Variations and reduction of plastome are associated with the evolution of parasitism in Convolvulaceae.

Parasitic lifestyle can often relax the constraint on the plastome, leading to gene pseudogenization and loss, and resulting in diverse genomic structures and rampant genome degradation. Although several plastomes of parasitic Cuscuta have  been reported, the evolution of parasitism in the family Convolvulaceae which is linked to structural variations and reduction of plastome has not been well investigated. In this study, we assembled and collected 40 plastid genomes belonging to 23 species representing four subgenera of Cuscuta and ten species of autotrophic Convolvulaceae. Our findings revealed nine types of structural variations and six types of inverted repeat (IR) boundary variations in the plastome of Convolvulaceae spp. These structural variations were associated with the shift of parasitic lifestyle, and IR boundary shift, as well as the abundance of long repeats. Overall, the degradation of Cuscuta plastome proceeded gradually, with one clade exhibiting an accelerated degradation rate. We observed five stages of gene loss in Cuscuta, including NAD(P)H complex → PEP complex → Photosynthesis-related → Ribosomal protein subunits → ATP synthase complex. Based on our results, we speculated that the shift of parasitic lifestyle in early divergent time promoted relaxed selection on plastomes, leading to the accumulation of microvariations, which ultimately resulted in the plastome reduction. This study provides new evidence towards a better understanding of plastomic evolution, variation, and reduction in the genus Cuscuta.

A combination of conserved and diverged responses underlies Theobroma cacao's defense response to Phytophthora palmivora.

BACKGROUND: Plants have complex and dynamic immune systems that have evolved to resist pathogens. Humans have worked to enhance these defenses in crops through breeding. However, many crops harbor only a fraction of the genetic diversity present in wild relatives. Increased utilization of diverse germplasm to search for desirable traits, such as disease resistance, is therefore a valuable step towards breeding crops that are adapted to both current and emerging threats. Here, we examine diversity of defense responses across four populations of the long-generation tree crop Theobroma cacao L., as well as four non-cacao Theobroma species, with the goal of identifying genetic elements essential for protection against the oomycete pathogen Phytophthora palmivora. RESULTS: We began by creating a new, highly contiguous genome assembly for the P. palmivora-resistant genotype SCA 6 (Additional file 1: Tables S1-S5), deposited in GenBank under accessions CP139290-CP139299. We then used this high-quality assembly to combine RNA and whole-genome sequencing data to discover several genes and pathways associated with resistance. Many of these are unique, i.e., differentially regulated in only one of the four populations (diverged 40 k-900 k generations). Among the pathways shared across all populations is phenylpropanoid biosynthesis, a metabolic pathway with well-documented roles in plant defense. One gene in this pathway, caffeoyl shikimate esterase (CSE), was upregulated across all four populations following pathogen treatment, indicating its broad importance for cacao's defense response. Further experimental evidence suggests this gene hydrolyzes caffeoyl shikimate to create caffeic acid, an antimicrobial compound and known inhibitor of Phytophthora spp. CONCLUSIONS: Our results indicate most expression variation associated with resistance is unique to populations. Moreover, our findings demonstrate the value of using a broad sample of evolutionarily diverged populations for revealing the genetic bases of cacao resistance to P. palmivora. This approach has promise for further revealing and harnessing valuable genetic resources in this and other long-generation plants.

Transcriptomic approach to uncover dynamic events in the development of mid-season sunburn in apple fruit.

Apples grown in high heat, high light, and low humidity environments are at risk for sun injury disorders like sunburn and associated crop losses. Understanding the physiological and molecular mechanisms underlying sunburn will support improvement of mitigation strategies and breeding for more resilient varieties. Numerous studies have highlighted key biochemical processes involved in sun injury, such as the phenylpropanoid and reactive oxygen species (ROS) pathways, demonstrating both enzyme activities and expression of related genes in response to sunburn conditions. Most previous studies have focused on at-harvest activity of a small number of genes in response to heat stress. Thus, it remains unclear how stress events earlier in the season affect physiology and gene expression. Here, we applied heat stress to mid-season apples in the field and collected tissue along a time course-24, 48, and 72 h following a heat stimulus-to investigate dynamic gene expression changes using a transcriptomic lens. We found a relatively small number of differentially expressed genes (DEGs) and enriched functional terms in response to heat treatments. Only a few of these belonged to pathways previously described to be involved in sunburn, such as the AsA-GSH pathway, while most DEGs had not yet been implicated in sunburn or heat stress in pome fruit.

