Temporal patterns of diversification in Brassicaceae demonstrate decoupling of rate shifts and mesopolyploidization events

New beginnings for dead ends: polyploidy, -SSE models and the dead-end hypothesis

BACKGROUND

Since the mid-20th century, it has been argued by some that the transition from diploidy to polyploidy is an 'evolutionary dead end' in plants. Although this point has been debated ever since, multiple definitions of 'dead end' have been used in the polyploidy literature, without sufficient differentiation between alternative uses.

SCOPE

Here, we focus on the two most common conceptions of the dead-end hypothesis currently discussed: the 'lowering diversification' hypothesis and the 'rarely successful' hypothesis. We discuss the evidence for both hypotheses, and we use a recently developed method of inferring tip diversification rates to demonstrate tests for the effect of ploidy on diversification in Solanaceae.

CONCLUSIONS

We find that diversification rates in the family are not strongly correlated with ploidy or with the closely related trait of breeding system. We also outline recent work in the field that moves beyond the relatively simple question of whether polyploidy increases, decreases or does not significantly affect diversification rates in plants.

Polyploids of Brassicaceae: Genomic Insights and Assembly Strategies

The Brassicaceae family is distinguished by its inclusion of high-value crops such as cabbage, broccoli, mustard, and wasabi, all noted for their glucosinolates. In this family, many polyploidy species are distributed and shaped by numerous whole-genome duplications, independent genome doublings, and hybridization events. The evolutionary trajectory of the family is marked by enhanced diversification and lineage splitting after paleo- and meso-polyploidization, with discernible remnants of whole-genome duplications within their genomes. The recent neopolyploidization events notably increased the proportion of polyploid species within the family. Although sequencing efforts for the Brassicaceae genome have been robust, accurately distinguishing sub-genomes remains a significant challenge, frequently complicating the assembly process. Assembly strategies include comparative analyses with ancestral species and examining k-mers, long terminal repeat retrotransposons, and pollen sequencing. This review comprehensively explores the unique genomic characteristics of the Brassicaceae family, with a particular emphasis on polyploidization events and the latest strategies for sequencing and assembly. This review will significantly improve our understanding of polyploidy in the Brassicaceae family and assist in future genome assembly methods.

Genomes of Meniocus linifolius and Tetracme quadricornis reveal the ancestral karyotype and genomic features of core Brassicaceae

Brassicaceae represents an important plant family from both a scientific and economic perspective. However, genomic features related to the early diversification of this family have not been fully characterized, especially upon the uplift of the Tibetan Plateau, which was followed by increasing aridity in the Asian interior, intensifying monsoons in Eastern Asia, and significantly fluctuating daily temperatures. Here, we reveal the genomic architecture that accompanied early Brassicaceae diversification by analyzing two high-quality chromosome-level genomes for Meniocus linifolius (Arabodae; clade D) and Tetracme quadricornis (Hesperodae; clade E), together with genomes representing all major Brassicaceae clades and the basal Aethionemeae. We reconstructed an ancestral core Brassicaceae karyotype (CBK) containing 9 pseudochromosomes with 65 conserved syntenic genomic blocks and identified 9702 conserved genes in Brassicaceae. We detected pervasive conflicting phylogenomic signals accompanied by widespread ancient hybridization events, which correlate well with the early divergence of core Brassicaceae. We identified a successive Brassicaceae-specific expansion of the class I TREHALOSE-6-PHOSPHATE SYNTHASE 1 (TPS1) gene family, which encodes enzymes with essential regulatory roles in flowering time and embryo development. The TPS1s were mainly randomly amplified, followed by expression divergence. Our results provide fresh insights into historical genomic features coupled with Brassicaceae evolution and offer a potential model for broad-scale studies of adaptive radiation under an ever-changing environment.

Complementing model species with model clades

Model species continue to underpin groundbreaking plant science research. At the same time, the phylogenetic resolution of the land plant tree of life continues to improve. The intersection of these 2 research paths creates a unique opportunity to further extend the usefulness of model species across larger taxonomic groups. Here we promote the utility of the Arabidopsis thaliana model species, especially the ability to connect its genetic and functional resources, to species across the entire Brassicales order. We focus on the utility of using genomics and phylogenomics to bridge the evolution and diversification of several traits across the Brassicales to the resources in Arabidopsis, thereby extending scope from a model species by establishing a "model clade." These Brassicales-wide traits are discussed in the context of both the model species Arabidopsis and the family Brassicaceae. We promote the utility of such a "model clade" and make suggestions for building global networks to support future studies in the model order Brassicales.

The evolution of ephemeral flora in Xinjiang, China: insights from plastid phylogenomic analyses of Brassicaceae

BACKGROUND

The ephemeral flora of northern Xinjiang, China, plays an important role in the desert ecosystems. However, the evolutionary history of this flora remains unclear. To gain new insights into its origin and evolutionary dynamics, we comprehensively sampled ephemeral plants of Brassicaceae, one of the essential plant groups of the ephemeral flora.

RESULTS

We reconstructed a phylogenetic tree using plastid genomes and estimated their divergence times. Our results indicate that ephemeral species began to colonize the arid areas in north Xinjiang during the Early Miocene and there was a greater dispersal of ephemeral species from the surrounding areas into the ephemeral community of north Xinjiang during the Middle and Late Miocene, in contrast to the Early Miocene or Pliocene periods.

CONCLUSIONS

Our findings, together with previous studies, suggest that the ephemeral flora originated in the Early Miocene, and species assembly became rapid from the Middle Miocene onwards, possibly attributable to global climate changes and regional geological events.

Research progress on Brassicaceae plants: a bibliometrics analysis

The Brassicaceae is a worldwide family that produces ornamental flowers, edible vegetables, and oilseed plants, with high economic value in agriculture, horticulture, and landscaping. This study used the Web of Science core dataset and the CiteSpace bibliometric tool to quantitatively visualize the number of publications, authors, institutions, and countries of 3139 papers related to Brassicaceae plants from 2002 to 2022. The keywords and references were divided into two phases: Phase 1 (2002-2011) and Phase 2 (2012-2022) for quantitative and qualitative analysis. The results showed: An average annual publication volume of 149 articles, with an overall fluctuating upward trend; the research force was mainly led by Professor Ihsan A. Al-shehbaz from Missouri Botanical Garden; and the United States had the highest number of publications. In the first phase, research focused on the phylogeny of Brassicaceae plants, while the second phase delved into diverse research based on previous studies, research in areas such as polyploidy, molecular technique, physiology, and hyperaccumulator has been extended. Based on this research, we propounded some ideas for future studies on Brassicaceae plants and summarized the research gaps.

