warts and cancers in different parts of the body in the mammalian lineage. Therefore, these viruses are proposed as model organisms to study host immune responses to pathogens causing chronic infections. The virus-associated cancer progression depends on two integral processes namely angiogenesis and immune response (AIR). The angiogenesis process aids in tumour progression through vessel formation and maturation but the host immune response, in contrast, makes every attempt to eliminate pathogens and thereby maintain healthy tissues. However, the evolutionary contribution of individual viral genes and host AIR genes in carcinogenesis is yet to be explored. Here, we applied the evolutionary genomics approach to find correlated evolution between six PV genes and 23 host AIR-related genes. We estimated that IFN-γ is the only host gene evolving in a correlated manner with all six PV genes under study. Furthermore, three papillomavirus genes, L2, E6, and E7, are found to interact with two third of host AIR-related genes. Moreover, a combined differential gene expression analysis and network analysis showed that inflammatory cytokine IFN-γ is the key regulator of hub genes in the PPI network of the differentially expressed genes. Functional enrichment of these hub genes is consistent with their established role in different cancers and viral infections. Overall, we conclude that IFN-γ maintains selective pressure on mammalian PV genes and seems to be a potential biomarker for PV-related cancers. This study demonstrates the evolutionary importance of IFN-γ in deciding the fate of carcinogenic PV variants.

Discovering novel reproductive genes in a non-model fly using de novo GridION transcriptomics

Gene discovery has important implications for investigating phenotypic trait evolution, adaptation, and speciation. Male reproductive tissues, such as accessory glands (AGs), are hotspots for recruitment of novel genes that diverge rapidly even among closely related species/populations. These genes synthesize seminal fluid proteins that often affect post-copulatory sexual selection-they can mediate male-male sperm competition, ejaculate-female interactions that modify female remating and even influence reproductive incompatibilities among diverging species/populations. Although de novo transcriptomics has facilitated gene discovery in non-model organisms, reproductive gene discovery is still challenging without a reference database as they are often novel and bear no homology to known proteins. Here, we use reference-free GridION long-read transcriptomics, from Oxford Nanopore Technologies (ONT), to discover novel AG genes and characterize their expression in the widespread dung fly, Sepsis punctum. Despite stark population differences in male reproductive traits (e.g.: Body size, testes size, and sperm length) as well as female re-mating, the male AG genes and their secretions of S. punctum are still unknown. We implement a de novo ONT transcriptome pipeline incorporating quality-filtering and rigorous error-correction procedures, and we evaluate gene sequence and gene expression results against high-quality Illumina short-read data. We discover highly-expressed reproductive genes in AG transcriptomes of S. punctum consisting of 40 high-quality and high-confidence ONT genes that cross-verify against Illumina genes, among which 26 are novel and specific to S. punctum. Novel genes account for an average of 81% of total gene expression and may be functionally relevant in seminal fluid protein production. For instance, 80% of genes encoding secretory proteins account for 74% total gene expression. In addition, median sequence similarities of ONT nucleotide and protein sequences match within-Illumina sequence similarities. Read-count based expression quantification in ONT is congruent with Illumina's Transcript per Million (TPM), both in overall pattern and within functional categories. Rapid genomic innovation followed by recruitment of de novo genes for high expression in S. punctum AG tissue, a pattern observed in other insects, could be a likely mechanism of evolution of these genes. The study also demonstrates the feasibility of adapting ONT transcriptomics for gene discovery in non-model systems.

A microRNA-microRNA crosstalk network inferred from genome-wide single nucleotide polymorphism variants in natural populations of Arabidopsis thaliana

