Predicting photosynthetic pathway from anatomy using machine learning

Plants with Crassulacean acid metabolism (CAM) have long been associated with a specialized anatomy, including succulence and thick photosynthetic tissues. Firm, quantitative boundaries between non-CAM and CAM plants have yet to be established - if they indeed exist. Using novel computer vision software to measure anatomy, we combined new measurements with published data across flowering plants. We then used machine learning and phylogenetic comparative methods to investigate relationships between CAM and anatomy. We found significant differences in photosynthetic tissue anatomy between plants with differing CAM phenotypes. Machine learning-based classification was over 95% accurate in differentiating CAM from non-CAM anatomy, and had over 70% recall of distinct CAM phenotypes. Phylogenetic least squares regression and threshold analyses revealed that CAM evolution was significantly correlated with increased mesophyll cell size, thicker leaves, and decreased intercellular airspace. Our findings suggest that machine learning may be used to aid the discovery of new CAM species and that the evolutionary trajectory from non-CAM to strong, obligate CAM requires continual anatomical specialization.

The CAM lineages of planet Earth

BACKGROUND AND SCOPE

The growth of experimental studies of crassulacean acid metabolism (CAM) in diverse plant clades, coupled with recent advances in molecular systematics, presents an opportunity to re-assess the phylogenetic distribution and diversity of species capable of CAM. It has been more than two decades since the last comprehensive lists of CAM taxa were published, and an updated survey of the occurrence and distribution of CAM taxa is needed to facilitate and guide future CAM research. We aimed to survey the phylogenetic distribution of these taxa, their diverse morphology, physiology and ecology, and the likely number of evolutionary origins of CAM based on currently known lineages.

RESULTS AND CONCLUSIONS

We found direct evidence (in the form of experimental or field observations of gas exchange, day-night fluctuations in organic acids, carbon isotope ratios and enzymatic activity) for CAM in 370 genera of vascular plants, representing 38 families. Further assumptions about the frequency of CAM species in CAM clades and the distribution of CAM in the Cactaceae and Crassulaceae bring the currently estimated number of CAM-capable species to nearly 7 % of all vascular plants. The phylogenetic distribution of these taxa suggests a minimum of 66 independent origins of CAM in vascular plants, possibly with dozens more. To achieve further insight into CAM origins, there is a need for more extensive and systematic surveys of previously unstudied lineages, particularly in living material to identify low-level CAM activity, and for denser sampling to increase phylogenetic resolution in CAM-evolving clades. This should allow further progress in understanding the functional significance of this pathway by integration with studies on the evolution and genomics of CAM in its many forms.

The diverse diaspora of CAM: a pole-to-pole sketch

BACKGROUND

Crassulacean acid metabolism (CAM) photosynthesis is a successful adaptation that has evolved often in angiosperms, gymnosperms, ferns and lycophytes. Present in ~5 % of vascular plants, the CAM diaspora includes all continents apart from Antarctica. Species with CAM inhabit most landscapes colonized by vascular plants, from the Arctic Circle to Tierra del Fuego, from below sea level to 4800 m a.s.l., from rainforests to deserts. They have colonized terrestrial, epiphytic, lithophytic, palustrine and aquatic systems, developing perennial, annual or geophyte strategies that can be structurally arborescent, shrub, forb, cladode, epiphyte, vine or leafless with photosynthetic roots. CAM can enhance survival by conserving water, trapping carbon, reducing carbon loss and/or via photoprotection.

SCOPE

This review assesses the phylogenetic diversity and historical biogeography of selected lineages with CAM, i.e. ferns, gymnosperms and eumagnoliids, Orchidaceae, Bromeliaceae, Crassulaceae, Euphorbiaceae, Aizoaceae, Portulacineae (Montiaceae, Basellaceae, Halophytaceae, Didiereaceae, Talinaceae, Portulacaceae, Anacampserotaceae and Cactaceae) and aquatics.

CONCLUSIONS

Most extant CAM lineages diversified after the Oligocene/Miocene, as the planet dried and CO2 concentrations dropped. Radiations exploited changing ecological landscapes, including Andean emergence, Panamanian Isthmus closure, Sundaland emergence and submergence, changing climates and desertification. Evidence remains sparse for or against theories that CAM biochemistry tends to evolve before pronounced changes in anatomy and that CAM tends to be a culminating xerophytic trait. In perennial taxa, any form of CAM can occur depending upon the lineage and the habitat, although facultative CAM appears uncommon in epiphytes. CAM annuals lack strong CAM. In CAM annuals, C3 + CAM predominates, and inducible or facultative CAM is common.