A High-Throughput Approach for Photosynthesis Studies in a Brassicaceae Panel

The study of natural variations in photosynthesis in the Brassicaceae family offers the possibility of identifying mechanisms to enhance photosynthetic efficiency in crop plants. Indeed, this family, and particularly its tribe Brassiceae, has been shown to harbor species that have a higher-than-expected photosynthetic efficiency, possibly as a result of a complex evolutionary history. Over the past two decades, methods have been developed to measure photosynthetic efficiency based on chlorophyll fluorescence. Chlorophyll fluorescence measurements are performed with special cameras, such as the FluorCams, which can be included in robotic systems to create high-throughput phenotyping platforms. While these platforms have so far demonstrated high efficiency in measuring small model species like Arabidopsis thaliana, they have the drawback of limited adaptability to accommodate different plant sizes. As a result, the range of species that can be analyzed is restricted. This chapter presents our approach to analyze the photosynthetic parameters: ϕPSII and Fv/Fm for a panel of Brassicaceae species, including a high-photosynthesis species, Hirschfeldia incana, and the adaptations to the phenotyping platform that are required to accommodate this varied group of plants.

The evolutionary responses of life-history strategies to climatic variability in flowering plants

The evolution of annual or perennial strategies in flowering plants likely depends on a broad array of temperature and precipitation variables. Previous documented climate life-history correlations in explicit phylogenetic frameworks have been limited to certain clades and geographic regions. To gain insights which generalize to multiple lineages we employ a multi-clade approach analyzing 32 groups of angiosperms across eight climatic variables. We utilize a recently developed method that accounts for the joint evolution of continuous and discrete traits to evaluate two hypotheses: annuals tend to evolve in highly seasonal regions prone to extreme heat and drought; and annuals tend to have faster rates of climatic niche evolution than perennials. We find that temperature, particularly highest temperature of the warmest month, is the most consistent climatic factor influencing the evolution of annual strategy in flowering plants. Unexpectedly, we do not find significant differences in rates of climatic niche evolution between perennial and annual lineages. We propose that annuals are consistently favored in areas prone to extreme heat due to their ability to escape heat stress as seeds, but they tend to be outcompeted by perennials in regions where extreme heat is uncommon or nonexistent.

Global Brassicaceae phylogeny based on filtering of 1,000-gene dataset

The mustard family (Brassicaceae) is a scientifically and economically important family, containing the model plant Arabidopsis thaliana and numerous crop species that feed billions worldwide. Despite its relevance, most phylogenetic trees of the family are incompletely sampled and often contain poorly supported branches. Here, we present the most complete Brassicaceae genus-level family phylogenies to date (Brassicaceae Tree of Life or BrassiToL) based on nuclear (1,081 genes, 319 of the 349 genera; 57 of the 58 tribes) and plastome (60 genes, 265 genera; all tribes) data. We found cytonuclear discordance between the two, which is likely a result of rampant hybridization among closely and more distantly related lineages. To evaluate the impact of such hybridization on the nuclear phylogeny reconstruction, we performed five different gene sampling routines, which increasingly removed putatively paralog genes. Our cleaned subset of 297 genes revealed high support for the tribes, whereas support for the main lineages (supertribes) was moderate. Calibration based on the 20 most clock-like nuclear genes suggests a late Eocene to late Oligocene origin of the family. Finally, our results strongly support a recently published new family classification, dividing the family into two subfamilies (one with five supertribes), together representing 58 tribes. This includes five recently described or re-established tribes, including Arabidopsideae, a monogeneric tribe accommodating Arabidopsis without any close relatives. With a worldwide community of thousands of researchers working on Brassicaceae and its diverse members, our new genus-level family phylogeny will be an indispensable tool for studies on biodiversity and plant biology.

Evolutionary dynamics of Euphorbia carniolica suggest a complex Plio-Pleistocene history of understorey species of deciduous forest in southeastern Europe

Deciduous forests form the dominant natural vegetation of Europe today, but were restricted to small refugia during Pleistocene cold stages, implying an evolutionary past shaped by recurrent range contractions and expansions. Cold-stage forest refugia were probably widespread in southern and central Europe, with the northwestern Balkan Peninsula being of particular importance. However, the actual number and location of deciduous forest refugia, as well as the connections between them, remain disputed. Here, we address the evolutionary dynamics of the deciduous forest understorey species Euphorbia carniolica as a proxy for past forest dynamics. To do so, we obtained genomic and morphometric data from populations representing the species' entire range, investigated phylogenetic position and intraspecific genetic variation, tested explicit demographic scenarios and applied species distribution models. Our data support two disjoint groups linked to separate refugia on the northwestern and central Balkan Peninsula. We find that genetic differentiation between groups started in the early Pleistocene via vicariance, suggesting a larger distribution in the past. Both refugia acted as sources for founder events to the southeastern Alps and the Carpathians; the latter were likely colonised before the last cold stage. In line with traditional views on the pre-Pleistocene origin of many southeastern European deciduous forest species, the origin of E. carniolica was dated to the late Pliocene. The fact that E. carniolica evolved at a time when a period of continuous forestation was ending in much of Eurasia provides an interesting biogeographical perspective on the past links between Eurasian deciduous forests and their biota.

Evolution of hygrophytic plant species in the Anatolia-Caucasus region: insights from phylogenomic analyses of Cardamine perennials

BACKGROUND AND AIMS

Southwestern Asia is a significant centre of biodiversity and a cradle of diversification for many plant groups, especially xerophytic elements. In contrast, little is known about the evolution and diversification of its hygrophytic flora. To fill this gap, we focus on Cardamine (Brassicaceae) species that grow in wetlands over a wide altitudinal range. We aimed to elucidate their evolution, assess the extent of presumed historical gene flow between species, and draw inferences about intraspecific structure.

METHODS

We applied the phylogenomic Hyb-Seq approach, ecological niche analyses and multivariate morphometrics to a total of 85 Cardamine populations from the target region of Anatolia-Caucasus, usually treated as four to six species, and supplemented them with close relatives from Europe.

