The genome of a cave plant, Primulina huaijiensis, provides insights into adaptation to limestone karst habitats

Chromosome-level assembly of Lindenbergia philippensis and comparative genomic analyses shed light on genome evolution in Lamiales

Lamiales, comprising over 23,755 species across 24 families, stands as a highly diverse and prolific plant group, playing a significant role in the cultivation of horticultural, ornamental, and medicinal plant varieties. Whole-genome duplication (WGD) and its subsequent post-polyploid diploidization (PPD) process represent the most drastic type of karyotype evolution, injecting significant potential for promoting the diversity of this lineage. However, polyploidization histories, as well as genome and subgenome fractionation following WGD events in Lamiales species, are still not well investigated. In this study, we constructed a chromosome-level genome assembly of Lindenbergia philippensis (Orobanchaceae) and conducted comparative genomic analyses with 14 other Lamiales species. L. philippensis is positioned closest to the parasitic lineage within Orobanchaceae and has a conserved karyotype. Through a combination of Ks analysis and syntenic depth analysis, we reconstructed and validated polyploidization histories of Lamiales species. Our results indicated that Primulina huaijiensis underwent three rounds of diploidization events following the γ-WGT event, rather than two rounds as reported. Besides, we reconfirmed that most Lamiales species shared a common diploidization event (L-WGD). Subsequently, we constructed the Lamiales Ancestral Karyotype (LAK), comprising 11 proto-chromosomes, and elucidated its evolutionary trajectory, highlighting the highly flexible reshuffling of the Lamiales paleogenome. We identified biased fractionation of subgenomes following the L-WGD event across eight species, and highlighted the positive impacts of non-WGD genes on gene family expansion. This study provides novel genomic resources and insights into polyploidy and karyotype remodeling of Lamiales species, essential for advancing our understanding of species diversification and genome evolution.

Landform and lithospheric development contribute to the assembly of mountain floras in China

Although it is well documented that mountains tend to exhibit high biodiversity, how geological processes affect the assemblage of montane floras is a matter of ongoing research. Here, we explore landform-specific differences among montane floras based on a dataset comprising 17,576 angiosperm species representing 140 Chinese mountain floras, which we define as the collection of all angiosperm species growing on a specific mountain. Our results show that igneous bedrock (granitic and karst-granitic landforms) is correlated with higher species richness and phylogenetic overdispersion, while the opposite is true for sedimentary bedrock (karst, Danxia, and desert landforms), which is correlated with phylogenetic clustering. Furthermore, we show that landform type was the primary determinant of the assembly of evolutionarily older species within floras, while climate was a greater determinant for younger species. Our study indicates that landform type not only affects montane species richness, but also contributes to the composition of montane floras. To explain the assembly and differentiation of mountain floras, we propose the 'floristic geo-lithology hypothesis', which highlights the role of bedrock and landform processes in montane floristic assembly and provides insights for future research on speciation, migration, and biodiversity in montane regions.

Incomplete lineage sorting and gene flow within Allium (Amayllidaceae)

The phylogeny and systematics of the genus Allium have been studied with a variety of diverse data types, including an increasing amount of molecular data. However, strong phylogenetic discordance and high levels of uncertainty have prevented the identification of a consistent phylogeny. The difficulty in establishing phylogenetic consensus and evidence for genealogical discordance make Allium a compelling test case to assess the relative contribution of incomplete lineage sorting (ILS), gene flow and gene tree estimation error on phylogenetic reconstruction. In this study, we obtained 75 transcriptomes of 38 Allium species across 10 subgenera. Whole plastid genome, single copy genes and consensus CDS were generated to estimate phylogenetic trees both using coalescence and concatenation methods. Multiple approaches including coalescence simulation, quartet sampling, reticulate network inference, sequence simulation, theta of ILS and reticulation index were carried out across the CDS gene trees to investigate the degrees of ILS, gene flow and gene tree estimation error. Afterward, a regression analysis was used to test the relative contributions of each of these forms of uncertainty to the final phylogeny. Despite extensive topological discordance among gene trees, we found a fully supported species tree that agrees with the most of well-accepted relationships and establishes monophyly of the genus Allium. We presented clear evidence for substantial ILS across the phylogeny of Allium. Further, we identified two ancient hybridization events for the formation of the second evolutionary line and subg. Butomissa as well as several introgression events between recently diverged species. Our regression analysis revealed that gene tree inference error and gene flow were the two most dominant factors explaining for the overall gene tree variation, with the difficulty in disentangling the effects of ILS and gene tree estimation error due to a positive correlation between them. Based on our efforts to mitigate the methodological errors in reconstructing trees, we believed ILS and gene flow are two principal reasons for the oft-reported phylogenetic heterogeneity of Allium. This study presents a strongly-supported and well-resolved phylogenetic backbone for the sampled Allium species, and exemplifies how to untangle heterogeneity in phylogenetic signal and reconstruct the true evolutionary history of the target taxa.