Extra-large G proteins have extra-large effects on agronomic traits and stress tolerance in maize and rice.

Heterotrimeric G proteins - comprising Gα, Gß, and Gγ subunits - are ubiquitous elements in eukaryotic cell signaling. Plant genomes contain both canonical Gα subunit genes and a family of plant-specific extra-large G protein genes (XLGs) that encode proteins consisting of a domain with Gα-like features downstream of a long N-terminal domain. In this review we summarize phenotypes modulated by the canonical Gα and XLG proteins of arabidopsis and highlight recent studies in maize and rice that reveal dramatic phenotypic consequences of XLG clustered regularly interspaced short palindromic repeats (CRISPR) mutagenesis in these important crop species. XLGs have both redundant and specific roles in the control of agronomically relevant plant architecture and resistance to both abiotic and biotic stresses. We also point out areas of current controversy, suggest future research directions, and propose a revised, phylogenetically-based nomenclature for XLG protein genes.

De novo Assembly and Comparative Analyses of Mitochondrial Genomes in Piperales.

The mitochondrial genome of Liriodendron tulipifera exhibits many ancestral angiosperm features and a remarkably slow evolutionary rate, while mitochondrial genomes of other magnoliids remain yet to be characterized. We assembled nine new mitochondrial genomes, representing all genera of perianth-bearing Piperales, as well as for a member of the sister clade: three complete or nearly complete mitochondrial genomes from Aristolochiaceae and six additional draft assemblies including Thottea, Asaraceae, Lactoridaceae, and Hydnoraceae. For comparative purpose, a complete mitochondrial genome was assembled for Saururus, a member of the perianth-less Piperales. The average number of short repeats (50-99 bp) was much larger in genus Aristolochia than in other angiosperm mitochondrial genomes, and approximately 30% of repeats (<350 bp) were found to have the capacity to mediate recombination. We found mitochondrial genomes in perianth-bearing Piperales comprising conserved repertories of protein-coding genes and rRNAs but variable copy numbers of tRNA genes. We identified several shifts from cis- to trans-splicing of the Group II introns of nad1i728, cox2i373, and nad7i209. Two short regions of the cox1 and atp8 genes were likely derived from independent horizontal gene transfer events in perianth-bearing Piperales. We found biased enrichment of specific substitution types in different lineages of magnoliids and the Aristolochiaceae family showed the highest ratio of A:T > T:A substitutions of all other investigated angiosperm groups. Our study reports the first mitochondrial genomes for Piperales and uses this new information for a better understanding of the evolutionary patterns of magnoliids and angiosperms in general.

Phylotranscriptomic Analyses of Mycoheterotrophic Monocots Show a Continuum of Convergent Evolutionary Changes in Expressed Nuclear Genes From Three Independent Nonphotosynthetic Lineages.