KEY RESULTS

Five diploids are recognized in the focus area, three of which occur in regions adjacent to the Black and/or Caspian Sea (C. penzesii, C. tenera, C. lazica), one species widely distributed from the Caucasus to Lebanon and Iran (C. uliginosa), and one western Anatolian entity (provisionally C. cf. uliginosa). Phylogenomic data suggest recent speciation during the Pleistocene, likely driven by both geographic separation (allopatry) and ecological divergence. With the exception of a single hybrid (allotetraploid) speciation event proven for C. wiedemanniana, an endemic of southern Turkey, no significant traces of past or present interspecific gene flow were observed. Genetic variation within the studied species is spatially structured, suggesting reduced gene flow due to geographic and ecological barriers, but also glacial survival in different refugia.

CONCLUSIONS

This study highlights the importance of the refugial regions of the Black and Caspian Seas for both harbouring and generating hygrophytic species diversity in Southwestern Asia. It also supports the significance of evolutionary links between Anatolia and the Balkan Peninsula. Reticulation and polyploidization played a minor evolutionary role here in contrast to the European relatives.

Genome assembly of the Brassicaceae diploid Orychophragmus violaceus reveals complex whole-genome duplication and evolution of dihydroxy fatty acid metabolism

Orychophragmus violaceus is a Brassicaceae species widely cultivated in China, particularly as a winter cover crop in northern China because of its low-temperature tolerance and low water demand. Recently, O. violaceus has also been cultivated as a potential industrial oilseed crop because of its abundant 24-carbon dihydroxy fatty acids (diOH-FAs), which contribute to superior high-temperature lubricant properties. In this study, we performed de novo assembly of the O. violaceus genome. Whole-genome synteny analysis of the genomes of its relatives demonstrated that O. violaceus is a diploid that has undergone an extra whole-genome duplication (WGD) after the Brassicaceae-specific α-WGD event, with a basic chromosome number of x = 12. Formation of diOH-FAs is hypothesized to have occurred after the WGD event. Based on the genome and the transcriptome data from multiple stages of seed development, we predicted that OvDGAT1-1 and OvDGAT1-2 are candidate genes for the regulation of diOH-FA storage in O. violaceus seeds. These results may greatly facilitate the development of heat-tolerant and eco-friendly plant-based lubricants using O. violaceus seed oil and improve our understanding of the genomic evolution of Brassicaceae.

An updated classification of the Brassicaceae (Cruciferae)

Based on recent achievements in phylogenetic studies of the Brassicaceae, a novel infrafamilial classification is proposed that includes major improvements at the subfamilial and supertribal levels. Herein, the family is subdivided into two subfamilies, Aethionemoideae (subfam. nov.) and Brassicoideae. The Brassicoideae, with 57 of the 58 tribes of Brassicaceae, are further partitioned into five supertribes, including the previously recognized Brassicodae and the newly established Arabodae, Camelinodae, Heliophilodae, and Hesperodae. Additional tribus-level contributions include descriptions of the newly recognized Arabidopsideae, Asperuginoideae, Hemilophieae, Schrenkielleae, and resurrection of the Chamireae and Subularieae. Further detailed comments on 17 tribes in need of clarifications are provided.

Phylotranscriptomic analyses reveal multiple whole-genome duplication events, the history of diversification and adaptations in the Araceae

BACKGROUND AND AIMS

The Araceae are one of the most diverse monocot families with numerous morphological and ecological novelties. Plastid and mitochondrial genes have been used to investigate the phylogeny and to interpret shifts in the pollination biology and biogeography of the Araceae. In contrast, the role of whole-genome duplication (WGD) in the evolution of eight subfamilies remains unclear.

METHODS

New transcriptomes or low-depth whole-genome sequences of 65 species were generated through Illumina sequencing. We reconstructed the phylogenetic relationships of Araceae using concatenated and species tree methods, and then estimated the age of major clades using TreePL. We inferred the WGD events by Ks and gene tree methods. We investigated the diversification patterns applying time-dependent and trait-dependent models. The expansions of gene families and functional enrichments were analysed using CAFE and InterProScan.

KEY RESULTS

Gymnostachydoideae was the earliest diverging lineage followed successively by Orontioideae, Lemnoideae and Lasioideae. In turn, they were followed by the clade of 'bisexual climbers' comprised of Pothoideae and Monsteroideae, which was resolved as the sister to the unisexual flowers clade of Zamioculcadoideae and Aroideae. A special WGD event ψ (psi) shared by the True-Araceae clade occurred in the Early Cretaceous. Net diversification rates first declined and then increased through time in the Araceae. The best diversification rate shift along the stem lineage of the True-Araceae clade was detected, and net diversification rates were enhanced following the ψ-WGD. Functional enrichment analyses revealed that some genes, such as those encoding heat shock proteins, glycosyl hydrolase and cytochrome P450, expanded within the True-Araceae clade.

CONCLUSIONS

Our results improve our understanding of aroid phylogeny using the large number of single-/low-copy nuclear genes. In contrast to the Proto-Araceae group and the lemnoid clade adaption to aquatic environments, our analyses of WGD, diversification and functional enrichment indicated that WGD may play a more important role in the evolution of adaptations to tropical, terrestrial environments in the True-Araceae clade. These insights provide us with new resources to interpret the evolution of the Araceae.

The evolutionary history of Cardamine bulbifera shows a successful rapid postglacial Eurasian range expansion in the absence of sexual reproduction

BACKGROUND AND AIMS

Sexual reproduction is known to drive plant diversification and adaptation. Here we investigate the evolutionary history and spatiotemporal origin of a dodecaploid (2n = 12x = 96) Eurasian deciduous woodland species, Cardamine bulbifera, which reproduces and spreads via vegetative bulb-like structures only. The species has been among the most successful range-expanding understorey woodland plants in Europe, which raises the question of the genetic architecture of its gene pool, since its hexaploid (2n = 6x = 48) but putatively outcrossing closest relative, C. quinquefolia, displays a smaller distribution range in Eastern Europe towards the Caucasus region. Cardamine bulbifera belongs to a small monophyletic clade of four species comprising also C. abchasica (2n = 2x = 16) and C. bipinnata (unknown ploidy) from the Caucasus region.