To adapt to variable natural conditions, plants have evolved several strategies to respond to different environmental stresses. MicroRNA (miRNA)-mediated gene regulation is one of such strategies. Variants, e.g., single nucleotide polymorphisms (SNPs) within the mature miRNAs or their target sites may cause the alteration of regulatory networks and serious phenotype changes. In this study, we proposed a novel approach to construct a miRNA-miRNA crosstalk network in Arabidopsis thaliana based on the notion that two cooperative miRNAs toward common targets are under a strong pressure to be inherited together across ecotypes. By performing a genome-wide scan of the SNPs within the mature miRNAs and their target sites, we defined a "regulation fate profile" to describe a miRNA-target regulation being static (kept) or dynamic (gained or lost) across 1,135 ecotypes compared with the reference genome of Col-0. The cooperative miRNA pairs were identified by estimating the similarity of their regulation fate profiles toward the common targets. The reliability of the cooperative miRNA pairs was supported by solid expressional correlation, high PPImiRFS scores, and similar stress responses. Different combinations of static and dynamic miRNA-target regulations account for the cooperative miRNA pairs acting on various biological characteristics of miRNA conservation, expression, homology, and stress response. Interestingly, the targets that are co-regulated dynamically by both cooperative miRNAs are more likely to be responsive to stress. Hence, stress-related genes probably bear selective pressures in a certain group of ecotypes, in which miRNA regulations on the stress genes reprogram. Finally, three case studies showed that reprogramming miRNA-miRNA crosstalk toward the targets in specific ecotypes was associated with these ecotypes' climatic variables and geographical locations. Our study highlights the potential of miRNA-miRNA crosstalk as a genetic basis underlying environmental adaptation in natural populations.

Phylotranscriptomic and Evolutionary Analyses of the Green Algal Order Chaetophorales (Chlorophyceae, Chlorophyta)

Considering the phylogenetic differences in the taxonomic framework of the Chaetophorales as determined by the use of nuclear molecular markers or chloroplast genes, the current study was the first to use phylotranscriptomic analyses comparing the transcriptomes of 12 Chaetophorales algal species. The results showed that a total of 240,133 gene families and 143 single-copy orthogroups were identified. Based on the single-copy orthogroups, supergene analysis and the coalescent-based approach were adopted to perform phylotranscriptomic analysis of the Chaetophorales. The phylogenetic relationships of most species were consistent with those of phylogenetic analyses based on the chloroplast genome data rather than nuclear molecular markers. The Schizomeriaceae and the Aphanochaetaceae clustered into a well-resolved basal clade in the Chaetophorales by either strategy. Evolutionary analyses of divergence time and substitution rate also revealed that the closest relationships existed between the Schizomeriaceae and Aphanochaetaceae. All species in the Chaetophorales exhibited a large number of expanded and contracted gene families, in particular the common ancestor of the Schizomeriaceae and Aphanochaetaceae. The only terrestrial alga, Fritschiella tuberosa, had the greatest number of expanded gene families, which were associated with increased fatty acid biosynthesis. Phylotranscriptomic and evolutionary analyses all robustly identified the unique taxonomic relationship of Chaetophorales consistent with chloroplast genome data, proving the advantages of high-throughput data in phylogeny.

Independent Evolution of the MYB Family in Brown Algae

Myeloblastosis (MYB) proteins represent one of the largest families of eukaryotic transcription factors and regulate important processes in growth and development. Studies on MYBs have mainly focused on animals and plants; however, comprehensive analysis across other supergroups such as SAR (stramenopiles, alveolates, and rhizarians) is lacking. This study characterized the structure, evolution, and expression of MYBs in four brown algae, which comprise the biggest multicellular lineage of SAR. Subfamily 1R-MYB comprised heterogeneous proteins, with fewer conserved motifs found outside the MYB domain. Unlike the SHAQKY subgroup of plant 1R-MYB, THAQKY comprised the largest subgroup of brown algal 1R-MYBs. Unlike the expansion of 2R-MYBs in plants, brown algae harbored more 3R-MYBs than 2R-MYBs. At least ten 2R-MYBs, fifteen 3R-MYBs, and one 6R-MYB orthologs existed in the common ancestor of brown algae. Phylogenetic analysis showed that brown algal MYBs had ancient origins and a diverged evolution. They showed strong affinity with stramenopile species, while not with red algae, green algae, or animals, suggesting that brown algal MYBs did not come from the secondary endosymbiosis of red and green plastids. Sequence comparison among all repeats of the three types of MYB subfamilies revealed that the repeat of 1R-MYBs showed higher sequence identity with the R3 of 2R-MYBs and 3R-MYBs, which supports the idea that 1R-MYB was derived from loss of the first and second repeats of the ancestor MYB. Compared with other species of SAR, brown algal MYB proteins exhibited a higher proportion of intrinsic disordered regions, which might contribute to multicellular evolution. Expression analysis showed that many MYB genes are responsive to different stress conditions and developmental stages. The evolution and expression analyses provided a comprehensive analysis of the phylogeny and functions of MYBs in brown algae.