Mycoheterotrophy is an alternative nutritional strategy whereby plants obtain sugars and other nutrients from soil fungi. Mycoheterotrophy and associated loss of photosynthesis have evolved repeatedly in plants, particularly in monocots. Although reductive evolution of plastomes in mycoheterotrophs is well documented, the dynamics of nuclear genome evolution remains largely unknown. Transcriptome datasets were generated from four mycoheterotrophs in three families (Orchidaceae, Burmanniaceae, Triuridaceae) and related green plants and used for phylogenomic analyses to resolve relationships among the mycoheterotrophs, their relatives, and representatives across the monocots. Phylogenetic trees based on 602 genes were mostly congruent with plastome phylogenies, except for an Asparagales + Liliales clade inferred in the nuclear trees. Reduction and loss of chlorophyll synthesis and photosynthetic gene expression and relaxation of purifying selection on retained genes were progressive, with greater loss in older nonphotosynthetic lineages. One hundred seventy-four of 1375 plant benchmark universally conserved orthologous genes were undetected in any mycoheterotroph transcriptome or the genome of the mycoheterotrophic orchid Gastrodia but were expressed in green relatives, providing evidence for massively convergent gene loss in nonphotosynthetic lineages. We designate this set of deleted or undetected genes Missing in Mycoheterotrophs (MIM). MIM genes encode not only mainly photosynthetic or plastid membrane proteins but also a diverse set of plastid processes, genes of unknown function, mitochondrial, and cellular processes. Transcription of a photosystem II gene (psb29) in all lineages implies a nonphotosynthetic function for this and other genes retained in mycoheterotrophs. Nonphotosynthetic plants enable novel insights into gene function as well as gene expression shifts, gene loss, and convergence in nuclear genomes.

Building a foundation for gene family analysis in Rosaceae genomes with a novel workflow: A case study in Pyrus architecture genes.

The rapid development of sequencing technologies has led to a deeper understanding of plant genomes. However, direct experimental evidence connecting genes to important agronomic traits is still lacking in most non-model plants. For instance, the genetic mechanisms underlying plant architecture are poorly understood in pome fruit trees, creating a major hurdle in developing new cultivars with desirable architecture, such as dwarfing rootstocks in European pear (Pyrus communis). An efficient way to identify genetic factors for important traits in non-model organisms can be to transfer knowledge across genomes. However, major obstacles exist, including complex evolutionary histories and variable quality and content of publicly available plant genomes. As researchers aim to link genes to traits of interest, these challenges can impede the transfer of experimental evidence across plant species, namely in the curation of high-quality, high-confidence gene models in an evolutionary context. Here we present a workflow using a collection of bioinformatic tools for the curation of deeply conserved gene families of interest across plant genomes. To study gene families involved in tree architecture in European pear and other rosaceous species, we used our workflow, plus a draft genome assembly and high-quality annotation of a second P. communis cultivar, 'd'Anjou.' Our comparative gene family approach revealed significant issues with the most recent 'Bartlett' genome - primarily thousands of missing genes due to methodological bias. After correcting assembly errors on a global scale in the 'Bartlett' genome, we used our workflow for targeted improvement of our genes of interest in both P. communis genomes, thus laying the groundwork for future functional studies in pear tree architecture. Further, our global gene family classification of 15 genomes across 6 genera provides a valuable and previously unavailable resource for the Rosaceae research community. With it, orthologs and other gene family members can be easily identified across any of the classified genomes. Importantly, our workflow can be easily adopted for any other plant genomes and gene families of interest.

Phylogenomic resolution of order- and family-level monocot relationships using 602 single-copy nuclear genes and 1375 BUSCO genes.

We assess relationships among 192 species in all 12 monocot orders and 72 of 77 families, using 602 conserved single-copy (CSC) genes and 1375 benchmarking single-copy ortholog (BUSCO) genes extracted from genomic and transcriptomic datasets. Phylogenomic inferences based on these data, using both coalescent-based and supermatrix analyses, are largely congruent with the most comprehensive plastome-based analysis, and nuclear-gene phylogenomic analyses with less comprehensive taxon sampling. The strongest discordance between the plastome and nuclear gene analyses is the monophyly of a clade comprising Asparagales and Liliales in our nuclear gene analyses, versus the placement of Asparagales and Liliales as successive sister clades to the commelinids in the plastome tree. Within orders, around six of 72 families shifted positions relative to the recent plastome analysis, but four of these involve poorly supported inferred relationships in the plastome-based tree. In Poales, the nuclear data place a clade comprising Ecdeiocoleaceae+Joinvilleaceae as sister to the grasses (Poaceae); Typhaceae, (rather than Bromeliaceae) are resolved as sister to all other Poales. In Commelinales, nuclear data place Philydraceae sister to all other families rather than to a clade comprising Haemodoraceae+Pontederiaceae as seen in the plastome tree. In Liliales, nuclear data place Liliaceae sister to Smilacaceae, and Melanthiaceae are placed sister to all other Liliales except Campynemataceae. Finally, in Alismatales, nuclear data strongly place Tofieldiaceae, rather than Araceae, as sister to all the other families, providing an alternative resolution of what has been the most problematic node to resolve using plastid data, outside of those involving achlorophyllous mycoheterotrophs. As seen in numerous prior studies, the placement of orders Acorales and Alismatales as successive sister lineages to all other extant monocots. Only 21.2% of BUSCO genes were demonstrably single-copy, yet phylogenomic inferences based on BUSCO and CSC genes did not differ, and overall functional annotations of the two sets were very similar. Our analyses also reveal significant gene tree-species tree discordance despite high support values, as expected given incomplete lineage sorting (ILS) related to rapid diversification. Our study advances understanding of monocot relationships and the robustness of phylogenetic inferences based on large numbers of nuclear single-copy genes that can be obtained from transcriptomes and genomes.