Haplotype-resolved genome assembly of Phanera championii reveals molecular mechanisms of flavonoid synthesis and adaptive evolution

Phanera championii is a medicinal liana plant that has successfully adapted to hostile karst habitats. Despite extensive research on its medicinal components and pharmacological effects, the molecular mechanisms underlying the biosynthesis of critical flavonoids and its adaptation to karst habitats remain elusive. In this study, we performed high-coverage PacBio and Hi-C sequencing of P. championii, which revealed its high heterozygosity and phased the genome into two haplotypes: Hap1 (384.60 Mb) and Hap2 (383.70 Mb), encompassing a total of 58 612 annotated genes. Comparative genomes analysis revealed that P. championii experienced two whole-genome duplications (WGDs), with approximately 59.59% of genes originating from WGD events, thereby providing a valuable genetic resource for P. championii. Moreover, we identified a total of 112 genes that were strongly positively selected. Additionally, about 81.60 Mb of structural variations between the two haplotypes. The allele-specific expression patterns suggested that the dominant effect of P. championii was the elimination of deleterious mutations and the promotion of beneficial mutations to enhance fitness. Moreover, our transcriptome and metabolome analysis revealed alleles in different tissues or different haplotypes collectively regulate the synthesis of flavonoid metabolites. In summary, our comprehensive study highlights the significance of genomic and morphological adaptation in the successful adaptation of P. championii to karst habitats. The high-quality phased genomes obtained in this study serve as invaluable genomic resources for various applications, including germplasm conservation, breeding, evolutionary studies, and elucidation of pathways governing key biological traits of P. championii.

The chromosome-level genome of the submerged plant Cryptocoryne crispatula provides insights into the terrestrial-freshwater transition in Araceae

Plant terrestrialization (i.e. the transition to a terrestrial environment) is a significant evolutionary event that has been intensively studied. While certain plant lineages, particularly in angiosperms, have re-adapted to freshwater habitats after colonizing terrene, however, the molecular mechanism of the terrestrial-freshwater (T-F) transition remains limited. Here, the basal monocot Araceae was selected as the study object to explore the T-F transition adaptation mechanism by comparative genomic analysis. Our findings revealed that the substitution rates significantly increased in the lineage of freshwater Araceae, which may promote their adaptation to the freshwater habitat. Additionally, 20 gene sets across all four freshwater species displayed signs of positive selection contributing to tissue development and defense responses in freshwater plants. Comparative synteny analysis showed that genes specific to submerged plants were enriched in cellular respiration and photosynthesis. In contrast, floating plants were involved in regulating gene expression, suggesting that gene and genome duplications may provide the original material for plants to adapt to the freshwater environment. Our study provides valuable insights into the genomic aspects of the transition from terrestrial to aquatic environments in Araceae, laying the groundwork for future research in the angiosperm.

Global Analysis of the WOX Transcription Factor Family in Akebia trifoliata

Akebia trifoliata is an economically important, self-incompatible fruit tree in the Lardizabalaceae family. Asexual propagation is the main strategy used to maintain excellent agronomic traits. However, the generation of adventitious roots during asexual propagation is very difficult. To study the important role of the WUSCHEL-related homeobox (WOX) transcription factor in adventitious root growth and development, we characterized this transcription factor family in the whole genome of A. trifoliata. A total of 10 AktWOXs were identified, with the following characteristics: length (657~11,328 bp), exon number (2~5), isoelectric point (5.65~9.03), amino acid number (176~361 AA) and molecular weight (20.500~40.173 kDa), and their corresponding expression sequence could also be detectable in the public transcriptomic data for A. trifoliata fruit. A total of 10 AktWOXs were classified into modern (6), intermediate (2) and ancient clades (2) and all AktWOXs had undergone strong purifying selection during evolution. The expression profile of AktWOXs during A. trifoliata adventitious root formation indicated that AktWOXs play an important role in the regulation of adventitious root development. Overall, this is the first study to identify and characterize the WOX family in A. trifoliata and will be helpful for further research on A. trifoliata adventitious root formation.

Phylogenomic Analyses Reveal an Allopolyploid Origin of Core Didymocarpinae (Gesneriaceae) Followed by Rapid Radiation

Allopolyploid plants have long been regarded as possessing genetic advantages under certain circumstances due to the combined effects of their hybrid origins and duplicated genomes. However, the evolutionary consequences of allopolyploidy in lineage diversification remain to be fully understood. Here, we investigate the evolutionary consequences of allopolyploidy using 138 transcriptomic sequences of Gesneriaceae, including 124 newly sequenced, focusing particularly on the largest subtribe Didymocarpinae. We estimated the phylogeny of Gesneriaceae using concatenated and coalescent-based methods based on five different nuclear matrices and 27 plastid genes, focusing on relationships among major clades. To better understand the evolutionary affinities in this family, we applied a range of approaches to characterize the extent and cause of phylogenetic incongruence. We found that extensive conflicts between nuclear and chloroplast genomes and among nuclear genes were caused by both incomplete lineage sorting (ILS) and reticulation, and we found evidence of widespread ancient hybridization and introgression. Using the most highly supported phylogenomic framework, we revealed multiple bursts of gene duplication throughout the evolutionary history of Gesneriaceae. By incorporating molecular dating and analyses of diversification dynamics, our study shows that an ancient allopolyploidization event occurred around the Oligocene-Miocene boundary, which may have driven the rapid radiation of core Didymocarpinae.

Phylogenomics reveals widespread hybridization and polyploidization in Henckelia (Gesneriaceae)

BACKGROUND AND AIMS

Hybridization has long been recognized as an important process for plant evolution and is often accompanied by polyploidization, another prominent force in generating biodiversity. Despite its pivotal importance in evolution, the actual prevalence and distribution of hybridization across the tree of life remain unclear.