Gene duplications facilitate C4-CAM compatibility in common purslane

Common purslane (Portulaca oleracea) integrates both C4 and crassulacean acid metabolism (CAM) photosynthesis pathways and is a promising model plant to explore C4-CAM plasticity. Here, we report a high-quality chromosome-level genome of nicotinamide adenine dinucleotide (NAD)-malic enzyme (ME) subtype common purslane that provides evidence for 2 rounds of whole-genome duplication (WGD) with an ancient WGD (P-β) in the common ancestor to Portulacaceae and Cactaceae around 66.30 million years ago (Mya) and another (Po-α) specific to common purslane lineage around 7.74 Mya. A larger number of gene copies encoding key enzymes/transporters involved in C4 and CAM pathways were detected in common purslane than in related species. Phylogeny, conserved functional site, and collinearity analyses revealed that the Po-α WGD produced the phosphoenolpyruvate carboxylase-encoded gene copies used for photosynthesis in common purslane, while the P-β WGD event produced 2 ancestral genes of functionally differentiated (C4- and CAM-specific) beta carbonic anhydrases involved in the C4 + CAM pathways. Additionally, cis-element enrichment analysis in the promoters showed that CAM-specific genes have recruited both evening and midnight circadian elements as well as the Abscisic acid (ABA)-independent regulatory module mediated by ethylene-response factor cis-elements. Overall, this study provides insights into the origin and evolutionary process of C4 and CAM pathways in common purslane, as well as potential targets for engineering crops by integrating C4 or CAM metabolism.

Phylogenomics of Aralia sect. Aralia (Araliaceae): Signals of hybridization and insights into its species delimitations and intercontinental biogeography

Genome-scale data have significantly increased the number of informative characters for phylogenetic analyses and recent studies have also revealed widespread phylogenomic discordance in many plant lineages. Aralia sect. Aralia is a small plant lineage (14 spp.) of the ginseng family Araliaceae with a disjunct distribution between eastern Asia (11 spp.) and North America (3 spp.). We herein employ sequences of hundreds of nuclear loci and the complete plastomes using targeted sequence capture and genome skimming to reconstruct the phylogenetic and biogeographic history of this section. We detected substantial conflicts among nuclear genes, yet different analytical strategies generated largely congruent topologies from the nuclear data. Significant cytonuclear discordance was detected, especially concerning the positions of the three North American species. The phylogenomic results support two intercontinental disjunctions: (1) Aralia californica of western North America is sister to the eastern Asian clade consisting of A. cordata and A. continentalis in the nuclear tree, and (2) the eastern North American A. racemosa forms a clade with A. bicrenata from southwestern North America, and the North American A. racemosa - A. bicrenata clade is then sister to the eastern Asian clade consisting of A. glabra (Japan), A. fargesii (C China), and A. apioides and A. atropurpurea (the Hengduan Mountains). Aralia cordata is supported to be disjunctly distributed in Japan, Taiwan, the Ulleung island of Korea, and in Central, Southwest and South China, and Aralia continentalis is redefined with a narrower distribution in Northeast China, eastern Russia and peninsular Korea.

A nuclear target sequence capture probe set for phylogeny reconstruction of the charismatic plant family Bignoniaceae

The plant family Bignoniaceae is a conspicuous and charismatic element of the tropical flora. The family has a complex taxonomic history, with substantial changes in the classification of the group during the past two centuries. Recent re-classifications at the tribal and generic levels have been largely possible by the availability of molecular phylogenies reconstructed using Sanger sequencing data. However, our complete understanding of the systematics, evolution, and biogeography of the family remains incomplete, especially due to the low resolution and support of different portions of the Bignoniaceae phylogeny. To overcome these limitations and increase the amount of molecular data available for phylogeny reconstruction within this plant family, we developed a bait kit targeting 762 nuclear genes, including 329 genes selected specifically for the Bignoniaceae; 348 genes obtained from the Angiosperms353 with baits designed specifically for the family; and, 85 low-copy genes of known function. On average, 77.4% of the reads mapped to the targets, and 755 genes were obtained per species. After removing genes with putative paralogs, 677 loci were used for phylogenetic analyses. On-target genes were compared and combined in the Exon-Only dataset, and on-target + off-target regions were combined in the Supercontig dataset. We tested the performance of the bait kit at different taxonomic levels, from family to species-level, using 38 specimens of 36 different species of Bignoniaceae, representing: 1) six (out of eight) tribal level-clades (e.g., Bignonieae, Oroxyleae, Tabebuia Alliance, Paleotropical Clade, Tecomeae, and Jacarandeae), only Tourrettieae and Catalpeae were not sampled; 2) all 20 genera of Bignonieae; 3) seven (out of nine) species of Dolichandra (e.g., D. chodatii, D. cynanchoides, D. dentata, D. hispida, D. quadrivalvis, D. uncata, and D. uniguis-cati), only D. steyermarkii and D. unguiculata were not sampled; and 4) three individuals of Dolichandra unguis-cati. Our data reconstructed a well-supported phylogeny of the Bignoniaceae at different taxonomic scales, opening new perspectives for a comprehensive phylogenetic framework for the family as a whole.