METHODS

We sequenced the genomes of the two polyploids and their two putative ancestors using Illumina short-read sequencing technology (×7-8 coverage). Covering the entire distribution range, genomic data were generated for 67 samples of the two polyploids (51 samples of C. bulbifera, 16 samples of C. quinquefolia) and 6 samples of the putative diploid taxa (4 samples of C. abchasica, 2 samples of C. bipinnata) to unravel the evolutionary origin of the polyploid taxa using phylogenetic reconstructions of biparentally and maternally inherited genetic sequence data. Ploidy levels of C. bulbifera and C. quinquefolia were analysed by comparative chromosome painting. We used genetic assignment analysis (STRUCTURE) and approximate Bayesian computation (ABC) modelling to test whether C. bulbifera represents genetically differentiated lineages and addressed the hypothesis of its hybrid origin. Comparative ecological modelling was applied to unravel possible niche differentiation among the two polyploid species.

KEY RESULTS

Cardamine bulbifera was shown to be a non-hybridogenous, auto-dodecaploid taxon of early Pleistocene origin, but with a history of past gene flow with its hexaploid sister species C. quinquefolia, likely during the last glacial maximum in shared refuge areas in Eastern Europe towards Western Turkey and the Crimean Peninsula region. The diploid Caucasian endemic C. abchasica is considered an ancestral species, which also provides evidence for the origin of the species complex in the Caucasus region. Cardamine bulbifera successfully expanded its distribution range postglacially towards Central and Western Europe accompanied by a transition to exclusively vegetative propagation.

CONCLUSIONS

A transition to vegetative propagation in C. bulbifera is hypothesized as the major innovation to rapidly expand its distribution range following postglacially progressing woodland vegetation throughout Europe. Preceding and introgressive gene flow from its sister species C. quinquefolia in the joint refuge area is documented. This transition and ecological differentiation may have been triggered by preceding introgressive gene flow from its sister species in the joint East European refuge areas.

Genome diploidization associates with cladogenesis, trait disparity, and plastid gene evolution

Angiosperm genome evolution was marked by many clade-specific whole-genome duplication events. The Microlepidieae is one of the monophyletic clades in the mustard family (Brassicaceae) formed after an ancient allotetraploidization. Postpolyploid cladogenesis has resulted in the extant c. 17 genera and 60 species endemic to Australia and New Zealand (10 species). As postpolyploid genome diploidization is a trial-and-error process under natural selection, it may proceed with different intensity and be associated with speciation events. In Microlepidieae, different extents of homoeologous recombination between the two parental subgenomes generated clades marked by slow ("cold") versus fast ("hot") genome diploidization. To gain a deeper understanding of postpolyploid genome evolution in Microlepidieae, we analyzed phylogenetic relationships in this tribe using complete chloroplast sequences, entire 35S rDNA units, and abundant repetitive sequences. The four recovered intra-tribal clades mirror the varied diploidization of Microlepidieae genomes, suggesting that the intrinsic genomic features underlying the extent of diploidization are shared among genera and species within one clade. Nevertheless, even congeneric species may exert considerable morphological disparity (e.g. in fruit shape), whereas some species within different clades experience extensive morphological convergence despite the different pace of their genome diploidization. We showed that faster genome diploidization is positively associated with mean morphological disparity and evolution of chloroplast genes (plastid-nuclear genome coevolution). Higher speciation rates in perennials than in annual species were observed. Altogether, our results confirm the potential of Microlepidieae as a promising subject for the analysis of postpolyploid genome diploidization in Brassicaceae.

Stability in the South, Turbulence Toward the North: Evolutionary History of Aurinia saxatilis (Brassicaceae) Revealed by Phylogenomic and Climatic Modelling Data

The Balkan Peninsula played an important role in the evolution of many Mediterranean plants and served as a major source for post-Pleistocene colonisation of central and northern Europe. Its complex geo-climatic history and environmental heterogeneity significantly influenced spatiotemporal diversification and resulted in intricate phylogeographic patterns. To explore the evolutionary dynamics and phylogeographic patterns within the widespread eastern Mediterranean and central European species Aurinia saxatilis, we used a combination of phylogenomic (restriction-site associated DNA sequencing, RADseq) and phylogenetic (sequences of the plastid marker ndhF) data as well as species distribution models generated for the present and the Last Glacial Maximum (LGM). The inferred phylogenies retrieved three main geographically distinct lineages. The southern lineage is restricted to the eastern Mediterranean, where it is distributed throughout the Aegean area, the southern Balkan Peninsula, and the southern Apennine Peninsula, and corresponds to the species main distribution area during the LGM. The eastern lineage extends from the eastern Balkan Peninsula over the Carpathians to central Europe, while the central lineage occupies the central Balkan Peninsula. Molecular dating places the divergence among all the three lineages to the early to middle Pleistocene, indicating their long-term independent evolutionary trajectories. Our data revealed an early divergence and stable in situ persistence of the southernmost, eastern Mediterranean lineage, whereas the mainland, south-east European lineages experienced more complex and turbulent evolutionary dynamics triggered by Pleistocene climatic oscillations. Our data also support the existence of multiple glacial refugia in southeast Europe and highlight the central Balkan Peninsula not only as a cradle of lineage diversifications but also as a source of lineage dispersal. Finally, the extant genetic variation within A. saxatilis is congruent with the taxonomic separation of peripatric A. saxatilis subsp. saxatilis and A. saxatilis subsp. orientalis, whereas the taxonomic status of A. saxatilis subsp. megalocarpa remains doubtful.

Phenotypic plasticity guides Moricandia arvensis divergence and convergence across the Brassicaceae floral morphospace

Many flowers exhibit phenotypic plasticity. By inducing the production of several phenotypes, plasticity may favour the rapid exploration of different regions of the floral morphospace. We investigated how plasticity drives Moricandia arvensis, a species displaying within-individual floral polyphenism, across the floral morphospace of the entire Brassicaceae family. We compiled the multidimensional floral phenotype, the phylogenetic relationships, and the pollination niche of over 3000 species to construct a family-wide floral morphospace. We assessed the disparity between the two M. arvensis floral morphs (as the distance between the phenotypic spaces occupied by each morph) and compared it with the family-wide disparity. We measured floral divergence by comparing disparity with the most common ancestor, and estimated the convergence of each floral morph with other species belonging to the same pollination niches. Moricandia arvensis exhibits a plasticity-mediated floral disparity greater than that found between species, genera and tribes. The novel phenotype of M. arvensis moves outside the region occupied by its ancestors and relatives, crosses into a new region where it encounters a different pollination niche, and converges with distant Brassicaceae lineages. Our study suggests that phenotypic plasticity favours floral divergence and rapid appearance of convergent flowers, a process which facilitates the evolution of generalist pollination systems.