METHODS

We used whole-genome shotgun (WGS) sequencing and cytological data to investigate the evolutionary history of Henckelia, a large genus in the family Gesneriaceae with a high frequency of suspected hybridization and polyploidization events. We generated WGS sequencing data at about 10× coverage for 26 Chinese Henckelia species plus one Sri Lankan species. To untangle the hybridization history, we separately extracted whole plastomes and thousands of single-copy nuclear genes from the sequencing data, and reconstructed phylogenies based on both nuclear and plastid data. We also explored sources of both genealogical and cytonuclear conflicts and identified signals of hybridization and introgression within our phylogenomic dataset using several statistical methods. Additionally, to test the polyploidization history, we evaluated chromosome counts for 45 populations of the 27 Henckelia species studied.

KEY RESULTS

We obtained well-supported phylogenetic relationships using both concatenation- and coalescent-based methods. However, the nuclear phylogenies were highly inconsistent with the plastid phylogeny, and we observed intensive discordance among nuclear gene trees. Further analyses suggested that both incomplete lineage sorting and gene flow contributed to the observed cytonuclear and genealogical discordance. Our analyses of introgression and phylogenetic networks revealed a complex history of hybridization within the genus Henckelia. In addition, based on chromosome counts for 27 Henckelia species, we found independent polyploidization events occurred within Henckelia after different hybridization events.

CONCLUSIONS

Our findings demonstrated that hybridization and polyploidization are common in Henckelia. Furthermore, our results revealed that H. oblongifolia is not a member of the redefined Henckelia and they suggested several other taxonomic treatments in this genus.

Genomic Insights into Adaptation to Karst Limestone and Incipient Speciation in East Asian Platycarya spp. (Juglandaceae)

When challenged by similar environmental conditions, phylogenetically distant taxa often independently evolve similar traits (convergent evolution). Meanwhile, adaptation to extreme habitats might lead to divergence between taxa that are otherwise closely related. These processes have long existed in the conceptual sphere, yet molecular evidence, especially for woody perennials, is scarce. The karst endemic Platycarya longipes and its only congeneric species, Platycarya strobilacea, which is widely distributed in the mountains in East Asia, provide an ideal model for examining the molecular basis of both convergent evolution and speciation. Using chromosome-level genome assemblies of both species, and whole-genome resequencing data from 207 individuals spanning their entire distribution range, we demonstrate that P. longipes and P. strobilacea form two species-specific clades, which diverged around 2.09 million years ago. We find an excess of genomic regions exhibiting extreme interspecific differentiation, potentially due to long-term selection in P. longipes, likely contributing to the incipient speciation of the genus Platycarya. Interestingly, our results unveil underlying karst adaptation in both copies of the calcium influx channel gene TPC1 in P. longipes. TPC1 has previously been identified as a selective target in certain karst-endemic herbs, indicating a convergent adaptation to high calcium stress among karst-endemic species. Our study reveals the genic convergence of TPC1 among karst endemics and the driving forces underneath the incipient speciation of the two Platycarya lineages.

Genome-wide analysis of the cellulose toolbox of Primulina eburnea, a calcium-rich vegetable

BACKGROUND

Human-guided crop domestication has lasted for more than 10,000 years. In terms of the domestication and breeding of vegetables, cellulose content in edible tissues is one of the most important traits. Primulina eburnea is a recently developed calcium-rich vegetable with a high soluble and bioavailable calcium content in its leaves. However, the high cellulose content in the leaves hampers the taste, and no research has been reported on the genetic basis of cellulose biosynthesis in this calcium-rich vegetable.

RESULTS

We identified 36 cellulose biosynthesis-involved genes belonging to eight gene families in the P. eburnea genome. The cellulose accumulated decreasingly throughout leaf development. Nineteen genes were considered core genes in cellulose biosynthesis, which were highly expressed in buds but lowly expressed in mature leaves. In the nitrogen fertilization experiment, exogenous nitrogen decreased the cellulose content in the buds. The expressing pattern of 14 genes were consistent with phenotypic variation in the nitrogen fertilization experiment, and thus they were proposed as cellulose toolbox genes.

CONCLUSIONS

The present study provides a strong basis for the subsequent functional research of cellulose biosynthesis-involved genes in P. eburnea, and provides a reference for breeding and/or engineering this calcium-rich vegetable with decreased leaf cellulose content to improve the taste.

Genetic and ecophysiological evidence that hybridization facilitated lineage diversification in yellow Camellia (Theaceae) species: a case study of natural hybridization between C. micrantha and C. flavida

BACKGROUND

Hybridization is generally considered an important creative evolutionary force, yet this evolutionary process is still poorly characterized in karst plants. In this study, we focus on natural hybridization in yellow Camellia species, a group of habitat specialists confined to karst/non-karst habitats in southwestern China.

RESULTS

Based on population genome data obtain from double digest restriction-site associated DNA (ddRAD) sequencing, we found evidence for natural hybridization and introgression between C. micrantha and C. flavida, and specifically confirmed their hybrid population, C. "ptilosperma". Ecophysiological results suggested that extreme hydraulic traits were fixed in C. "ptilosperma", these being consistent with its distinct ecological niche, which lies outside its parental ranges.

CONCLUSION

The identified hybridization event is expected to have played a role in generating novel variation during, in which the hybrid population displays different phenological characteristics and novel ecophysiological traits associated with the colonization of a new niche in limestone karst.