Utilizing paralogs for phylogenetic reconstruction has the potential to increase species tree support and reduce gene tree discordance in target enrichment data

The analysis of target enrichment data in phylogenetics lacks optimization toward using paralogs for phylogenetic reconstruction. We developed a novel approach of detecting paralogs and utilizing them for phylogenetic tree inference, by retrieving both ortho- and paralogous copies and creating orthologous alignments, from which the gene trees are built. We implemented this approach in ParalogWizard and demonstrate its performance in plant groups that underwent a whole genome duplication relatively recently: the subtribe Malinae (family Rosaceae), using Angiosperms353 as well as Malinae481 probes, the genus Oritrophium (family Asteraceae), using Compositae1061 probes, and the genus Amomum (family Zingiberaceae), using Zingiberaceae1180 probes. Discriminating between orthologs and paralogs reduced gene tree discordance and increased the species tree support in the case of the Malinae, but not for Oritrophium and Amomum. This may relate to the difference in the proportion of paralogous loci between the datasets, which was highest for the Malinae. Overall, retrieving paralogs for phylogenetic reconstruction following ParalogWizard has the potential to increase the species tree support and reduce gene tree discordance in target enrichment data, particularly if the proportion of paralogous loci is high.

Evolutionary Genetics of Cacti: Research Biases, Advances and Prospects

Here, we present a review of the studies of evolutionary genetics (phylogenetics, population genetics, and phylogeography) using genetic data as well as genome scale assemblies in Cactaceae (Caryophyllales, Angiosperms), a major lineage of succulent plants with astonishing diversity on the American continent. To this end, we performed a literature survey (1992-2021) to obtain detailed information regarding key aspects of studies investigating cactus evolution. Specifically, we summarize the advances in the following aspects: molecular markers, species delimitation, phylogenetics, hybridization, biogeography, and genome assemblies. In brief, we observed substantial growth in the studies conducted with molecular markers in the past two decades. However, we found biases in taxonomic/geographic sampling and the use of traditional markers and statistical approaches. We discuss some methodological and social challenges for engaging the cactus community in genomic research. We also stressed the importance of integrative approaches, coalescent methods, and international collaboration to advance the understanding of cactus evolution.

Phylogenomic analysis of 997 nuclear genes reveals the need for extensive generic re-delimitation in Caesalpinioideae (Leguminosae)

Subfamily Caesalpinioideae with ca. 4,600 species in 152 genera is the second-largest subfamily of legumes (Leguminosae) and forms an ecologically and economically important group of trees, shrubs and lianas with a pantropical distribution. Despite major advances in the last few decades towards aligning genera with clades across Caesalpinioideae, generic delimitation remains in a state of considerable flux, especially across the mimosoid clade. We test the monophyly of genera across Caesalpinioideae via phylogenomic analysis of 997 nuclear genes sequenced via targeted enrichment (Hybseq) for 420 species and 147 of the 152 genera currently recognised in the subfamily. We show that 22 genera are non-monophyletic or nested in other genera and that non-monophyly is concentrated in the mimosoid clade where ca. 25% of the 90 genera are found to be non-monophyletic. We suggest two main reasons for this pervasive generic non-monophyly: (i) extensive morphological homoplasy that we document here for a handful of important traits and, particularly, the repeated evolution of distinctive fruit types that were historically emphasised in delimiting genera and (ii) this is an artefact of the lack of pantropical taxonomic syntheses and sampling in previous phylogenies and the consequent failure to identify clades that span the Old World and New World or conversely amphi-Atlantic genera that are non-monophyletic, both of which are critical for delimiting genera across this large pantropical clade. Finally, we discuss taxon delimitation in the phylogenomic era and especially how assessing patterns of gene tree conflict can provide additional insights into generic delimitation. This new phylogenomic framework provides the foundations for a series of papers reclassifying genera that are presented here in Advances in Legume Systematics (ALS) 14 Part 1, for establishing a new higher-level phylogenetic tribal and clade-based classification of Caesalpinioideae that is the focus of ALS14 Part 2 and for downstream analyses of evolutionary diversification and biogeography of this important group of legumes which are presented elsewhere.

Analysis of Paralogs in Target Enrichment Data Pinpoints Multiple Ancient Polyploidy Events in Alchemilla s.l. (Rosaceae)