Pan-genome of Raphanus highlights genetic variation and introgression among domesticated, wild, and weedy radishes

Post-polyploid diploidization associated with descending dysploidy and interspecific introgression drives plant genome evolution by unclear mechanisms. Raphanus is an economically and ecologically important Brassiceae genus and model system for studying post-polyploidization genome evolution and introgression. Here, we report the de novo sequence assemblies for 11 genomes covering most of the typical sub-species and varieties of domesticated, wild and weedy radishes from East Asia, South Asia, Europe, and America. Divergence among the species, sub-species, and South/East Asian types coincided with Quaternary glaciations. A genus-level pan-genome was constructed with family-based, locus-based, and graph-based methods, and whole-genome comparisons revealed genetic variations ranging from single-nucleotide polymorphisms (SNPs) to inversions and translocations of whole ancestral karyotype (AK) blocks. Extensive gene flow occurred between wild, weedy, and domesticated radishes. High frequencies of genome reshuffling, biased retention, and large-fragment translocation have shaped the genomic diversity. Most variety-specific gene-rich blocks showed large structural variations. Extensive translocation and tandem duplication of dispensable genes were revealed in two large rearrangement-rich islands. Disease resistance genes mostly resided on specific and dispensable loci. Variations causing the loss of function of enzymes modulating gibberellin deactivation were identified and could play an important role in phenotype divergence and adaptive evolution. This study provides new insights into the genomic evolution underlying post-polyploid diploidization and lays the foundation for genetic improvement of radish crops, biological control of weeds, and protection of wild species' germplasms.

Arabis alpina: A perennial model plant for ecological genomics and life-history evolution

Many model organisms were chosen and achieved prominence because of an advantageous combination of their life‐history characteristics, genetic properties and also practical considerations. Discoveries made in Arabidopsis thaliana, the most renowned noncrop plant model species, have markedly stimulated studies in other species with different biology. Within the family Brassicaceae, the arctic–alpine Arabis alpina has become a model complementary to Arabidopsis thaliana to study the evolution of life‐history traits, such as perenniality, and ecological genomics in harsh environments. In this review, we provide an overview of the properties that facilitated the rapid emergence of A. alpina as a plant model. We summarize the evolutionary history of A. alpina, including genomic aspects, the diversification of its mating system and demographic properties, and we discuss recent progress in the molecular dissection of developmental traits that are related to its perennial life history and environmental adaptation. From this published knowledge, we derive open questions that might inspire future research in A. alpina, other Brassicaceae species or more distantly related plant families.

Comparison of glucosinolate diversity in the crucifer tribe Cardamineae and the remaining order Brassicales highlights repetitive evolutionary loss and gain of biosynthetic steps

We review glucosinolate (GSL) diversity and analyze phylogeny in the crucifer tribe Cardamineae as well as selected species from Brassicaceae (tribe Brassiceae) and Resedaceae. Some GSLs occur widely, while there is a scattered distribution of many less common GSLs, tentatively sorted into three classes: ancient, intermediate and more recently evolved. The number of conclusively identified GSLs in the tribe (53 GSLs) constitute 60% of all GSLs known with certainty from any plant (89 GSLs) and apparently unique GSLs in the tribe constitute 10 of those GSLs conclusively identified (19%). Intraspecific, qualitative GSL polymorphism is known from at least four species in the tribe. The most ancient GSL biosynthesis in Brassicales probably involved biosynthesis from Phe, Val, Leu, Ile and possibly Trp, and hydroxylation at the β-position. From a broad comparison of families in Brassicales and tribes in Brassicaceae, we estimate that a common ancestor of the tribe Cardamineae and the family Brassicaceae exhibited GSL biosynthesis from Phe, Val, Ile, Leu, possibly Tyr, Trp and homoPhe (ancient GSLs), as well as homologs of Met and possibly homoIle (intermediate age GSLs). From the comparison of phylogeny and GSL diversity, we also suggest that hydroxylation and subsequent methylation of indole GSLs and usual modifications of Met-derived GSLs (formation of sulfinyls, sulfonyls and alkenyls) occur due to conserved biochemical mechanisms and was present in a common ancestor of the family. Apparent loss of homologs of Met as biosynthetic precursors was deduced in the entire genus Barbarea and was frequent in Cardamine (e.g. C. pratensis, C. diphylla, C. concatenata, possibly C. amara). The loss was often associated with appearance of significant levels of unique or rare GSLs as well as recapitulation of ancient types of GSLs. Biosynthetic traits interpreted as de novo evolution included hydroxylation at rare positions, acylation at the thioglucose and use of dihomoIle and possibly homoIle as biosynthetic precursors. Biochemical aspects of the deduced evolution are discussed and testable hypotheses proposed. Biosyntheses from Val, Leu, Ile, Phe, Trp, homoPhe and homologs of Met are increasingly well understood, while GSL biosynthesis from mono- and dihomoIle is poorly understood. Overall, interpretation of known diversity suggests that evolution of GSL biosynthesis often seems to recapitulate ancient biosynthesis. In contrast, unprecedented GSL biosynthetic innovation seems to be rare.