Identification of Photoperiod- and Phytohormone-Responsive DNA-Binding One Zinc Finger (Dof) Transcription Factors in Akebia trifoliata via Genome-Wide Expression Analysis

As a kind of plant-specific transcription factor (TF), DNA-Binding One Zinc Finger (Dof) is widely involved in the response to environmental change, and as an evolutionarily important perennial plant species, Akebia trifoliata is ideal for studying environmental adaptation. In this study, a total of 41 AktDofs were identified in the A. trifoliata genome. First, the characteristics, including the length, exon number, and chromosomal distribution of the AktDofs and the isoelectric point (PI), amino acid number, molecular weight (MW), and conserved motifs of their putative proteins, were reported. Second, we found that all AktDofs evolutionarily underwent strong purifying selection, and many (33, 80.5%) of them were generated by whole-genome duplication (WGD). Third, we outlined their expression profiles by the use of available transcriptomic data and RT-qPCR analysis. Finally, we identified four candidate genes (AktDof21, AktDof20, AktDof36, and AktDof17) and three other candidate genes (AktDof26, AktDof16, and AktDof12) that respond to long day (LD) and darkness, respectively, and that are closely associated with phytohormone-regulating pathways. Overall, this research is the first to identify and characterize the AktDofs family and is very helpful for further research on A. trifoliata adaptation to environmental factors, especially photoperiod changes.

Chromosomal-level genome assembly of Melastoma candidum provides insights into trichome evolution

Melastoma, consisting of ~100 species diversified in tropical Asia and Oceania in the past 1-2 million years, represents an excellent example of rapid speciation in flowering plants. Trichomes on hypanthia, twigs and leaves vary markedly among species of this genus and are the most important diagnostic traits for species identification. These traits also play critical roles in contributing to differential adaptation of these species to their own habitats. Here we sequenced the genome of M. candidum, a common, erect-growing species from southern China, with the aim to provide genomic insights into trichome evolution in this genus. We generated a high-quality, chromosome-level genome assembly of M. candidum, with the genome size of 256.2 Mb and protein-coding gene number of 40,938. The gene families specific to, and significantly expanded in Melastoma are enriched for GO terms related to trichome initiation and differentiation. We provide evidence that Melastoma and its sister genus Osbeckia have undergone two whole genome duplications (WGDs) after the triplication event (γ) shared by all core eudicots. Preferential retention of trichome development-related transcription factor genes such as C2H2, bHLH, HD-ZIP, WRKY, and MYB after both WGDs might provide raw materials for trichome evolution and thus contribute to rapid species diversification in Melastoma. Our study provides candidate transcription factor genes related to trichome evolution in Melastoma, which can be used to evolutionary and functional studies of trichome diversification among species of this genus.

PlantRNA 2.0: an updated database dedicated to tRNAs of photosynthetic eukaryotes

PlantRNA (http://plantrna.ibmp.cnrs.fr/) is a comprehensive database of transfer RNA (tRNA) gene sequences retrieved from fully annotated nuclear, plastidial and mitochondrial genomes of photosynthetic organisms. In the first release (PlantRNA 1.0), tRNA genes from 11 organisms were annotated. In this second version, the annotation was implemented to 51 photosynthetic species covering the whole phylogenetic tree of photosynthetic organisms, from the most basal group of Archeplastida, the glaucophyte Cyanophora paradoxa, to various land plants. tRNA genes from lower photosynthetic organisms such as streptophyte algae or lycophytes as well as extremophile photosynthetic species such as Eutrema parvulum were incorporated in the database. As a whole, about 37 000 tRNA genes were accurately annotated. In the frame of the tRNA genes annotation from the genome of the Rhodophyte Chondrus crispus, non-canonical splicing sites in the D- or T-regions of tRNA molecules were identified and experimentally validated. As for PlantRNA 1.0, comprehensive biological information including 5'- and 3'-flanking sequences, A and B box sequences, region of transcription initiation and poly(T) transcription termination stretches, tRNA intron sequences and tRNA mitochondrial import are included.

Plastomes of limestone karst gesneriad genera Petrocodon and Primulina, and the comparative plastid phylogenomics of Gesneriaceae

Petrocodon and Primulina are two characteristic genera of Gesneriaceae that exhibit remarkable species and floral diversity, and high endemism across the Sino-Vietnamese Limestone Karsts. To better understand the evolution of limestone gesneriad plastomes, we report nine complete plastomes of seven Primulina and two Petrocodon which have never been assembled before. The newly generated plastomes range from 152,323 to 153,786 bp in size and display a typical quadripartite structure. To further explore the plastome evolution across Gesneriaceae, we assembled five additional plastomes from public reads data and incorporated 38 complete Gesneriaceae plastomes available online into comparative and phylogenomic analyses. The comparison of 52 Gesneriaceae plastomes reveals that not only Primulina and Petrocodon but all gesneriad genera analyzed are highly conserved in genome size, genome structure, gene contents, IR boundary configurations, and codon usage bias. Additionally, sliding window analyses were implemented across alignments of Primulina and Petrocodon for identifying highly variable regions, providing informative markers for future studies. Meanwhile, the SSRs and long repeats of Gesneriaceae plastomes were characterized, serving as useful data in studying population and repetitive sequence evolutions. The results of plastome phylogenetics represent a preliminary but highly resolved maternal backbone genealogy of Primulina and the Old World subtribes of Gesneriaceae.