Target enrichment is becoming increasingly popular for phylogenomic studies. Although baits for enrichment are typically designed to target single-copy genes, paralogs are often recovered with increased sequencing depth, sometimes from a significant proportion of loci, especially in groups experiencing whole-genome duplication (WGD) events. Common approaches for processing paralogs in target enrichment data sets include random selection, manual pruning, and mainly, the removal of entire genes that show any evidence of paralogy. These approaches are prone to errors in orthology inference or removing large numbers of genes. By removing entire genes, valuable information that could be used to detect and place WGD events is discarded. Here, we used an automated approach for orthology inference in a target enrichment data set of 68 species of Alchemilla s.l. (Rosaceae), a widely distributed clade of plants primarily from temperate climate regions. Previous molecular phylogenetic studies and chromosome numbers both suggested ancient WGDs in the group. However, both the phylogenetic location and putative parental lineages of these WGD events remain unknown. By taking paralogs into consideration and inferring orthologs from target enrichment data, we identified four nodes in the backbone of Alchemilla s.l. with an elevated proportion of gene duplication. Furthermore, using a gene-tree reconciliation approach, we established the autopolyploid origin of the entire Alchemilla s.l. and the nested allopolyploid origin of four major clades within the group. Here, we showed the utility of automated tree-based orthology inference methods, previously designed for genomic or transcriptomic data sets, to study complex scenarios of polyploidy and reticulate evolution from target enrichment data sets.[Alchemilla; allopolyploidy; autopolyploidy; gene tree discordance; orthology inference; paralogs; Rosaceae; target enrichment; whole genome duplication.].

Development and phylogenetic utilities of a new set of single-/low-copy nuclear genes in Senecioneae (Asteraceae), with new insights into the tribal position and the relationships within subtribe Tussilagininae

The tribe Senecioneae is one of the largest tribes in Asteraceae, with a nearly cosmopolitan distribution. Despite great efforts devoted to elucidate the evolution of Senecioneae, many questions still remain concerning the systematics of this group, from the tribal circumscription and position to species relationships in many genera. The hybridization-based target enrichment method of next-generation sequencing has been accepted as a promising approach to resolve phylogenetic problems. We herein develop a set of single-/low-copy genes for Senecioneae, and test their phylogenetic utilities. Our results demonstrate that these genes work highly efficiently for Senecioneae, with a high average gene recovery of 98.8% across the tribe and recovering robust phylogenetic hypotheses at different levels. In particular, the delimitation of the Senecioneae has been confirmed to include Abrotanella and exclude Doronicum, with the former sister to core Senecioneae and the latter shown to be more closely related to Calenduleae. Moreover, Doronicum and Calenduleae are inferred to be the closest relatives of Senecioneae, which is a new hypothesis well supported by statistical topology tests, morphological evidence, and the profile of pyrrolizidine alkaloids, a special kind of chemical characters generally used to define Senecioneae. Furthermore, this study suggests a complex reticulation history in the diversification of Senecioneae, accounting for the prevalence of polyploid groups in the tribe. With subtribe Tussilagininae s.str. as a case study showing a more evident pattern of gene duplication, we further explored reconstructing the phylogeny in the groups with high ploidy levels. Our results also demonstrate that tree topologies based on sorted paralogous copies are stable across different methods of phylogenetic inference, and more congruent with the morphological evidence and the results of previous phylogenetic studies.

Phylogenomics of Gesneriaceae using targeted capture of nuclear genes

Gesneriaceae (ca. 3400 species) is a pantropical plant family with a wide range of growth form and floral morphology that are associated with repeated adaptations to different environments and pollinators. Although Gesneriaceae systematics has been largely improved by the use of Sanger sequencing data, our understanding of the evolutionary history of the group is still far from complete due to the limited number of informative characters provided by this type of data. To overcome this limitation, we developed here a Gesneriaceae-specific gene capture kit targeting 830 single-copy loci (776,754 bp in total), including 279 genes from the Universal Angiosperms-353 kit. With an average of 557,600 reads and 87.8% gene recovery, our target capture was successful across the family Gesneriaceae and also in other families of Lamiales. From our bait set, we selected the most informative 418 loci to resolve phylogenetic relationships across the entire Gesneriaceae family using maximum likelihood and coalescent-based methods. Upon testing the phylogenetic performance of our baits on 78 taxa representing 20 out of 24 subtribes within the family, we showed that our data provided high support for the phylogenetic relationships among the major lineages, and were able to provide high resolution within more recent radiations. Overall, the molecular resources we developed here open new perspectives for the study of Gesneriaceae phylogeny at different taxonomical levels and the identification of the factors underlying the diversification of this plant group.

Hybrid capture of 964 nuclear genes resolves evolutionary relationships in the mimosoid legumes and reveals the polytomous origins of a large pantropical radiation

PREMISE

Targeted enrichment methods facilitate sequencing of hundreds of nuclear loci to enhance phylogenetic resolution and elucidate why some parts of the "tree of life" are difficult (if not impossible) to resolve. The mimosoid legumes are a prominent pantropical clade of ~3300 species of woody angiosperms for which previous phylogenies have shown extensive lack of resolution, especially among the species-rich and taxonomically challenging ingoids.

METHODS

We generated transcriptomes to select low-copy nuclear genes, enrich these via hybrid capture for representative species of most mimosoid genera, and analyze the resulting data using de novo assembly and various phylogenomic tools for species tree inference. We also evaluate gene tree support and conflict for key internodes and use phylogenetic network analysis to investigate phylogenetic signal across the ingoids.