Pervasive Phylogenomic Incongruence Underlies Evolutionary Relationships in Eyebrights (Euphrasia, Orobanchaceae).

Disentangling the phylogenetic relationships of taxonomically complex plant groups is often mired by challenges associated with recent speciation, hybridization, complex mating systems, and polyploidy. Here, we perform a phylogenomic analysis of eyebrights (Euphrasia), a group renowned for taxonomic complexity, with the aim of documenting the extent of phylogenetic discordance at both deep and at shallow phylogenetic scales. We generate whole-genome sequencing data and integrate this with prior genomic data to perform a comprehensive analysis of nuclear genomic, nuclear ribosomal (nrDNA), and complete plastid genomes from 57 individuals representing 36 Euphrasia species. The species tree analysis of 3,454 conserved nuclear scaffolds (46 Mb) reveals that at shallow phylogenetic scales postglacial colonization of North Western Europe occurred in multiple waves from discrete source populations, with most species not being monophyletic, and instead combining genomic variants from across clades. At a deeper phylogenetic scale, the Euphrasia phylogeny is structured by geography and ploidy, and partially by taxonomy. Comparative analyses show Southern Hemisphere tetraploids include a distinct subgenome indicative of independent polyploidy events from Northern Hemisphere taxa. In contrast to the nuclear genome analyses, the plastid genome phylogeny reveals limited geographic structure, while the nrDNA phylogeny is informative of some geographic and taxonomic affinities but more thorough phylogenetic inference is impeded by the retention of ancestral polymorphisms in the polyploids. Overall our results reveal extensive phylogenetic discordance at both deeper and shallower nodes, with broad-scale geographic structure of genomic variation but a lack of definitive taxonomic signal. This suggests that Euphrasia species either have polytopic origins or are maintained by narrow genomic regions in the face of extensive homogenizing gene flow. Moreover, these results suggest genome skimming will not be an effective extended barcode to identify species in groups such as Euphrasia, or many other postglacial species groups.

PlantTribes2: Tools for comparative gene family analysis in plant genomics.

Plant genome-scale resources are being generated at an increasing rate as sequencing technologies continue to improve and raw data costs continue to fall; however, the cost of downstream analyses remains large. This has resulted in a considerable range of genome assembly and annotation qualities across plant genomes due to their varying sizes, complexity, and the technology used for the assembly and annotation. To effectively work across genomes, researchers increasingly rely on comparative genomic approaches that integrate across plant community resources and data types. Such efforts have aided the genome annotation process and yielded novel insights into the evolutionary history of genomes and gene families, including complex non-model organisms. The essential tools to achieve these insights rely on gene family analysis at a genome-scale, but they are not well integrated for rapid analysis of new data, and the learning curve can be steep. Here we present PlantTribes2, a scalable, easily accessible, highly customizable, and broadly applicable gene family analysis framework with multiple entry points including user provided data. It uses objective classifications of annotated protein sequences from existing, high-quality plant genomes for comparative and evolutionary studies. PlantTribes2 can improve transcript models and then sort them, either genome-scale annotations or individual gene coding sequences, into pre-computed orthologous gene family clusters with rich functional annotation information. Then, for gene families of interest, PlantTribes2 performs downstream analyses and customizable visualizations including, (1) multiple sequence alignment, (2) gene family phylogeny, (3) estimation of synonymous and non-synonymous substitution rates among homologous sequences, and (4) inference of large-scale duplication events. We give examples of PlantTribes2 applications in functional genomic studies of economically important plant families, namely transcriptomics in the weedy Orobanchaceae and a core orthogroup analysis (CROG) in Rosaceae. PlantTribes2 is freely available for use within the main public Galaxy instance and can be downloaded from GitHub or Bioconda. Importantly, PlantTribes2 can be readily adapted for use with genomic and transcriptomic data from any kind of organism.