Integrated multi-omics analysis and microbial recombinant protein system reveal hydroxylation and glycosylation involving nevadensin biosynthesis in Lysionotus pauciflorus

Background

Karst-adapted plant, Lysionotus pauciflours accumulates special secondary metabolites with a wide range of pharmacological effects for surviving in drought and high salty areas, while researchers focused more on their environmental adaptations and evolutions. Nevadensin (5,7-dihydroxy-6,8,4'-trimethoxyflavone), the main active component in L. pauciflours, has unique bioactivity of such as anti-inflammatory, anti-tubercular, and anti-tumor or cancer. Complex decoration of nevadensin, such as hydroxylation and glycosylation of the flavone skeleton determines its diversity and biological activities. The lack of omics data limits the exploration of accumulation mode and biosynthetic pathway. Herein, we integrated transcriptomics, metabolomics, and microbial recombinant protein system to reveal hydroxylation and glycosylation involving nevadensin biosynthesis in L. pauciflours.

Results

Up to 275 flavonoids were found to exist in L. pauciflorus by UPLC-MS/MS based on widely targeted metabolome analysis. The special flavone nevadensin (5,7-dihydroxy-6,8,4'-trimethoxyflavone) is enriched in different tissues, as are its related glycosides. The flavonoid biosynthesis pathway was drawn based on differential transcripts analysis, including 9 PAL, 5 C4H, 8 4CL, 6 CHS, 3 CHI, 1 FNSII, and over 20 OMTs.

Total 310 LpCYP450s were classified into 9 clans, 36 families, and 35 subfamilies, with 56% being A-type CYP450s by phylogenetic evolutionary analysis. According to the phylogenetic tree with AtUGTs, 187 LpUGTs clustered into 14 evolutionary groups (A-N), with 74% being E, A, D, G, and K groups.

Two LpCYP82D members and LpUGT95 were functionally identified in Saccharomyces cerevisiae and Escherichia coli, respectively. CYP82D-8 and CYP82D-1 specially hydroxylate the 6- or 8-position of A ring in vivo and in vitro, dislike the function of F6H or F8H discovered in basil which functioned depending on A-ring substituted methoxy. These results refreshed the starting mode that apigenin can be firstly hydroxylated on A ring in nevadensin biosynthesis. Furthermore, LpUGT95 clustered into the 7-OGT family was verified to catalyze 7-O glucosylation of nevadensin accompanied with weak nevadensin 5-O glucosylation function, firstly revealed glycosylation modification of flavones with completely substituted A-ring.

Conclusions

Metabolomic and full-length transcriptomic association analysis unveiled the accumulation mode and biosynthetic pathway of the secondary metabolites in the karst-adapted plant L. pauciflorus. Moreover, functional identification of two LpCYP82D members and one LpUGT in microbe reconstructed the pathway of nevadensin biosynthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01921-2.

Genome-wide analysis of MYB family genes in Tripterygium wilfordii and their potential roles in terpenoid biosynthesis

Terpenoids are a class of significant bioactive components in the woody vine of Tripterygium wilfordii. Previous studies have shown that MYB transcription factors play important roles in plant secondary metabolism, growth, and developmental processes. However, the MYB involved in terpenoid biosynthesis in Tripterygium wilfordii are unknown. To identify Tripterygium wilfordii MYB (TwMYB) genes that are involved in terpenoid biosynthesis, we conducted the genome-wide analysis of the TwMYB gene family. A total of 207 TwMYBs were identified including 84 1R-TwMYB, 117 R2R3-TwMYB, four 3R-TwMYB, and two 4R-TwMYB genes. The most abundant R2R3-TwMYBs together with their Arabidopsis homologs were categorized into 26 subgroups. Intraspecific collinearity analysis found that the 74.9% of the TwMYBs may be generated by segmental duplication events, and 36.7% of duplicated gene pairs were derived from the specific whole genome duplication (WGD) event in Tripterygium wilfordii. In addition, interspecies collinearity analysis found that 16 TwMYB genes formed homologous gene pairs with MYB genes in seven representative species, which indicated they may have a key role in evolution. Notably, we found that the TwMYB genes were differentially expressed in various tissues by expression pattern analysis. In order to further select the candidate genes related to terpenoid biosynthesis, the assay of Methyl jasmonate (MeJA) induction and analysis of phylogenetic tree was conducted. It was speculated that six candidate TwMYB genes (TwMYB33, TwMYB34, TwMYB45, TwMYB67, TwMYB102, and TwMYB103) are involved in regulating terpenoid biosynthesis. This study is the first systematic analysis of the TwMYB gene family and will lay a foundation for the functional characterization of TwMYB genes in the regulation of terpenoid biosynthesis.