RESULTS

Our selection of 964 nuclear genes greatly improves phylogenetic resolution across the mimosoid phylogeny and shows that the ingoid clade can be resolved into several well-supported clades. However, nearly all loci show lack of phylogenetic signal for some of the deeper internodes within the ingoids.

CONCLUSIONS

Lack of resolution in the ingoid clade is most likely the result of hyperfast diversification, potentially causing a hard polytomy of six or seven lineages. The gene set for targeted sequencing presented here offers great potential to further enhance the phylogeny of mimosoids and the wider Caesalpinioideae with denser taxon sampling, to provide a framework for taxonomic reclassification, and to study the ingoid radiation.

Strategies for reducing per-sample costs in target capture sequencing for phylogenomics and population genomics in plants

The reduced cost of high-throughput sequencing and the development of gene sets with wide phylogenetic applicability has led to the rise of sequence capture methods as a plausible platform for both phylogenomics and population genomics in plants. An important consideration in large targeted sequencing projects is the per-sample cost, which can be inflated when using off-the-shelf kits or reagents not purchased in bulk. Here, we discuss methods to reduce per-sample costs in high-throughput targeted sequencing projects. We review the minimal equipment and consumable requirements for targeted sequencing while comparing several alternatives to reduce bulk costs in DNA extraction, library preparation, target enrichment, and sequencing. We consider how each of the workflow alterations may be affected by DNA quality (e.g., fresh vs. herbarium tissue), genome size, and the phylogenetic scale of the project. We provide a cost calculator for researchers considering targeted sequencing to use when designing projects, and identify challenges for future development of low-cost sequencing in non-model plant systems.

Phylogenetic Relationships and Evolutionary Trends in the Cactus Family

Members of the cactus family are keystone species of arid and semiarid biomes in the Americas, as they provide shelter and resources to support other members of ecosystems. Extraordinary examples are the several species of flies of the genus Drosophila that lay eggs and feed in their rotting stems, which provide a model system for studying evolutionary processes. Although there is significant progress in understanding the evolution of Drosophila species, there are gaps in our knowledge about the cactus lineages hosting them. Here, we review the current knowledge about the evolution of Cactaceae, focusing on phylogenetic relationships and trends revealed by the study of DNA sequence data. During the last several decades, the availability of molecular phylogenies has considerably increased our understanding of the relationships, biogeography, and evolution of traits in the family. Remarkably, although succulent cacti have very low growth rates and long generation times, they underwent some of the fastest diversifications observed in the plant kingdom, possibly fostered by strong ecological interactions. We have a better understanding of the reproductive biology, population structure and speciation mechanisms in different clades. The recent publication of complete genomes for some species has revealed the importance of phenomena such as incomplete lineage sorting. Hybridization and polyploidization are common in the family, and have been studied using a variety of phylogenetic methods. We discuss potential future avenues for research in Cactaceae, emphasizing the need of a concerted effort among scientists in the Americas, together with the analyses of data from novel sequencing techniques.

Large-scale genomic sequence data resolve the deepest divergences in the legume phylogeny and support a near-simultaneous evolutionary origin of all six subfamilies

Phylogenomics is increasingly used to infer deep-branching relationships while revealing the complexity of evolutionary processes such as incomplete lineage sorting, hybridization/introgression and polyploidization. We investigate the deep-branching relationships among subfamilies of the Leguminosae (or Fabaceae), the third largest angiosperm family. Despite their ecological and economic importance, a robust phylogenetic framework for legumes based on genome-scale sequence data is lacking. We generated alignments of 72 chloroplast genes and 7621 homologous nuclear-encoded proteins, for 157 and 76 taxa, respectively. We analysed these with maximum likelihood, Bayesian inference, and a multispecies coalescent summary method, and evaluated support for alternative topologies across gene trees. We resolve the deepest divergences in the legume phylogeny despite lack of phylogenetic signal across all chloroplast genes and the majority of nuclear genes. Strongly supported conflict in the remainder of nuclear genes is suggestive of incomplete lineage sorting. All six subfamilies originated nearly simultaneously, suggesting that the prevailing view of some subfamilies as 'basal' or 'early-diverging' with respect to others should be abandoned, which has important implications for understanding the evolution of legume diversity and traits. Our study highlights the limits of phylogenetic resolution in relation to rapid successive speciation.