A derived ZW chromosome system in Amborella trichopoda, representing the sister lineage to all other extant flowering plants.

The genetic basis and evolution of sex determination in dioecious plants is emerging as an active area of research with exciting advances in genome sequencing and analysis technologies. As the sole species within the sister lineage to all other extant flowering plants, Amborella trichopoda is an important model for understanding the evolution and development of flowers. Plants typically produce only male or female flowers, but sex determination mechanisms are unknown for the species. Sequence data derived from plants of natural origin and an F1 mapping population were used to identify sex-linked genes and the nonrecombining region. Amborella trichopoda has a ZW sex determination system. Analysis of genes in a 4 Mb nonrecombining sex-determination region reveals recent divergence of Z and W gametologs, and few Z- and W-specific genes. The sex chromosomes of A. trichopoda evolved less than 16.5 Myr ago, long after the divergence of the extant angiosperms.

Insights into angiosperm evolution, floral development and chemical biosynthesis from the Aristolochia fimbriata genome.

Aristolochia, a genus in the magnoliid order Piperales, has been famous for centuries for its highly specialized flowers and wide medicinal applications. Here, we present a new, high-quality genome sequence of Aristolochia fimbriata, a species that, similar to Amborella trichopoda, lacks further whole-genome duplications since the origin of extant angiosperms. As such, the A. fimbriata genome is an excellent reference for inferences of angiosperm genome evolution, enabling detection of two novel whole-genome duplications in Piperales and dating of previously reported whole-genome duplications in other magnoliids. Genomic comparisons between A. fimbriata and other angiosperms facilitated the identification of ancient genomic rearrangements suggesting the placement of magnoliids as sister to monocots, whereas phylogenetic inferences based on sequence data we compiled yielded ambiguous relationships. By identifying associated homologues and investigating their evolutionary histories and expression patterns, we revealed highly conserved floral developmental genes and their distinct downstream regulatory network that may contribute to the complex flower morphology in A. fimbriata. Finally, we elucidated the genetic basis underlying the biosynthesis of terpenoids and aristolochic acids in A. fimbriata.

Genomic structural variants constrain and facilitate adaptation in natural populations of Theobroma cacao, the chocolate tree.

Genomic structural variants (SVs) can play important roles in adaptation and speciation. Yet the overall fitness effects of SVs are poorly understood, partly because accurate population-level identification of SVs requires multiple high-quality genome assemblies. Here, we use 31 chromosome-scale, haplotype-resolved genome assemblies of Theobroma cacao-an outcrossing, long-lived tree species that is the source of chocolate-to investigate the fitness consequences of SVs in natural populations. Among the 31 accessions, we find over 160,000 SVs, which together cover eight times more of the genome than single-nucleotide polymorphisms and short indels (125 versus 15 Mb). Our results indicate that a vast majority of these SVs are deleterious: they segregate at low frequencies and are depleted from functional regions of the genome. We show that SVs influence gene expression, which likely impairs gene function and contributes to the detrimental effects of SVs. We also provide empirical support for a theoretical prediction that SVs, particularly inversions, increase genetic load through the accumulation of deleterious nucleotide variants as a result of suppressed recombination. Despite the overall detrimental effects, we identify individual SVs bearing signatures of local adaptation, several of which are associated with genes differentially expressed between populations. Genes involved in pathogen resistance are strongly enriched among these candidates, highlighting the contribution of SVs to this important local adaptation trait. Beyond revealing empirical evidence for the evolutionary importance of SVs, these 31 de novo assemblies provide a valuable resource for genetic and breeding studies in Tcacao.