Allele Sorting as a Novel Approach to Resolving the Origin of Allotetraploids Using Hyb-Seq Data: A Case Study of the Balkan Mountain Endemic Cardamine barbaraeoides

Mountains of the Balkan Peninsula are significant biodiversity hotspots with great species richness and a large proportion of narrow endemics. Processes that have driven the evolution of the rich Balkan mountain flora, however, are still insufficiently explored and understood. Here we focus on a group of Cardamine (Brassicaceae) perennials growing in wet, mainly mountainous habitats. It comprises several Mediterranean endemics, including those restricted to the Balkan Peninsula. We used target enrichment with genome skimming (Hyb-Seq) to infer their phylogenetic relationships, and, along with genomic in situ hybridization (GISH), to resolve the origin of tetraploid Cardamine barbaraeoides endemic to the Southern Pindos Mts. (Greece). We also explored the challenges of phylogenomic analyses of polyploid species and developed a new approach of allele sorting into homeologs that allows identifying subgenomes inherited from different progenitors. We obtained a robust phylogenetic reconstruction for diploids based on 1,168 low-copy nuclear genes, which suggested both allopatric and ecological speciation events. In addition, cases of plastid-nuclear discordance, in agreement with divergent nuclear ribosomal DNA (nrDNA) copy variants in some species, indicated traces of interspecific gene flow. Our results also support biogeographic links between the Balkan and Anatolian-Caucasus regions and illustrate the contribution of the latter region to high Balkan biodiversity. An allopolyploid origin was inferred for C. barbaraeoides, which highlights the role of mountains in the Balkan Peninsula both as refugia and melting pots favoring species contacts and polyploid evolution in response to Pleistocene climate-induced range dynamics. Overall, our study demonstrates the importance of a thorough phylogenomic approach when studying the evolution of recently diverged species complexes affected by reticulation events at both diploid and polyploid levels. We emphasize the significance of retrieving allelic and homeologous variation from nuclear genes, as well as multiple nrDNA copy variants from genome skim data.

Novelty and convergence in adaptation to whole genome duplication

Whole genome duplication (WGD) can promote adaptation but is disruptive to conserved processes, especially meiosis. Studies in Arabidopsis arenosa revealed a coordinated evolutionary response to WGD involving interacting proteins controlling meiotic crossovers, which are minimised in an autotetraploid (within-species polyploid) to avoid mis-segregation. Here we test whether this surprising flexibility of a conserved essential process, meiosis, is recapitulated in an independent WGD system, Cardamine amara, 17 million years diverged from A. arenosa. We assess meiotic stability and perform population-based scans for positive selection, contrasting the genomic response to WGD in C. amara with that of A. arenosa. We found in C. amara the strongest selection signals at genes with predicted functions thought important to adaptation to WGD: meiosis, chromosome remodelling, cell cycle, and ion transport. However, genomic responses to WGD in the two species differ: minimal ortholog-level convergence emerged, with none of the meiosis genes found in A. arenosa exhibiting strong signal in C. amara. This is consistent with our observations of lower meiotic stability and occasional clonal spreading in diploid C. amara, suggesting that nascent C. amara autotetraploid lineages were preadapted by their diploid lifestyle to survive while enduring reduced meiotic fidelity. However, in contrast to a lack of ortholog convergence, we see process-level and network convergence in DNA management, chromosome organisation, stress signalling, and ion homeostasis processes. This gives the first insight into the salient adaptations required to meet the challenges of a WGD state and shows that autopolyploids can utilize multiple evolutionary trajectories to adapt to WGD.

A story from the Miocene: Clock-dated phylogeny of Sisymbrium L. (Sisymbrieae, Brassicaceae)

Morphological variability and imprecise generic boundaries have hindered systematic, taxonomical, and nomenclatural studies of Sisymbrium L. (Brassicaceae, Sisymbrieae DC.). The members of this almost exclusively Old-World genus grow mostly on highly porous substrates across open steppe, semidesert, or ruderal habitats in the temperate zone of the Northern Hemisphere and African subtropics. The present study placed the biological history of Sisymbrium L. into time and space and rendered the tribus Sisymbrieae as monotypic. Five nuclear-encoded and three chloroplast-encoded loci of approximately 85% of all currently accepted species were investigated. Several accessions per species covering their whole distribution range allowed for a more representative assessment of intraspecific genetic diversity. In the light of fossil absence, the impact of different secondary calibration methods and taxon sets on time spans was tested, and we showed that such a combinatorial nested dating approach is beneficial. Multigene phylogeny accompanied with a time divergence estimation analysis placed the onset and development of this tribus into the western Irano-Turanian floristic region during the Miocene. Continuous increase in continentality and decrease in temperatures promoted the diversity of the Sisymbrieae, which invaded the open grasslands habitats in Eurasia, Mediterranean, and South Africa throughout the Pliocene and Pleistocene. Our results support the assumption of the Irano-Turanian region as a biodiversity reservoir for adjacent regions.

Multiple Drivers of High Species Diversity and Endemism Among Alyssum Annuals in the Mediterranean: The Evolutionary Significance of the Aegean Hotspot

The Mediterranean Basin is a significant hotspot of species diversity and endemism, with various distribution patterns and speciation mechanisms observed in its flora. High species diversity in the Mediterranean is also manifested in the monophyletic lineage of Alyssum annuals (Brassicaceae), but little is known about its origin. These species include both diploids and polyploids that grow mainly in open and disturbed sites across a wide elevational span and show contrasting distribution patterns, ranging from broadly distributed Eurasian species to narrow island endemics. Here, we investigated the evolution of European representatives of this lineage, and aimed to reconstruct their phylogeny, polyploid and genome size evolution using flow cytometric analyses, chloroplast and nuclear high- and low-copy DNA markers. The origin and early diversification of the studied Alyssum lineage could be dated back to the Late Miocene/Pliocene and were likely promoted by the onset of the Mediterranean climate, whereas most of the extant species originated during the Pleistocene. The Aegean region represents a significant diversity center, as it hosts 12 out of 16 recognized European species and comprises several (sub)endemics placed in distinct phylogenetic clades. Because several species, including the closest relatives, occur here sympatrically without apparent niche differences, we can reject simple allopatric speciation via vicariance as well as ecological speciation for most cases. Instead, we suggest scenarios of more complex speciation processes that involved repeated range shifts in response to sea-level changes and recurrent land connections and disconnections since the Pliocene. In addition, multiple polyploidization events significantly contributed to species diversity across the entire distribution range. All seven polyploids, representing both widespread species and endemics to the western or eastern Mediterranean, were inferred to be allopolyploids. Finally, the current distribution patterns have likely been affected also by the human factor (farming and grazing). This study illustrates the complexity of evolutionary and speciation processes in the Mediterranean flora.