Facultative crassulacean acid metabolism in a C3-C4 intermediate

The Portulacaceae enable the study of the evolutionary relationship between C4 and crassulacean acid metabolism (CAM) photosynthesis. Shoots of well-watered plants of the C3-C4 intermediate species Portulaca cryptopetala Speg. exhibit net uptake of CO2 solely during the light. CO2 fixation is primarily via the C3 pathway as indicated by a strong stimulation of CO2 uptake when shoots were provided with air containing 2% O2. When plants were subjected to water stress, daytime CO2 uptake was reduced and CAM-type net CO2 uptake in the dark occurred. This was accompanied by nocturnal accumulation of acid in both leaves and stems, also a defining characteristic of CAM. Following rewatering, net CO2 uptake in the dark ceased in shoots, as did nocturnal acidification of the leaves and stems. With this unequivocal demonstration of stress-related reversible, i.e. facultative, induction of CAM, P. cryptopetala becomes the first C3-C4 intermediate species reported to exhibit CAM. Portulaca molokiniensis Hobdy, a C4 species, also exhibited CAM only when subjected to water stress. Facultative CAM has now been demonstrated in all investigated species of Portulaca, which are well sampled from across the phylogeny. This strongly suggests that in Portulaca, a lineage in which species engage predominately in C4 photosynthesis, facultative CAM is ancestral to C4. In a broader context, it has now been demonstrated that CAM can co-exist in leaves that exhibit any of the other types of photosynthesis known in terrestrial plants: C3, C4 and C3-C4 intermediate.

An empirical assessment of a single family-wide hybrid capture locus set at multiple evolutionary timescales in Asteraceae

PREMISE

Hybrid capture with high-throughput sequencing (Hyb-Seq) is a powerful tool for evolutionary studies. The applicability of an Asteraceae family-specific Hyb-Seq probe set and the outcomes of different phylogenetic analyses are investigated here.

METHODS

Hyb-Seq data from 112 Asteraceae samples were organized into groups at different taxonomic levels (tribe, genus, and species). For each group, data sets of non-paralogous loci were built and proportions of parsimony informative characters estimated. The impacts of analyzing alternative data sets, removing long branches, and type of analysis on tree resolution and inferred topologies were investigated in tribe Cichorieae.

RESULTS

Alignments of the Asteraceae family-wide Hyb-Seq locus set were parsimony informative at all taxonomic levels. Levels of resolution and topologies inferred at shallower nodes differed depending on the locus data set and the type of analysis, and were affected by the presence of long branches.

DISCUSSION

The approach used to build a Hyb-Seq locus data set influenced resolution and topologies inferred in phylogenetic analyses. Removal of long branches improved the reliability of topological inferences in maximum likelihood analyses. The Astereaceae Hyb-Seq probe set is applicable at multiple taxonomic depths, which demonstrates that probe sets do not necessarily need to be lineage-specific.

Hyb-Seq for Flowering Plant Systematics

High-throughput DNA sequencing (HTS) presents great opportunities for plant systematics, yet genomic complexity needs to be reduced for HTS to be effectively applied. We highlight Hyb-Seq as a promising approach, especially in light of the recent development of probes enriching 353 low-copy nuclear genes from any flowering plant taxon.

Phylogenomics in Cactaceae: A case study using the chollas sensu lato (Cylindropuntieae, Opuntioideae) reveals a common pattern out of the Chihuahuan and Sonoran deserts

PREMISE

Although numerous phylogenetic studies have been conducted in Cactaceae, whole-plastome datasets have not been employed. We used the chollas to develop a plastome dataset for phylogeny reconstruction to test species relationships, biogeography, clade age, and morphological evolution.

METHODS

We developed a plastome dataset for most known diploid members of the chollas (42 taxa) as well as for other members of Cylindropuntieae. Paired-end, raw reads from genome skimming were reference-mapped onto a de novo plastome assembly of one species of cholla, Cylindropuntia bigelovii, and were used to build our plastome dataset, which was analyzed using various methods.

RESULTS

Our plastome dataset resolved the phylogeny of the chollas, including most interspecific and intraspecific relationships. Tribe Cylindropuntieae arose ~18 mya, during the early Miocene in southern South America, and is supported as sister to the South American clade Tephrocacteae. The (Micropuntia (Cylindropuntia + Grusonia)) clade most likely originated in the Chihuahuan Desert region around 16 mya and then migrated into other North American desert regions. Key morphological characters for recognizing traditional taxonomic series in Cylindropuntia (e.g., spiny fruit) are mostly homoplasious.

CONCLUSIONS

This study provides the first comprehensive plastome phylogeny for any clade within Cactaceae. Although the chollas s.l. are widespread throughout western North American deserts, their most recent common ancestor likely arose in the Chihuahuan Desert region during the mid-Miocene, with much of their species diversity arising in the early to mid-Pliocene, a pattern strikingly similar to those found in other western North American desert groups.