Phylogenomic Analyses of Alismatales Shed Light into Adaptations to Aquatic Environments

Land plants first evolved from freshwater algae, and flowering plants returned to water as early as the Cretaceous and multiple times subsequently. Alismatales is the largest clade of aquatic angiosperms including all marine angiosperms, as well as terrestrial plants. We used Alismatales to explore plant adaptations to aquatic environments by analyzing a data set that included 95 samples (89 Alismatales species) covering four genomes and 91 transcriptomes (59 generated in this study). To provide a basis for investigating adaptations, we assessed phylogenetic conflict and whole-genome duplication (WGD) events in Alismatales. We recovered a relationship for the three main clades in Alismatales as (Tofieldiaceae, Araceae) + core Alismatids. We also found phylogenetic conflict among the three main clades that was best explained by incomplete lineage sorting and introgression. Overall, we identified 18 putative WGD events across Alismatales. One of them occurred at the most recent common ancestor of core Alismatids, and three occurred at seagrass lineages. We also found that lineage and life-form were both important for different evolutionary patterns for the genes related to freshwater and marine adaptation. For example, several light- or ethylene-related genes were lost in the seagrass Zosteraceae, but are present in other seagrasses and freshwater species. Stomata-related genes were lost in both submersed freshwater species and seagrasses. Nicotianamine synthase genes, which are important in iron intake, expanded in both submersed freshwater species and seagrasses. Our results advance the understanding of the adaptation to aquatic environments and WGDs using phylogenomics.

The first genome for the Cape Primrose Streptocarpus rexii (Gesneriaceae), a model plant for studying meristem-driven shoot diversity

Cape Primroses (Streptocarpus, Gesneriaceae) are an ideal study system for investigating the genetics underlying species diversity in angiosperms. Streptocarpus rexii has served as a model species for plant developmental research for over five decades due to its unusual extended meristem activity present in the leaves. In this study, we sequenced and assembled the complete nuclear, chloroplast, and mitochondrial genomes of S. rexii using Oxford Nanopore Technologies long read sequencing. Two flow cells of PromethION sequencing resulted in 32 billion reads and were sufficient to generate a draft assembly including the chloroplast, mitochondrial and nuclear genomes, spanning 776 Mbp. The final nuclear genome assembly contained 5,855 contigs, spanning 766 Mbp of the 929-Mbp haploid genome with an N50 of 3.7 Mbp and an L50 of 57 contigs. Over 70% of the draft genome was identified as repeats. A genome repeat library of Gesneriaceae was generated and used for genome annotation, with a total of 45,045 genes annotated in the S. rexii genome. Ks plots of the paranomes suggested a recent whole genome duplication event, shared between S. rexii and Primulina huaijiensis. A new chloroplast and mitochondrial genome assembly method, based on contig coverage and identification, was developed, and successfully used to assemble both organellar genomes of S. rexii. This method was developed into a pipeline and proved widely applicable. The nuclear genome of S. rexii and other datasets generated and reported here will be invaluable resources for further research to aid in the identification of genes involved in morphological variation underpinning plant diversification.

Strong Genetic Structure Observed in Primulina danxiaensis, a Small Herb Endemic to Mount Danxia With Extremely Small Populations

Danxia landform occurring sporadically in southern China is a unique type of petrographic geomorphology. It has nurtured about 400 rare or threatened plant and animal species, whose diversity, endemism, and conservation have called increasing scientific and public attentions. Among them, Primulina danxiaensis (W. B. Liao, S. S. Lin, and R. J. Shen) W. B. Liao and K. F. Chung is a tiny perennial grass species recorded only in Mount Danxia, a natural World Heritage Site as part of China’s Danxia. In this study, restriction site-associated DNA sequencing (RAD-seq) was performed to investigate genetic diversity among these 12 populations of P. danxiaensis. A total of 432,041 variant sites were detected in 84,779 loci across 94 samples. The expected heterozygosity (H_E) ranged from 0.017 to 0.139. Bottleneck signals were detected in most populations, Tajima’s D tests showed that most loci could be under recent positive selection, and one of the six positively selected loci identified by BayeScan was annotated as tRNA^Glu, which may contribute to the species’ adaptation to shady environment. STRUCTURE analysis and phylogenetic tree showed that the 12 populations of P. danxiaensis could be divided into four gene pools (clades) corresponding to their geographic locations, and significant correlation was observed between genetic and geographic distances. Our study demonstrated that P. danxiaensis maintained a middle level of genetic diversity and strong population structure; geographic distance could be an important factor limiting gene flow among populations of P. danxiaensis, which were only sporadically recorded in Mount Danxia.

Multi-tissue transcriptome analysis of two Begonia species reveals dynamic patterns of evolution in the chalcone synthase gene family

Begonia is an important horticultural plant group, as well as one of the most speciose Angiosperm genera, with over 2000 described species. Genus wide studies of genome size have shown that Begonia has a highly variable genome size, and analysis of paralog pairs has previously suggested that Begonia underwent a whole genome duplication. We address the contribution of gene duplication to the generation of diversity in Begonia using a multi-tissue RNA-seq approach. We chose to focus on chalcone synthase (CHS), a gene family having been shown to be involved in biotic and abiotic stress responses in other plant species, in particular its importance in maximising the use of variable light levels in tropical plants. We used RNA-seq to sample six tissues across two closely related but ecologically and morphologically divergent species, Begonia conchifolia and B. plebeja, yielding 17,012 and 19,969 annotated unigenes respectively. We identified the chalcone synthase gene family members in our Begonia study species, as well as in Hillebrandia sandwicensis, the monotypic sister genus to Begonia, Cucumis sativus, Arabidopsis thaliana, and Zea mays. Phylogenetic analysis suggested the CHS gene family has high duplicate turnover, all members of CHS identified in Begonia arising recently, after the divergence of Begonia and Cucumis. Expression profiles were similar within orthologous pairs, but we saw high inter-ortholog expression variation. Sequence analysis showed relaxed selective constraints on some ortholog pairs, with substitutions at conserved sites. Evidence of pseudogenisation and species specific duplication indicate that lineage specific differences are already beginning to accumulate since the divergence of our study species. We conclude that there is evidence for a role of gene duplication in generating diversity through sequence and expression divergence in Begonia.