Angiosperms at the edge: Extremity, diversity, and phylogeny

A hallmark of flowering plants is their ability to invade some of the most extreme and dynamic habitats, including cold and dry biomes, to a far greater extent than other land plants. Recent work has provided insight to the phylogenetic distribution and evolutionary mechanisms which have enabled this success, yet needed is a synthesis of evolutionary perspectives with plant physiological traits, morphology, and genomic diversity. Linking these disparate components will not only lead to better understand the evolutionary parallelism and diversification of plants with these two strategies, but also to provide the framework needed for directing future research. We summarize the primary physiological and structural traits involved in response to cold- and drought stress, outline the phylogenetic distribution of these adaptations, and describe the recurring association of these changes with rapid diversification events that occurred in multiple lineages over the past 15 million years. Across these threefold facets of dry-cold correlation (traits, phylogeny, and time) we stress the contrast between (a) the amazing diversity of solutions flowering plants have developed in the face of extreme environments and (b) a broad correlation between cold and dry adaptations that in some cases may hint at deep common origins.

The Quaternary range dynamics ofNoccaea iberidea(Brassicaceae), a typical representative of subalpine/alpine steppe communities of Anatolian mountains

The responses of Anatolian plants to global climate change have been poorly investigated. In this study, we aimed to understand how climatic oscillation during the Quaternary period helped to shape the current distribution patterns of the Anatolian endemic Noccaea iberidea, a typical representative of tragacanthic (thorny cushion) steppe communities of Anatolia. We used ecological niche modelling combined with statistical phylogeography, based on nuclear ribosomal ITS and plastidic trnL-F and trnS-ycf9 regions. Both the structure of the haplotype networks and the results of the extended Bayesian skyline plot analysis clearly indicated that N. iberidea has been through a recent population expansion. This interpretation was also supported by ecological niche modelling analysis, which showed that the availability of areas of high suitability expanded from the Last Interglacial to the Last Glacial Maximum, suggesting that N. iberidea might have expanded its range during the glacial periods. In conclusion, the study improves our understanding of the demographic history and responses of steppe plant communities of Anatolia to global climate changes through the Late Quaternary glacial–interglacial cycles, which in turn might aid in the development of future conservation strategies.

Parallel ddRAD and Genome Skimming Analyses Reveal a Radiative and Reticulate Evolutionary History of the Temperate Bamboos

Rapid evolutionary radiations are among the most challenging phylogenetic problems, wherein different types of data (e.g., morphology and molecular) or genetic markers (e.g., nuclear and organelle) often yield inconsistent results. The tribe Arundinarieae, that is, the temperate bamboos, is a clade of tetraploid originated 22 Ma and subsequently radiated in East Asia. Previous studies of Arundinarieae have found conflicting relationships and/or low support. Here, we obtain nuclear markers from ddRAD data for 213 Arundinarieae taxa and parallel sampling of chloroplast genomes from genome skimming for 147 taxa. We first assess the feasibility of using ddRAD-seq data for phylogenetic estimates of paleopolyploid and rapidly radiated lineages, optimize clustering thresholds, and analysis workflow for orthology identification. Reference-based ddRAD data assembly approaches perform well and yield strongly supported relationships that are generally concordant with morphology-based taxonomy. We recover five major lineages, two of which are notable (the pachymorph and leptomorph lineages), in that they correspond with distinct rhizome morphologies. By contrast, the phylogeny from chloroplast genomes differed significantly. Based on multiple lines of evidence, the ddRAD tree is favored as the best species tree estimation for temperate bamboos. Using a time-calibrated ddRAD tree, we find that Arundinarieae diversified rapidly around the mid-Miocene corresponding with intensification of the East Asian monsoon and the evolution of key innovations including the leptomorph rhizomes. Our results provide a highly resolved phylogeny of Arundinarieae, shed new light on the radiation and reticulate evolutionary history of this tribe, and provide an empirical example for the study of recalcitrant plant radiations. [Arundinarieae; ddRAD; paleopolyploid; genome skimming; rapid diversification; incongruence.]

Polyploids increase overall diversity despite higher turnover than diploids in the Brassicaceae

Although polyploidy is widespread across the plant Tree of Life, its long-term evolutionary significance is still poorly understood. Here, we examine the effects of polyploidy in explaining the large-scale evolutionary patterns within angiosperms by focusing on a single family exhibiting extensive interspecific variation in chromosome numbers. We inferred ploidy from haploid chromosome numbers for 80% of species in the most comprehensive species-level chronogram for the Brassicaceae. After evaluating a total of 94 phylogenetic models of diversification, we found that ploidy influences diversification rates across the Brassicaceae. We also found that despite diversifying at a similar rate to diploids, polyploids have played a significant role in driving present-day differences in species richness among clades. Overall, in addition to highlighting the complexity in the evolutionary consequences of polyploidy, our results suggest that rare successful polyploids persist while significantly contributing to the long-term evolution of clades. Our findings further indicate that polyploidy has played a major role in driving the long-term evolution of the Brassicaceae and highlight the potential of polyploidy in shaping present-day diversity patterns across the plant Tree of Life.

Phylogeny of Euclidieae (Brassicaceae) based on plastome and nuclear ribosomal DNA data

Euclidieae, a morphologically diverse tribe in the family Brassicaceae (Cruciferae), consists of 29 genera and more than 150 species distributed mainly in Asia. Prior phylogenetic analyses on Euclidieae are inadequate. In this study, sequence data from the plastid genome and nuclear ribosomal DNA of 72 species in 27 genera of Euclidieae were used to infer the inter- and intra-generic relationships within. The well-resolved and strongly supported plastome phylogenies revealed that Euclidieae could be divided into five clades. Both Cymatocarpus and Neotorularia are polyphyletic in nuclear and plastome phylogenies. Besides, the conflicts of systematic positions of three species of Braya and two species of Solms-laubachia s.l. indicated that hybridization and or introgression might have happened during the evolutionary history of the tribe. Results from divergence-time analyses suggested an early Miocene origin of Euclidieae, and it probably originated from the Central Asia, Pamir Plateau and West Himalaya. In addition, multiple ndh genes loss and pseudogenization were detected in eight species based on comparative genomic study.

Nested whole-genome duplications coincide with diversification and high morphological disparity in Brassicaceae

Angiosperms have become the dominant terrestrial plant group by diversifying for ~145 million years into a broad range of environments. During the course of evolution, numerous morphological innovations arose, often preceded by whole genome duplications (WGD). The mustard family (Brassicaceae), a successful angiosperm clade with ~4000 species, has been diversifying into many evolutionary lineages for more than 30 million years. Here we develop a species inventory, analyze morphological variation, and present a maternal, plastome-based genus-level phylogeny. We show that increased morphological disparity, despite an apparent absence of clade-specific morphological innovations, is found in tribes with WGDs or diversification rate shifts. Both are important processes in Brassicaceae, resulting in an overall high net diversification rate. Character states show frequent and independent gain and loss, and form varying combinations. Therefore, Brassicaceae pave the way to concepts of phylogenetic genome-wide association studies to analyze the evolution of morphological form and function.