Ketogenic diet-mediated steroid metabolism reprogramming improves the immune microenvironment and myelin growth in spinal cord injury rats according to gene and co-expression network analyses

The ketogenic diet has been widely used in the treatment of various nervous system and metabolic-related diseases. Our previous research found that a ketogenic diet exerts a protective effect and promotes functional recovery after spinal cord injury. However, the mechanism of action is still unclear. In this study, different dietary feeding methods were used, and myelin expression and gene level changes were detected among different groups. We established 15 RNA-seq cDNA libraries from among 4 different groups. First, KEGG pathway enrichment of upregulated differentially expressed genes and gene set enrichment analysis of the ketogenic diet and normal diet groups indicated that a ketogenic diet significantly improved the steroid anabolic pathway in rats with spinal cord injury. Through cluster analysis, protein-protein interaction analysis and visualization of iPath metabolic pathways, it was determined that Sqle, Sc5d, Cyp51, Dhcr24, Msmo1, Hsd17b7, and Fdft1 expression changed significantly. Second, through weighted gene co-expression network analysis showed that rats fed a ketogenic diet showed a significant reduction in the expression of genes involved in immune-related pathways, including those associated with immunity and infectious diseases. A ketogenic diet may improve the immune microenvironment and myelin growth in rats with spinal cord injury through reprogramming of steroid metabolism.

Integrative analysis of chloroplast DNA methylation in a marine alga-Saccharina japonica

KEY MESSAGE

We applied an integrative approach using multiple methods to verify cytosine methylation in the chloroplast DNA of the multicellular brown alga Saccharina japonica. Cytosine DNA methylation is a heritable process which plays important roles in regulating development throughout the life cycle of an organism. Although methylation of nuclear DNA has been studied extensively, little is known about the state and role of DNA methylation in chloroplast genomes, especially in marine algae. Here, we have applied an integrated approach encompassing whole-genome bisulfite sequencing, methylated DNA immunoprecipitation, gene co-expression networks and photophysiological analyses to provide evidence for the role of chloroplast DNA methylation in a marine alga, the multicellular brown alga Saccharina japonica. Although the overall methylation level was relatively low in the chloroplast genome of S. japonica, gametophytes exhibited higher methylation levels than sporophytes. Gene-specific bisulfite-cloning sequencing provided additional evidence for the methylation of key photosynthetic genes. Many of them were highly expressed in sporophytes whereas genes involved in transcription, translation and biosynthesis were strongly expressed in gametophytes. Nucleus-encoded photosynthesis genes were co-expressed with their chloroplast-encoded counterparts potentially contributing to the higher photosynthetic performance in sporophytes compared to gametophytes where these co-expression networks were less pronounced. A nucleus-encoded DNA methyltransferase of the DNMT2 family is assumed to be responsible for the methylation of the chloroplast genome because it is predicted to possess a plastid transit peptide.

Divergent evolution and purifying selection of the Type 2 diabetes gene sequences in Drosophila: a phylogenomic study

The recently developed phylogenomic approach provides a unique way to identify disease risk or protective allele in any organism. While risk alleles evolve mostly under purifying selection, protective alleles are evolving either under balancing or positive selection. Owing to insufficient information, authors employed the phylogenomic approach to detect the nature of selection acting on type 2 diabetes (T2D) genes in Drosophila genus using various models of CODEML utility of PAML. The obtained result revealed that T2D gene sequences are evolving under purifying selection. However, only a few sites in membrane proteins encoded via CG8051, ZnT35C, and kar, are significantly evolving under positive selection under specific scenarios, which might be because of positive or adaptive evolution in response to changing niche, diet or other factors. In the near future, this information will be highly useful in the field of evolutionary medicine and the drug discovery process.

Relaxed Selection and the Rapid Evolution of Reproductive Genes

Evolutionary genomic studies find that reproductive protein genes, those directly involved in reproductive processes, diversify more rapidly than most other gene categories. Strong postcopulatory sexual selection acting within species is the predominant hypothesis proposed to account for the observed pattern. Recently, relaxed selection due to sex-specific gene expression has also been put forward to explain the relatively rapid diversification. We contend that relaxed selection due to sex-limited gene expression is the correct null model for tests of molecular evolution of reproductive genes and argue that it may play a more significant role in the evolutionary diversification of reproductive genes than previously recognized. We advocate for a re-evaluation of adaptive explanations for the rapid diversification of reproductive genes.