Genomic insights into molecular adaptation to intertidal environments in the mangrove Aegiceras corniculatum

Mangroves have colonised extreme intertidal environments characterised by high salinity, hypoxia and other abiotic stresses. Aegiceras corniculatum, a pioneer mangrove species that has evolved two specialised adaptive traits (salt secretion and crypto-vivipary) is an attractive ecological model to investigate molecular mechanisms underlying adaptation to intertidal environments. We assembled de novo a high-quality reference genome of A. corniculatum and performed comparative genomic and transcriptomic analyses to investigate molecular mechanisms underlying adaptation to intertidal environments. We provide evidence that A. corniculatum experienced a whole-genome duplication (WGD) event c. 35 Ma. We infer that maintenance of cellular environmental homeostasis is an important adaptive process in A. corniculatum. The 14-3-3 and H⁺ -ATPase protein-coding genes, essential for the salt homeostasis, were preferentially retained after the recent WGD event. Using comparative transcriptomics, we show that genes upregulated under high-salt conditions are involved in salt transport and ROS scavenging. We also found that all homologues of DELAY OF GERMINATION1 (DOG1) had lost their heme-binding ability in A. corniculatum, and that this may contribute to crypto-vivipary. Our study provides insight into the genomic correlates of phenotypic adaptation to intertidal environments. This could contribute not only within the genomics community, but also to the field of plant evolution.

Phylogenomic and Macroevolutionary Evidence for an Explosive Radiation of a Plant Genus in the Miocene

Mountain systems harbor a substantial fraction of global biodiversity and, thus, provide excellent opportunities to study rapid diversification and to understand the historical processes underlying the assembly of biodiversity hotspots. The rich biodiversity in mountains is widely regarded as having arisen under the influence of geological and climatic processes as well as the complex interactions among them. However, the relative contribution of geology and climate in driving species radiation is seldom explored. Here, we studied the evolutionary radiation of Oreocharis (Gesneriaceae), which has diversified extensively throughout East Asia, especially within the Hengduan Mountains (HDM), using transcriptomic data and a time calibrated phylogeny for 88% (111/126) of all species of the genus. In particular, we applied phylogenetic reconstructions to evaluate the extent of incomplete lineage sorting accompanying the early and rapid radiation in the genus. We then fit macroevolutionary models to explore its spatial and diversification dynamics in Oreocharis and applied explicit birth–death models to investigate the effects of past environmental changes on its diversification. Evidence from 574 orthologous loci suggest that Oreocharis underwent an impressive early burst of speciation starting ca. 12 Ma in the Miocene, followed by a drastic decline in speciation toward the present. Although we found no evidence for a shift in diversification rate across the phylogeny of Oreocharis, we showed a difference in diversification dynamics between the HDM and non-HDM lineages, with higher diversification rates in the HDM. The diversification dynamic of Oreocharis is most likely positively associated with temperature-dependent speciation and dependency on the Asian monsoons. We suggest that the warm and humid climate of the mid-Miocene was probably the primary driver of the rapid diversification in Oreocharis, while mountain building of the HDM might have indirectly affected species diversification of the HDM lineage. This study highlights the importance of past climatic changes, combined with mountain building, in creating strong environmental heterogeneity and driving diversification of mountain plants, and suggests that the biodiversity in the HDM cannot directly be attributed to mountain uplift, contrary to many recent speculations.[East Asian monsoons; environmental heterogeneity; Hengduan Mountains; incomplete lineage sorting; Oreocharis; past climate change; rapid diversification; transcriptome.]

Tracing the Diploid Ancestry of the Cultivated Octoploid Strawberry

The commercial strawberry, Fragaria × ananassa, is a recent allo-octoploid that is cultivated worldwide. However, other than Fragaria vesca, which is universally accepted one of its diploid ancestors, its other early diploid progenitors remain unclear. Here, we performed comparative analyses of the genomes of five diploid strawberries, F. iinumae, F. vesca, F. nilgerrensis, F. nubicola, and F. viridis, of which the latter three are newly sequenced. We found that the genomes of these species share highly conserved gene content and gene order. Using an alignment-based approach, we show that F. iinumae and F. vesca are the diploid progenitors to the octoploid F. × ananassa, whereas the other three diploids that we analyzed in this study are not parental species. We generated a fully resolved, dated phylogeny of Fragaria, and determined that the genus arose ∼6.37 Ma. Our results effectively resolve conflicting hypotheses regarding the putative diploid progenitors of the cultivated strawberry, establish a reliable backbone phylogeny for the genus, and provide genetic resources for molecular breeding.