Nested whole-genome duplications coincide with diversification and high morphological disparity in Brassicaceae

Angiosperms have become the dominant terrestrial plant group by diversifying for ~145 million years into a broad range of environments. During the course of evolution, numerous morphological innovations arose, often preceded by whole genome duplications (WGD). The mustard family (Brassicaceae), a successful angiosperm clade with ~4000 species, has been diversifying into many evolutionary lineages for more than 30 million years. Here we develop a species inventory, analyze morphological variation, and present a maternal, plastome-based genus-level phylogeny. We show that increased morphological disparity, despite an apparent absence of clade-specific morphological innovations, is found in tribes with WGDs or diversification rate shifts. Both are important processes in Brassicaceae, resulting in an overall high net diversification rate. Character states show frequent and independent gain and loss, and form varying combinations. Therefore, Brassicaceae pave the way to concepts of phylogenetic genome-wide association studies to analyze the evolution of morphological form and function.

Recent accelerated diversification in rosids occurred outside the tropics

Conflicting relationships have been found between diversification rate and temperature across disparate clades of life. Here, we use a supermatrix comprising nearly 20,000 species of rosids-a clade of ~25% of all angiosperm species-to understand global patterns of diversification and its climatic association. Our approach incorporates historical global temperature, assessment of species' temperature niche, and two broad-scale characterizations of tropical versus non-tropical niche occupancy. We find the diversification rates of most subclades dramatically increased over the last 15 million years (Myr) during cooling associated with global expansion of temperate habitats. Climatic niche is negatively associated with diversification rates, with tropical rosids forming older communities and experiencing speciation rates ~2-fold below rosids in cooler climates. Our results suggest long-term cooling had a disproportionate effect on non-tropical diversification rates, leading to dynamic young communities outside of the tropics, while relative stability in tropical climes led to older, slower-evolving but still species-rich communities.

Genome Evolution in Arabideae Was Marked by Frequent Centromere Repositioning

Centromere position may change despite conserved chromosomal collinearity. Centromere repositioning and evolutionary new centromeres (ENCs) were frequently encountered during vertebrate genome evolution but only rarely observed in plants. The largest crucifer tribe, Arabideae (∼550 species; Brassicaceae, the mustard family), diversified into several well-defined subclades in the virtual absence of chromosome number variation. Bacterial artificial chromosome-based comparative chromosome painting uncovered a constancy of genome structures among 10 analyzed genomes representing seven Arabideae subclades classified as four genera: Arabis, Aubrieta, Draba, and Pseudoturritis Interestingly, the intra-tribal diversification was marked by a high frequency of ENCs on five of the eight homoeologous chromosomes in the crown-group genera, but not in the most ancestral Pseudoturritis genome. From the 32 documented ENCs, at least 26 originated independently, including 4 ENCs recurrently formed at the same position in not closely related species. While chromosomal localization of ENCs does not reflect the phylogenetic position of the Arabideae subclades, centromere seeding was usually confined to long chromosome arms, transforming acrocentric chromosomes to (sub)metacentric chromosomes. Centromere repositioning is proposed as the key mechanism differentiating overall conserved homoeologous chromosomes across the crown-group Arabideae subclades. The evolutionary significance of centromere repositioning is discussed in the context of possible adaptive effects on recombination and epigenetic regulation of gene expression.

So Closely Related and Yet So Different: Strong Contrasts Between the Evolutionary Histories of Species of the Cardamine pratensis Polyploid Complex in Central Europe

Recurrent polyploid formation and weak reproductive barriers between independent polyploid lineages generate intricate species complexes with high diversity and reticulate evolutionary history. Uncovering the evolutionary processes that formed their present-day cytotypic and genetic structure is a challenging task. We studied the species complex of Cardamine pratensis, composed of diploid endemics in the European Mediterranean and diploid-polyploid lineages more widely distributed across Europe, focusing on the poorly understood variation in Central Europe. To elucidate the evolution of Central European populations we analyzed ploidy level and genome size variation, genetic patterns inferred from microsatellite markers and target enrichment of low-copy nuclear genes (Hyb-Seq), and environmental niche differentiation. We observed almost continuous variation in chromosome numbers and genome size in C. pratensis s.str., which is caused by the co-occurrence of euploid and dysploid cytotypes, along with aneuploids, and is likely accompanied by inter-cytotype mating. We inferred that the polyploid cytotypes of C. pratensis s.str. are both of single and multiple, spatially and temporally recurrent origins. The tetraploid Cardamine majovskyi evolved at least twice in different regions by autopolyploidy from diploid Cardamine matthioli. The extensive genome size and genetic variation of Cardamine rivularis reflects differentiation induced by the geographic isolation of disjunct populations, establishment of triploids of different origins, and hybridization with sympatric C. matthioli. Geographically structured genetic lineages identified in the species under study, which are also ecologically divergent, are interpreted as descendants from different source populations in multiple glacial refugia. The postglacial range expansion was accompanied by substantial genetic admixture between the lineages of C. pratensis s.str., which is reflected by diffuse borders in their contact zones. In conclusion, we identified an interplay of diverse processes that have driven the evolution of the species studied, including allopatric and ecological divergence, hybridization, multiple polyploid origins, and genetic reshuffling caused by Pleistocene climate-induced range dynamics.

Zahora, a new monotypic genus from tribe Brassiceae (Brassicaceae) endemic to the Moroccan Sahara

Zahora ait-atta Lemmel & M.Koch, a new species from the Moroccan Sahara, is described and documented here and constitutes a monotypic new genus. The new taxon belongs to the tribe Brassiceae (Brassicaceae), and cytogenetic and phylogenetic analyses reveal that this diploid species has a remote status of Miocene origin in the northwestern Sahara Desert. We examined the morphological differences between morphologically related genera and provide photographs of the new species. The new genus may play a key role in future Brassica-Raphanus crop research since it is placed phylogenetically at the base of a generically highly diverse clade including Raphanus sativus, and it shows affinities to various Brassica species.