Genome-wide analysis of the Saccharina japonica sulfotransferase genes and their transcriptional profiles during whole developmental periods and under abiotic stresses

BACKGROUND

As a unique sulfated polysaccharide, fucoidan is an important component of cell wall in brown seaweeds. Its biochemical properties are determined by the positions and quantity of sulfate groups. Sulfotransferases (STs) catalyze the sulfation process, which transfer the sulfuryl groups to carbohydrate backbones and are crucial for fucoidan biosynthesis. Nevertheless, the structures and functions of STs in brown seaweeds are rarely investigated.

RESULTS

There are a total of 44 ST genes identified from our genome and transcriptome analysis of Saccharina japonica, which were located in the 17 scaffolds and 11 contigs. The S. japonica ST genes have abundant introns and alternative splicing sites, and five tandem duplicated gene clusters were identified. Generally, the ST genes could be classified into five groups (Group I ~ V) based on phylogenetic analysis. Accordingly, the ST proteins, which were encoded by genes within the same group, contained similar conserved motifs. Members of the S. japonica ST gene family show various expression patterns in different tissues and developmental stages. Transcriptional profiles indicate that the transcriptional levels of more than half of the ST genes are higher in kelp basal blades than in distal blades. Except for ST5 and ST28, most ST genes are down-regulated with the kelp development stages. The expression levels of nine ST genes were detected by real-time quantitative PCR, which demonstrates that they responded to low salinity and drought stresses.

CONCLUSIONS

Various characteristics of the STs allow the feasibilities of S. japonica to synthesize fucoidans with different sulfate groups. This enables the kelp the potential to adapt to the costal environments and meet the needs of S. japonica growth.

Single-base methylome profiling of the giant kelp Saccharina japonica reveals significant differences in DNA methylation to microalgae and plants

Brown algae have convergently evolved plant-like body plans and reproductive cycles, which in plants are controlled by differential DNA methylation. This contribution provides the first single-base methylome profiles of haploid gametophytes and diploid sporophytes of a multicellular alga. Although only c. 1.4% of cytosines in Saccharina japonica were methylated mainly at CHH sites and characterized by 5-methylcytosine (5mC), there were significant differences between life-cycle stages. DNA methyltransferase 2 (DNMT2), known to efficiently catalyze tRNA methylation, is assumed to methylate the genome of S. japonica in the structural context of tRNAs as the genome does not encode any other DNA methyltransferases. Circular and long noncoding RNA genes were the most strongly methylated regulatory elements in S. japonica. Differential expression of genes was negatively correlated with DNA methylation with the highest methylation levels measured in both haploid gametophytes. Hypomethylated and highly expressed genes in diploid sporophytes included genes involved in morphogenesis and halogen metabolism. The data herein provide evidence that cytosine methylation, although occurring at a low level, is significantly contributing to the formation of different life-cycle stages, tissue differentiation and metabolism in brown algae.

Transcriptomic analysis reveals insights into deep-sea adaptations of the dominant species, Shinkaia crosnieri (Crustacea: Decapoda: Anomura), inhabiting both hydrothermal vents and cold seeps

Background

Hydrothermal vents and cold seeps are typical deep-sea chemosynthetically-driven ecosystems that allow high abundance of specialized macro-benthos. To gather knowledge about the genetic basis of adaptation to these extreme environments, species shared between different habitats, especially for the dominant species, are of particular interest. The galatheid squat lobster, Shinkaia crosnieri Baba and Williams, 1998, is one of the few dominant species inhabiting both deep-sea hydrothermal vents and cold seeps. In this study, we performed transcriptome analyses of S. crosnieri collected from the Iheya North hydrothermal vent (HV) and a cold seep in the South China Sea (CS) to provide insights into how this species has evolved to thrive in different deep-sea chemosynthetic ecosystems.

Results

We analyzed 5347 orthologs between HV and CS to identify genes under positive selection through the maximum likelihood approach. A total of 82 genes were identified to be positively selected and covered diverse functional categories, potentially indicating their importance for S. crosnieri to cope with environmental heterogeneity between deep-sea vents and seeps. Among 39,806 annotated unigenes, a large number of differentially expressed genes (DEGs) were identified between HV and CS, including 339 and 206 genes significantly up-regulated in HV and CS, respectively. Most of the DEGs associated with stress response and immunity were up-regulated in HV, possibly allowing S. crosnieri to increase its capability to manage more environmental stresses in the hydrothermal vents.