Transcriptomics of Differential Ripening in 'd'Anjou' Pear (Pyrus communis L.).

Estimating maturity in pome fruits is a critical task that directs virtually all postharvest supply chain decisions. This is especially important for European pear (Pyrus communis) cultivars because losses due to spoilage and senescence must be minimized while ensuring proper ripening capacity is achieved (in part by satisfying a fruit chilling requirement). Reliable methods are lacking for accurate estimation of pear fruit maturity, and because ripening is maturity dependent it makes predicting ripening capacity a challenge. In this study of the European pear cultivar 'd'Anjou', we sorted fruit at harvest based upon on-tree fruit position to build contrasts of maturity. Our sorting scheme showed clear contrasts of maturity between canopy positions, yet there was substantial overlap in the distribution of values for the index of absorbance difference (I AD ), a non-destructive spectroscopic measurement that has been used as a proxy for pome fruit maturity. This presented an opportunity to explore a contrast of maturity that was more subtle than I AD could differentiate, and thus guided our subsequent transcriptome analysis of tissue samples taken at harvest and during storage. Using a novel approach that tests for condition-specific differences of co-expressed genes, we discovered genes with a phased character that mirrored our sorting scheme. The expression patterns of these genes are associated with fruit quality and ripening differences across the experiment. Functional profiles of these co-expressed genes are concordant with previous findings, and also offer new clues, and thus hypotheses, about genes involved in pear fruit quality, maturity, and ripening. This work may lead to new tools for enhanced postharvest management based on activity of gene co-expression modules, rather than individual genes. Further, our results indicate that modules may have utility within specific windows of time during postharvest management of 'd'Anjou' pear.

GetOrganelle: a fast and versatile toolkit for accurate de novo assembly of organelle genomes.

GetOrganelle is a state-of-the-art toolkit to accurately assemble organelle genomes from whole genome sequencing data. It recruits organelle-associated reads using a modified "baiting and iterative mapping" approach, conducts de novo assembly, filters and disentangles the assembly graph, and produces all possible configurations of circular organelle genomes. For 50 published plant datasets, we are able to reassemble the circular plastomes from 47 datasets using GetOrganelle. GetOrganelle assemblies are more accurate than published and/or NOVOPlasty-reassembled plastomes as assessed by mapping. We also assemble complete mitochondrial genomes using GetOrganelle. GetOrganelle is freely released under a GPL-3 license ( https://github.com/Kinggerm/GetOrganelle ).

Genomics of sorghum local adaptation to a parasitic plant.

Host-parasite coevolution can maintain high levels of genetic diversity in traits involved in species interactions. In many systems, host traits exploited by parasites are constrained by use in other functions, leading to complex selective pressures across space and time. Here, we study genome-wide variation in the staple crop Sorghum bicolor (L.) Moench and its association with the parasitic weed Striga hermonthica (Delile) Benth., a major constraint to food security in Africa. We hypothesize that geographic selection mosaics across gradients of parasite occurrence maintain genetic diversity in sorghum landrace resistance. Suggesting a role in local adaptation to parasite pressure, multiple independent loss-of-function alleles at sorghum LOW GERMINATION STIMULANT 1 (LGS1) are broadly distributed among African landraces and geographically associated with S. hermonthica occurrence. However, low frequency of these alleles within S. hermonthica-prone regions and their absence elsewhere implicate potential trade-offs restricting their fixation. LGS1 is thought to cause resistance by changing stereochemistry of strigolactones, hormones that control plant architecture and below-ground signaling to mycorrhizae and are required to stimulate parasite germination. Consistent with trade-offs, we find signatures of balancing selection surrounding LGS1 and other candidates from analysis of genome-wide associations with parasite distribution. Experiments with CRISPR-Cas9-edited sorghum further indicate that the benefit of LGS1-mediated resistance strongly depends on parasite genotype and abiotic environment and comes at the cost of reduced photosystem gene expression. Our study demonstrates long-term maintenance of diversity in host resistance genes across smallholder agroecosystems, providing a valuable comparison to both industrial farming systems and natural communities.