A Target Capture-Based Method to Estimate Ploidy From Herbarium Specimens

Whole genome duplication (WGD) events are common in many plant lineages, but the ploidy status and possible occurrence of intraspecific ploidy variation are unknown for most species. Standard methods for ploidy determination are chromosome counting and flow cytometry approaches. While flow cytometry approaches typically use fresh tissue, an increasing number of studies have shown that recently dried specimens can be used to yield ploidy data. Recent studies have started to explore whether high-throughput sequencing (HTS) data can be used to assess ploidy levels by analyzing allelic frequencies from single copy nuclear genes. Here, we compare different approaches using a range of yam (Dioscorea) tissues of varying ages, drying methods and quality, including herbarium tissue. Our aims were to: (1) explore the limits of flow cytometry in estimating ploidy level from dried samples, including herbarium vouchers collected between 1831 and 2011, and (2) optimize a HTS-based method to estimate ploidy by considering allelic frequencies from nuclear genes obtained using a target-capture method. We show that, although flow cytometry can be used to estimate ploidy levels from herbarium specimens collected up to fifteen years ago, success rate is low (5.9%). We validated our HTS-based estimates of ploidy using 260 genes by benchmarking with dried samples of species of known ploidy (Dioscorea alata, D. communis, and D. sylvatica). Subsequently, we successfully applied the method to the 85 herbarium samples analyzed with flow cytometry, and successfully provided results for 91.7% of them, comprising species across the phylogenetic tree of Dioscorea. We also explored the limits of using this HTS-based approach for identifying high ploidy levels in herbarium material and the effects of heterozygosity and sequence coverage. Overall, we demonstrated that ploidy diversity within and between species may be ascertained from historical collections, allowing the determination of polyploidization events from samples collected up to two centuries ago. This approach has the potential to provide insights into the drivers and dynamics of ploidy level changes during plant evolution and crop domestication.

Evolution of Portulacineae Marked by Gene Tree Conflict and Gene Family Expansion Associated with Adaptation to Harsh Environments

Several plant lineages have evolved adaptations that allow survival in extreme and harsh environments including many families within the plant clade Portulacineae (Caryophyllales) such as the Cactaceae, Didiereaceae, and Montiaceae. Here, using newly generated transcriptomic data, we reconstructed the phylogeny of Portulacineae and examined potential correlates between molecular evolution and adaptation to harsh environments. Our phylogenetic results were largely congruent with previous analyses, but we identified several early diverging nodes characterized by extensive gene tree conflict. For particularly contentious nodes, we present detailed information about the phylogenetic signal for alternative relationships. We also analyzed the frequency of gene duplications, confirmed previously identified whole genome duplications (WGD), and proposed a previously unidentified WGD event within the Didiereaceae. We found that the WGD events were typically associated with shifts in climatic niche but did not find a direct association with WGDs and diversification rate shifts. Diversification shifts occurred within the Portulacaceae, Cactaceae, and Anacampserotaceae, and whereas these did not experience WGDs, the Cactaceae experienced extensive gene duplications. We examined gene family expansion and molecular evolutionary patterns with a focus on genes associated with environmental stress responses and found evidence for significant gene family expansion in genes with stress adaptation and clades found in extreme environments. These results provide important directions for further and deeper examination of the potential links between molecular evolutionary patterns and adaptation to harsh environments.

Phylogenomic analyses reveal a deep history of hybridization and polyploidy in the Neotropical genus Lachemilla (Rosaceae)

Hybridization, incomplete lineage sorting, and phylogenetic error produce similar incongruence patterns, representing a great challenge for phylogenetic reconstruction. Here, we use sequence capture data and multiple species tree and species network approaches to resolve the backbone phylogeny of the Neotropical genus Lachemilla, while distinguishing among sources of incongruence. We used 396 nuclear loci and nearly complete plastome sequences from 27 species to clarify the relationships among the major groups of Lachemilla, and explored multiple sources of conflict between gene trees and species trees inferred with a plurality of approaches. All phylogenetic methods recovered the four major groups previously proposed for Lachemilla, but species tree methods recovered different topologies for relationships between these four clades. Species network analyses revealed that one major clade, Orbiculate, is likely of ancient hybrid origin, representing one of the main sources of incongruence among the species trees. Additionally, we found evidence for a potential whole genome duplication event shared by Lachemilla and allied genera. Lachemilla shows clear evidence of ancient and recent hybridization throughout the evolutionary history of the group. Also, we show the necessity to use phylogenetic network approaches that can simultaneously accommodate incomplete lineage sorting and gene flow when studying groups that show patterns of reticulation.

Phylogeny, evolution, and biogeographic history of Calandrinia (Montiaceae)

PREMISE OF THE STUDY

Calandrinia are small, succulent herbs that vary broadly in habitat, morphology, life history, and photosynthetic metabolism. The lineage is placed within the Montiaceae, which in turn is sister to the rest of the Portulacineae (Caryophyllales). Calandrinia occupy two distinct biogeographic regions, one in the Americas (~14 species), and one in Australia (~74 species). Past analyses of the Montiaceae present conflicting hypotheses for the phylogenetic placement and monophyly of Calandrinia, and to date, there has been no molecular phylogenetic analysis of the Australian species.