A Combined Morphological and Molecular Evolutionary Analysis of Karst-Environment Adaptation for the Genus Urophysa (Ranunculaceae)

The karst environment is characterized by low soil water content, periodic water deficiency, and poor nutrient availability, which provides an ideal natural laboratory for studying the adaptive evolution of its inhabitants. However, how species adapt to such a special karst environment remains poorly understood. Here, transcriptome sequences of two Urophysa species (Urophysa rockii and Urophysa henryi), which are Chinese endemics with karst-specific distribution, and allied species in Semiaquilegia and Aquilegia (living in non-karst habitat) were collected. Single-copy genes (SCGs) were extracted to perform the phylogenetic analysis using concatenation and coalescent methods. Positively selected genes (PSGs) and clusters of paralogous genes (Mul_genes) were detected and subsequently used to conduct gene function annotation. We filtered 2,271 SCGs and the coalescent analysis revealed that 1,930 SCGs shared the same tree topology, which was consistent with the topology detected from the concatenated tree. Total of 335 PSGs and 243 Mul_genes were detected, and many were enriched in stress and stimulus resistance, transmembrane transport, cellular ion homeostasis, calcium ion transport, calcium signaling regulation, and water retention. Both molecular and morphological evidences indicated that Urophysa species evolved complex strategies for adapting to hostile karst environments. Our findings will contribute to a new understanding of genetic and phenotypic adaptive mechanisms of karst adaptation in plants.

Demographic history and local adaptation of Myripnois dioica (Asteraceae) provide insight on plant evolution in northern China flora

The flora of northern China forms the main part of the Sino-Japanese floristic region and is located in a south-north vegetative transect in East Asia. Phylogeographic studies have demonstrated that an arid belt in this region has promoted divergence of plants in East Asia. However, little is known about how plants that are restricted to the arid belt of flora in northern China respond to climatic oscillation and environmental change. Here, we used genomic-level data of Myripnois dioica across its distribution as a representative of northern China flora to reconstruct plant demographic history, examine local adaptation related to environmental disequilibrium, and investigate the factors related to effective population size change. Our results indicate M. dioica originated from the northern area and expanded to the southern area, with the Taihang Mountains serving as a physical barrier promoting population divergence. Genome-wide evidence found strong correlation between genomic variation and environmental factors, specifically signatures associated with local adaptation to drought stress in heterogeneous environments. Multiple linear regression analyses revealed joint effects of population age, mean temperature of coldest quarter, and precipitation of wettest month on effective population size (Ne). Our current study uses M. dioica as a case for providing new insights into the evolutionary history and local adaptation of northern China flora and provides qualitative strategies for plant conservation.

Genome-Wide Analysis of MYB Gene Family in Chinese Bayberry (Morella rubra) and Identification of Members Regulating Flavonoid Biosynthesis

Chinese bayberry (Morella rubra), the most economically important fruit tree in the Myricaceae family, is a rich source of natural flavonoids. Recently the Chinese bayberry genome has been sequenced, and this provides an opportunity to investigate the organization and evolutionary characteristics of MrMYB genes from a whole genome view. In the present study, we performed the genome-wide analysis of MYB genes in Chinese bayberry and identified 174 MrMYB transcription factors (TFs), including 122 R2R3-MYBs, 43 1R-MYBs, two 3R-MYBs, one 4R-MYB, and six atypical MYBs. Collinearity analysis indicated that both syntenic and tandem duplications contributed to expansion of the MrMYB gene family. Analysis of transcript levels revealed the distinct expression patterns of different MrMYB genes, and those which may play important roles in leaf and flower development. Through phylogenetic analysis and correlation analyses, nine MrMYB TFs were selected as candidates regulating flavonoid biosynthesis. By using dual-luciferase assays, MrMYB12 was shown to trans-activate the MrFLS1 promoter, and MrMYB39 and MrMYB58a trans-activated the MrLAR1 promoter. In addition, overexpression of 35S:MrMYB12 caused a significant increase in flavonol contents and induced the expression of NtCHS, NtF3H, and NtFLS in transgenic tobacco leaves and flowers and significantly reduced anthocyanin accumulation, resulting in pale-pink or pure white flowers. This indicates that MrMYB12 redirected the flux away from anthocyanin biosynthesis resulting in higher flavonol content. The present study provides valuable information for understanding the classification, gene and motif structure, evolution and predicted functions of the MrMYB gene family and identifies MYBs regulating different aspects of flavonoid biosynthesis in Chinese bayberry.

The identification of an R2R3-MYB transcription factor involved in regulating anthocyanin biosynthesis in Primulina swinglei flowers

Primulina genus is an ideal wild ornamental flower and emerging model for studying biosynthesis, diversity, and evolution of flower pigment. However, the molecular mechanism underlying anthocyanin biosynthesis and regulation in Primulina remains unknown. Here, changes in anthocyanin content and the expression profiles of anthocyanin biosynthetic structural genes were examined in developing Primulina swinglei flowers and three other organs. Seventy-three R2R3-MYB transcription factor genes were identified from transcriptome of P. swinglei flowers, two of which, PsMYB1 and PsMYB2, are candidate regulators of anthocyanin biosynthesis according to clustering analysis. Furthermore, transient over-expression studies using tobacco leaves showed distinct pigment accumulation following co-infection with PsMYB1 and MrbHLH1 (a previously confirmed anthocyanin regulator from Morella rubra). Additionally, dual luciferase assays showed that PsMYB1 trans-activated the PsANS promoter, with the addition of MrbHLH1 resulting in a 5-fold increase in the intensity of this interaction. PsMYB1 did not, however, have any effect on the PsF3H promoter. The expression profile and dual luciferase assays showed that PsMYB2 plays no roles in anthocyanin regulation. Therefore, PsMYB1 is proposed to be the transcription factor gene regulating anthocyanin biosynthesis in P. swinglei.