In silico identification and structure function analysis of a putative coclaurine N-methyltransferase from Aristolochia fimbriata.

In this study we isolated and performed in silico analysis of a putative coclaurine N-methyltransferase (CNMT) from the basal angiosperm Aristolochia fimbriata. The Aristolochiaceae plant family produces alkaloids similar to the Papavaraceae family, and CNMTs are central enzymes in biosynthesis pathways producing compounds of ethnopharmacological interest. We used bioinformatics and computational tools to predict a three-dimensional homology model and to investigate the putative function of the protein and its mechanism for methylation. The putative CNMT is a unique (S)-adenosyl-L-methionine (SAM)-dependent N-methyltransferase, catalyzing transfer of a methyl group from SAM to the amino group of coclaurine. The model revealed a mixed α/ß structure comprising seven twisted ß-strands surrounded by twelve α-helices. Sequence comparisons and the model indicate an N-terminal catalytic Core domain and a C-terminal domain, of which the latter forms a pocket for coclaurine. An additional binding pocket for SAM is connected to the coclaurine binding pocket by a small opening. CNMT activity is proposed to follow an SN2-type mechanism as observed for a similarly conformed enzyme. Residues predicted for the methyl transfer reaction are Tyr79 and Glu96, which are conserved in the sequence from A. fimbriata and in homologous N-methyltransferases. The isolated CNMT is the first to be investigated from any basal angiosperm.

Gene Expression Modularity Reveals Footprints of Polygenic Adaptation in Theobroma cacao.

Separating footprints of adaptation from demography is challenging. When selection has acted on a single locus with major effect, this issue can be alleviated through signatures left by selective sweeps. However, as adaptation is often driven by small allele frequency shifts at many loci, studies focusing on single genes are able to identify only a small portion of genomic variants responsible for adaptation. In face of this challenge, we utilize coexpression information to search for signals of polygenetic adaptation in Theobroma cacao, a tropical tree species that is the source of chocolate. Using transcriptomics and a weighted correlation network analysis, we group genes with similar expression patterns into functional modules. We then ask whether modules enriched for specific biological processes exhibit cumulative effects of differential selection in the form of high FST and dXY between populations. Indeed, modules putatively involved in protein modification, flowering, and water transport show signs of polygenic adaptation even though individual genes that are members of those groups do not bear strong signatures of selection. Modeling of demography, background selection, and the effects of genomic features reveal that these patterns are unlikely to arise by chance. We also find that specific modules are enriched for signals of strong or relaxed purifying selection, with one module bearing signs of adaptive differentiation and an excess of deleterious mutations. Our results provide insight into polygenic adaptation and contribute to understanding of population structure, demographic history, and genome evolution in T. cacao.

Compensatory sequence variation between trans-species small RNAs and their target sites.

Trans-species small regulatory RNAs (sRNAs) are delivered to host plants from diverse pathogens and parasites and can target host mRNAs. How trans-species sRNAs can be effective on diverse hosts has been unclear. Multiple species of the parasitic plant Cuscuta produce trans-species sRNAs that collectively target many host mRNAs. Confirmed target sites are nearly always in highly conserved, protein-coding regions of host mRNAs. Cuscuta trans-species sRNAs can be grouped into superfamilies that have variation in a three-nucleotide period. These variants compensate for synonymous-site variation in host mRNAs. By targeting host mRNAs at highly conserved protein-coding sites, and simultaneously expressing multiple variants to cover synonymous-site variation, Cuscuta trans-species sRNAs may be able to successfully target multiple homologous mRNAs from diverse hosts.