METHODS

Using a targeted gene enrichment approach, we sequenced 297 loci from multiple gene families across the Montiaceae, including all named and 16 putative new species of Australian Calandrinia, and the enigmatic monotypic genus Rumicastrum.

KEY RESULTS

All data sets and analyses reject the monophyly of Calandrinia, with Australian and New World Calandrinia each comprising distinct and well-supported clades, and Rumicastrum nested within Australian Calandrinia. We provide the first well-supported phylogeny for Australian Calandrinia, which includes all named species and several phrase-named taxa.

CONCLUSIONS

This study brings much needed clarity to relationships within Montiaceae and confirms that New World and Australian Calandrinia do not form a clade. Australian Calandrinia is a longtime resident of the continent, having diverged from its sister lineage ~30 Ma, concurrent with separation of Australia from Antarctica. Most diversification occurred during the middle Miocene, with lowered speciation and/or higher extinction rates coincident with the establishment of severe aridity by the late Miocene.

Crassulacean acid metabolism in the Basellaceae (Caryophyllales)

C₄ and crassulacean acid metabolism (CAM) have evolved in the order Caryophyllales many times but neither C₄ nor CAM have been recorded for the Basellaceae, a small family in the CAM-rich sub-order Portulacineae. 24 h gas exchange and day-night changes in titratable acidity were measured in leaves of Anredera baselloides exposed to wet-dry-wet cycles. While net CO₂ uptake was restricted to the light period in well-watered plants, net CO₂ fixation in the dark, accompanied by significant nocturnal increases in leaf acidity, developed in droughted plants. Plants reverted to solely C₃ photosynthesis upon rewatering. The reversible induction of nocturnal net CO₂ uptake by drought stress indicates that this species is able to exhibit CAM in a facultative manner. This is the first report of CAM in a member of the Basellaceae.

Molecular evolution of key metabolic genes during transitions to C4 and CAM photosynthesis

PREMISE OF THE STUDY

Next-generation sequencing facilitates rapid production of well-sampled phylogenies built from very large genetic data sets, which can then be subsequently exploited to examine the molecular evolution of the genes themselves. We present an evolutionary analysis of 83 gene families (19 containing carbon-concentrating mechanism (CCM) genes, 64 containing non-CCM genes) in the portullugo clade (Caryophyllales), a diverse lineage of mostly arid-adapted plants that contains multiple evolutionary origins of all known photosynthesis types in land plants (C₃ , C₄ , CAM, C₄ -CAM, and various intermediates).

METHODS

We inferred a phylogeny of 197 individuals from 167 taxa using coalescent-based approaches and individual gene family trees using maximum likelihood. Positive selection analyses were conducted on individual gene family trees with a mixed effects model of evolution (MEME). We devised new indices to compare levels of convergence and prevalence of particular residues between CCM and non-CCM genes and between species with different photosynthetic pathways.

KEY RESULTS

Contrary to expectations, there were no significant differences in the levels of positive selection detected in CCM versus non-CCM genes. However, we documented a significantly higher level of convergent amino acid substitutions in CCM genes, especially in C₄ taxa.

CONCLUSIONS

Our analyses reveal a new suite of amino acid residues putatively important for C₄ and CAM function. We discuss both the advantages and challenges of using targeted enrichment sequence data for exploratory studies of molecular evolution.

From cacti to carnivores: Improved phylotranscriptomic sampling and hierarchical homology inference provide further insight into the evolution of Caryophyllales

PREMISE OF THE STUDY

The Caryophyllales contain ~12,500 species and are known for their cosmopolitan distribution, convergence of trait evolution, and extreme adaptations. Some relationships within the Caryophyllales, like those of many large plant clades, remain unclear, and phylogenetic studies often recover alternative hypotheses. We explore the utility of broad and dense transcriptome sampling across the order for resolving evolutionary relationships in Caryophyllales.

METHODS

We generated 84 transcriptomes and combined these with 224 publicly available transcriptomes to perform a phylogenomic analysis of Caryophyllales. To overcome the computational challenge of ortholog detection in such a large data set, we developed an approach for clustering gene families that allowed us to analyze >300 transcriptomes and genomes. We then inferred the species relationships using multiple methods and performed gene-tree conflict analyses.

KEY RESULTS

Our phylogenetic analyses resolved many clades with strong support, but also showed significant gene-tree discordance. This discordance is not only a common feature of phylogenomic studies, but also represents an opportunity to understand processes that have structured phylogenies. We also found taxon sampling influences species-tree inference, highlighting the importance of more focused studies with additional taxon sampling.

CONCLUSIONS

Transcriptomes are useful both for species-tree inference and for uncovering evolutionary complexity within lineages. Through analyses of gene-tree conflict and multiple methods of species-tree inference, we demonstrate that phylogenomic data can provide unparalleled insight into the evolutionary history of Caryophyllales. We also discuss a method for overcoming computational challenges associated with homolog clustering in large data sets.