Conclusions

We provide the first comprehensive transcriptomic resource for the deep-sea squat lobster, S. crosnieri, inhabiting both hydrothermal vents and cold seeps. A number of stress response and immune-related genes were positively selected and/or differentially expressed, potentially indicating their important roles for S. crosnieri to thrive in both deep-sea vents and cold seeps. Our results indicated that genetic adaptation of S. crosnieri to different deep-sea chemosynthetic environments might be mediated by adaptive evolution of functional genes related to stress response and immunity, and alterations in their gene expression that lead to different stress resistance. However, further work is required to test these proposed hypotheses. All results can constitute important baseline data for further studies towards elucidating the adaptive mechanisms in deep-sea crustaceans.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5753-7) contains supplementary material, which is available to authorized users.

Diversity and evolution of cytochromes P450 in stramenopiles

MAIN CONCLUSION

Comparative genomic analysis of cytochromes P450 revealed high diversification and dynamic changes in stramenopiles, associated with transcriptional responsiveness to various environmental stimuli. Comparative genomic and molecular evolution approaches were used to characterize cytochromes P450 (P450) diversity in stramenopiles. Phylogenetic analysis pointed to a high diversity of P450 in stramenopiles and identified three major clans. The CYP51 and CYP97 clans were present in brown algae, diatoms and Nannochloropsis gaditana, whereas the CYP5014 clan mainly includes oomycetes. Gene gain and loss patterns revealed that six CYP families-CYP51, CYP97, CYP5160, CYP5021, CYP5022, and CYP5165-predated the split of brown algae and diatoms. After they diverged, diatoms gained more CYP families, especially in the cold-adapted species Fragilariopsis cylindrus, in which eight new CYP families were found. Selection analysis revealed that the expanded CYP51 family in the brown alga Cladosiphon okamuranus exhibited a more relaxed selection constraint compared with those of other brown algae and diatoms. Our RNA-seq data further evidenced that most of P450s in Saccharina japonica are highly expressed in large sporophytes, which could potentially promote the large kelp formation in this developmental stage. A survey of Ectocarpus siliculosus and diatom transcriptomes showed that many P450s are responsive to stress, nutrient limitation or light quality, suggesting pivotal roles in detoxification or metabolic processes under adverse environmental conditions. The information provided in this study will be helpful in designing functional experiments and interpreting P450 roles in this particular lineage.

Evolution and Expansion of the Prokaryote-Like Lipoxygenase Family in the Brown Alga Saccharina japonica

Lipoxygenase (LOX) plays important roles in fatty acid oxidation and lipid mediator biosynthesis. In this study, we give first insights into brown algal LOX evolution. Whole genome searches revealed four, three, and eleven LOXs in Ectocarpus siliculosus, Cladosiphon okamuranus, and Saccharina japonica, respectively. In phylogenetic analyses, LOXs from brown algae form a robust clade with those from prokaryotes, suggesting an ancestral origin and slow evolution. Brown algal LOXs were divided into two clades, C1 and C2 in a phylogenetic tree. Compared to the two species of Ectocarpales, LOX gene expansion occurred in the kelp S. japonica through tandem duplication and segmental duplication. Selection pressure analysis showed that LOX genes in brown algae have undergone strong purifying selection, while the selective constraint in the C2 clade was more relaxed than that in the C1 clade. Furthermore, within each clade, LOXs of S. japonica evolved under more relaxed selection constraints than E. siliculosus and C. okamuranus. Structural modeling showed that unlike LOXs of plants and animals, which contain a β barrel in the N-terminal part of the protein, LOXs in brown algae fold into a single domain. Analysis of previously published transcriptomic data showed that LOXs in E. siliculosus are responsive to hyposaline, hypersaline, oxidative, and copper stresses. Moreover, clear divergence of expression patterns was observed among different life stages, as well as between duplicate gene pairs. In E. siliculosus, all four LOXs are male-biased in immature gametophytes, and mature gametophytes showed significantly higher LOX mRNA levels than immature gametophytes and sporophytes. In S. japonica, however, our RNA-Seq data showed that most LOXs are highly expressed in sporophytes. Even the most recently duplicated gene pairs showed divergent expression patterns, suggesting that functional divergence has likely occurred since LOX genes duplicated, which potentially contributes to the production of various oxylipins in brown algae.